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Can I use a 16 ohm speaker w/ 8 ohm guitar amp, should I wire 16 ohm resistor in?
Old 2 weeks ago
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Can I use a 16 ohm speaker w/ 8 ohm guitar amp, should I wire 16 ohm resistor in?

I have a 16 ohm guitar speaker I'd like to use with some amps that are 8ohms.

First of all, what might happen if I did that without taking any further precautions?


Second of all, Could I simply wire a 16 ohm resistor in parallel with the speaker to approximate an 8 ohm load and would that be more safe for the amps?

Thanks!

-Kevin
Old 2 weeks ago
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Lastly, if it was "safe" to simply use as is without a resistor in parallel, would the affect be that the output would be lower/less efficient because the speaker/driver is "resisting" more ? If so that might actually be a desireable effect, meaning I could turn the amp up hotter with less volume/SPL output. Or am I misunderstanding the role of resistance .. thanks!
Old 2 weeks ago
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Depends on the amp. Most tube amps OTs can take a 1:2 mismatch either way. It will change the resonance point of the OT though. For a tube amp, going into a less resistive load is actually less stressful on the OT. As a general rule, I avoid going into a higher impedance load. If the OT sees too much resistance, that energy has to go somewhere...and in this case it is released as heat in the OT and at extremes, can even arc between windings, and fry the OT. Again, most likely not at a 1:2 mismatch, but I prefer to err on the side of caution.

Tube amp output wattage is not effected by speaker load nearly as extreamly transistor amps are. You are not likely to here a volume difference in a mismatch situation. You may hear a tonal difference due to the change in resonance peak of the OT though.
Old 2 weeks ago
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Quote:
Originally Posted by Joe Porto View Post
Depends on the amp. Most tube amps OTs can take a 1:2 mismatch either way. It will change the resonance point of the OT though. For a tube amp, going into a less resistive load is actually less stressful on the OT. As a general rule, I avoid going into a higher impedance load. If the OT sees too much resistance, that energy has to go somewhere...and in this case it is released as heat in the OT and at extremes, can even arc between windings, and fry the OT. Again, most likely not at a 1:2 mismatch, but I prefer to err on the side of caution.

Tube amp output wattage is not effected by speaker load nearly as extreamly transistor amps are. You are not likely to here a volume difference in a mismatch situation. You may hear a tonal difference due to the change in resonance peak of the OT though.

Thanks for the info. So the speaker measures about 13 ohms on my meter, this must be standard because my "8 ohm" speakers only measure about 5.5 ohms also.

But for the "16 ohm" speaker I could wire a resistor in parallell to make it less stressful for my OT right? thanks
Old 2 weeks ago
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Quote:
Originally Posted by klong View Post
Thanks for the info. So the speaker measures about 13 ohms on my meter, this must be standard because my "8 ohm" speakers only measure about 5.5 ohms also.

But for the "16 ohm" speaker I could wire a resistor in parallell to make it less stressful for my OT right? thanks
Technically, yes. You would need a power resistor greater than 1/2 the total wattage of the amp. And since the speaker is reactive, where the resistor is not, more power may go to the resistor than the speaker, so to be safe, I would go even higher. I'm not sure if you will be able to find a 16 ohm power resistor though. You may have to wire 2 8 ohm resistors in series, then wire the pair in parallel with the speaker. You'll want to build a box for it, as there will be a good deal of power on the exposed leads of the resistors and wiring.

In all honesty, I would just get an 8 ohm speaker.
Old 2 weeks ago
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Quote:
Originally Posted by Joe Porto View Post
For a tube amp, going into a less resistive load is actually less stressful on the OT. As a general rule, I avoid going into a higher impedance load.
That's exactly opposite of what I've always heard. Be careful...
Old 2 weeks ago
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Originally Posted by OldTools View Post
That's exactly opposite of what I've always heard. Be careful...
Tube amps ARE the opposite of transistor amps regarding speaker load. Transistor amps can tolerate infinite load (open circuit), so higher load than spec'd is not a problem, and only results in lost power.

Tube amps can tolerate no load (short circuit) or less load than spec'd better than higher loads or open circuit. That's why tube amp speaker jacks are designed to short when nothing is plugged in. Basically, due to the output transformer, if the secondary sees too much resistance, the magnetic field breaks down, and the energy builds up in the primary, essentially turning it into a heating element.

I found this post at the TDPRI forum, by a user named Jammin' John. It summarizes pretty well how tube amps handle load:


Actually a greater load is bad for a tube amp.
The reason is called reflected impedance and it's bad news.
If you are going to mismatch VALVE amps it is actually less bad, (for the amp or more specifically, the Output Transformer), to mismatch to a lower impedance, (16 ohm into 8 ohm cab), rather then higher. Unlike solid-state amps, valve amps are basically self limiting current wise (the valves!) into a lower impedance, though the valves will take more wear. Into a too high impedance the risk is different - potential very high flyback voltages can fry the OT. But most valve amps with strong output transformers will take a 1/2 to 2x mismatch without complaining.

Although not recommended, technically you can short circuit a valve amps output (0 ohms) without frying it - it is trying to put a signal into an open circuit (virtually infinite ohms) that is a real killer. Total opposite from solid-state amps of course - short circuits will kill the power transistors pronto, but they'll sit happily all day with no speaker load applied.

Solid State amps: Safe with rated load or with any higher impedance speaker load (up to & including an open circuit ie infinite ohms.) Develop less power as impedance increases. Do not short circuit (0 ohms) as this is sudden death for the output transistors.

Valve amps: Match impedance if possible. If mismatching, it is safer for the amp to mismatch low. This will wear the valves but the amp shouldn't suffer. A short circuit (0 ohms) is normally survivable.

If mismatching high there is a risk to the OT, which increases with the severity of the mismatch, & with how hard you are pushing the valve output section. The ultimate 'high' mismatch is no speaker load ie infinite ohms. If trying to pass a signal into this then there is a severe risk to the OT from high flashback voltages which can arc through the insulation layers & burn out the tranny. Mismatching between 1/2 & 2x the impedance the amp 'expects to see' is normally problem free for most amps with healthy OTs. It is never guaranteed safe though, & being manufacturer specific Marshalls fail much more often when doing this then Fenders do. There is a lot of misunderstanding about impedance & mismatching issues, but I repeat that the advice I'm giving is correct for valve amps. (The exact opposite is true for solid state amps, but they work very differently, usually having no output transformers for starters!)

For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes & leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.

Yes running 8 ohm amp --> 16 ohm cab is probably within normal safe limits (within 1/2 to 2x impedance range) for most valve amps. In fact you'll often get away with playing Russian Roulette by running a 4 ohm amp to a 16 ohm cab, but the risk to transformers is definitely greater when going into a higher then intended impedance. You really are always better to have the speaker impedance lower if mismatching! If you value your amp that is. Going lower strains the valves more then normal, but they are disposable in a way that the output transformer isn't. (Which is after all why valves are removable from their sockets.)

The thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, & acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer & short the windings. I would not go above double the rated load on any tap. & NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

It's almost never low impedance that kills an OT, it's too high an impedance. The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all. If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary & between both half- primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike. This voltage spike can punch through insulation or flash over sockets, & the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, & eventually erode the insulation to make a conductive path between layers. The sound goes south, & with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, & now you have a dead transformer.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high. For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper & hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break before make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted & the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

Too high impedances on the speaker outputs are much more dangerous for valve amps then too low impedances are. You can short circuit the + & - speaker output connections by connecting them together. (This gives very low impedance, close to zero ohms.) Valve amps can survive this. But if you don't believe me then try this: power up your own valve amp, unplug your speaker, take it off standby & crank that baby into an open circuit very high impedance load. (Fresh air) You stand a very good chance of frying your amps OT!

Speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.
Old 1 week ago
  #8
Quote:
Originally Posted by Joe Porto View Post
Tube amps ARE the opposite of transistor amps regarding speaker load. Transistor amps can tolerate infinite load (open circuit), so higher load than spec'd is not a problem, and only results in lost power.

Tube amps can tolerate no load (short circuit) or less load than spec'd better than higher loads or open circuit. That's why tube amp speaker jacks are designed to short when nothing is plugged in. Basically, due to the output transformer, if the secondary sees too much resistance, the magnetic field breaks down, and the energy builds up in the primary, essentially turning it into a heating element.

I found this post at the TDPRI forum, by a user named Jammin' John. It summarizes pretty well how tube amps handle load:


Actually a greater load is bad for a tube amp.
The reason is called reflected impedance and it's bad news.
If you are going to mismatch VALVE amps it is actually less bad, (for the amp or more specifically, the Output Transformer), to mismatch to a lower impedance, (16 ohm into 8 ohm cab), rather then higher. Unlike solid-state amps, valve amps are basically self limiting current wise (the valves!) into a lower impedance, though the valves will take more wear. Into a too high impedance the risk is different - potential very high flyback voltages can fry the OT. But most valve amps with strong output transformers will take a 1/2 to 2x mismatch without complaining.

Although not recommended, technically you can short circuit a valve amps output (0 ohms) without frying it - it is trying to put a signal into an open circuit (virtually infinite ohms) that is a real killer. Total opposite from solid-state amps of course - short circuits will kill the power transistors pronto, but they'll sit happily all day with no speaker load applied.

Solid State amps: Safe with rated load or with any higher impedance speaker load (up to & including an open circuit ie infinite ohms.) Develop less power as impedance increases. Do not short circuit (0 ohms) as this is sudden death for the output transistors.

Valve amps: Match impedance if possible. If mismatching, it is safer for the amp to mismatch low. This will wear the valves but the amp shouldn't suffer. A short circuit (0 ohms) is normally survivable.

If mismatching high there is a risk to the OT, which increases with the severity of the mismatch, & with how hard you are pushing the valve output section. The ultimate 'high' mismatch is no speaker load ie infinite ohms. If trying to pass a signal into this then there is a severe risk to the OT from high flashback voltages which can arc through the insulation layers & burn out the tranny. Mismatching between 1/2 & 2x the impedance the amp 'expects to see' is normally problem free for most amps with healthy OTs. It is never guaranteed safe though, & being manufacturer specific Marshalls fail much more often when doing this then Fenders do. There is a lot of misunderstanding about impedance & mismatching issues, but I repeat that the advice I'm giving is correct for valve amps. (The exact opposite is true for solid state amps, but they work very differently, usually having no output transformers for starters!)

For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes & leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.

Yes running 8 ohm amp --> 16 ohm cab is probably within normal safe limits (within 1/2 to 2x impedance range) for most valve amps. In fact you'll often get away with playing Russian Roulette by running a 4 ohm amp to a 16 ohm cab, but the risk to transformers is definitely greater when going into a higher then intended impedance. You really are always better to have the speaker impedance lower if mismatching! If you value your amp that is. Going lower strains the valves more then normal, but they are disposable in a way that the output transformer isn't. (Which is after all why valves are removable from their sockets.)

The thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, & acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer & short the windings. I would not go above double the rated load on any tap. & NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

It's almost never low impedance that kills an OT, it's too high an impedance. The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all. If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary & between both half- primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike. This voltage spike can punch through insulation or flash over sockets, & the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, & eventually erode the insulation to make a conductive path between layers. The sound goes south, & with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, & now you have a dead transformer.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high. For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper & hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break before make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted & the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

Too high impedances on the speaker outputs are much more dangerous for valve amps then too low impedances are. You can short circuit the + & - speaker output connections by connecting them together. (This gives very low impedance, close to zero ohms.) Valve amps can survive this. But if you don't believe me then try this: power up your own valve amp, unplug your speaker, take it off standby & crank that baby into an open circuit very high impedance load. (Fresh air) You stand a very good chance of frying your amps OT!

Speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.
Your electronics are correct but your terminology is backwards.

The lower the impedance, the greater the load (the amp has to do more work.) Short circuit is infinite load.

The higher the impedance, the less the load. Open circuit is no load.

I know it's counterintuitive at first, but when you think about it, it makes sense, especially when you use a solid state amp as an example. When the impedance goes down, the power output of the amp increases, so it's doing more work (greater load.) When the impedance goes up, power output goes down, so less work (less load.) Tube amps don't behave like this because of the transformer, so it's less obvious there but it still holds - higher impedance, lower load. Tube amps like to be loaded so those nasty things don't happen to the transformer - they want the load to take all they try to give it.

I usually try to use the terms "higher impedance" and "lower impedance" rather than greater or lesser load to avoid this common confusion.

Traditional Fender amps can usually go double or half rated impedance. I don't really know about the newer, semi affordable models, but given the other cost cutting measures in them I don't trust them. Most Brit type amps are much less tolerant, as are boutique amps that operate their power stages closer to the "bleeding edge".
Old 1 week ago
  #9
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Quote:
Originally Posted by John Eppstein View Post
Your electronics are correct but your terminology is backwards.

The lower the impedance, the greater the load (the amp has to do more work.) Short circuit is infinite load.
Right. I always screw that up...lol.
Old 1 week ago
  #10
Quote:
Originally Posted by Joe Porto View Post
Right. I always screw that up...lol.
So, it would appear, did the guy you quoted. LOTS of people do. I did until relatively recently.
Old 1 week ago
  #11
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foldback's Avatar
Back when I was doing electrical engineering for a job we study things like loads and current.

I have a couple of Boogie combo amps that only have 8 or 4 ohm output jacks.

I've been driving 16-ohm Marshall cabinets with them both for years, never had a problem.

Current is current, whether it's a solid state circuit or tube circuit.

A 16 ohm speaker draws less current than an 8-ohm speaker and puts less stress on the transformer secondary even if it is looking for an 8-ohm load. The transformer does not transfer power as efficiently in this set up but in 45 years of playing with tube amps I've never seen this direction and this level of mismatched impedance to be detrimental to the amplifier or the speaker.

Having a 4-ohm speaker load connected to an amp with 8-ohm specific secondaries is harder on the output transformer but I've seen it done all the time.

Ideally an 8-ohm load would certainly be preferable if it is a tube amp with a transformer that is 8-ohm specific.

The only tube amps I've ever seen smoked with improper impedance mismatches were also old amps from the 50's with barely adequate output transformer designs. Back in 1964 when I was young my brother ran his Super Reverb into a single showman cab with an 8-ohm JBL in it, that was a 2-ohm specific amp driving an 8-ohm load, never had a problem with it.

The op has never mentioned whether the amp in question is solid state or tube.

If the amp is solid state, it should easily drive a 16 ohm load. Perhaps the marking on the amp really means don't load the output below 8 ohms, that's a whole different thing though. I have a little solid state stomp box amp by Electro Harmonix, the 44 Magnum. It makes its full power at 8 ohms and specifically says "do not connect to 4 ohm load or else output protection will mute the amp". This little pedal drives a 16 ohm Marshall cabinet to very annoyingly loud levels though you only get 22 watts at 16 ohms.

Adding power resisters to the circuit is not recommended unless it's a tube amp you're trying to attenuate. Adding load resisters will actually make the output transformer work harder driving a non-reactive load than just playing the 16ohm speaker from the 8 ohm output of something like a Boogie Studio 22.

Hope that helps. Good luck and good music to all!
Old 1 week ago
  #12
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We don't need to be rewriting the text books here because people aren't used to using the terminology.

Conductance and resistance are opposites. As conductance goes up, resistance goes down.

A short circuit is never referred to as an infinite load in electronics. #1 reason being is it can too easily be confused with infinite resistance (No Load)

You could call a short maximum load, no resistance or maximum conductance.
You can call No load as Infinite resistance or No conductance.

Using any other terminology is simply going to confuse people who are trying to wrap they're heads around the problems matching impedance.

DC resistance is not impedance. Bot are read in ohms but the DC resistance is only for helping identify what an unlabeled speaker might be. Its usually off by a couple of ohms and the earlier examples are classic. Speakers are electromagnets and the inductances of the coil passing an AC current creates a back EMF which resists the changes in polarity as the AC swings.

The impedance in ohms varies depending on the frequencies being produced. This is why a straight resistor of any kind is a poor solution to impedance matching. Its not going to change resistance with frequency. A coil acts like a shock absorber to the signal, a DC resistor is like a brick wall and hard on an amp head.

Running a mis matched impedance really depends on the head type. If it was a Marshall Plexi Id say do not run it at anything besides a perfect match. (Of course the heads have an impedance switch/plug so you can run it at 3 different impedances so it wouldn't be a problem but you are much more likely to blow that head if you didn't switch the transformer taps)

Other Tune heads like my old Bassman is a 4 ohm head. It can be safely run at 8 ohms for years. If you run it at 16 ohms you'll blow the tubes and make them red plate within a week and blow the screen resistors. I've done it first hand 50 years ago before I was formally trained in electronics and knew what I was doing.

If this is a transistor head you shouldn't have any problems running 16 ohms. If you're worried about it simply buy another cheap 16 ohm speaker. and run 8 ohms. Not sure what wattage we're talking about. Using two speakers the speaker only needs to be half the heads maximum. These warehouse speakers for example is only $39. Ebay is loaded with good deals too. http://www.guitarcenter.com/Warehouse-Guitar-Speakers/G8C-8-20W-American-Vintage-Guitar-Speaker-16-ohms-1378824447929.gc?cntry=us&source=4WWRWXGP&gclid=EAIaIQobChMIt-nyy8aE1QIViQJpCh0lyQNtEAQYASABEgKLMPD_BwE&kwid=productads-adid^172488555108-device^c-plaid^145409138682-sku^1378824447929@ADL4GC-adType^PLA
Old 1 week ago
  #13
Quote:
Originally Posted by foldback View Post
Back when I was doing electrical engineering for a job we study things like loads and current.

I have a couple of Boogie combo amps that only have 8 or 4 ohm output jacks.

I've been driving 16-ohm Marshall cabinets with them both for years, never had a problem.

Current is current, whether it's a solid state circuit or tube circuit.

A 16 ohm speaker draws less current than an 8-ohm speaker and puts less stress on the transformer secondary even if it is looking for an 8-ohm load. The transformer does not transfer power as efficiently in this set up but in 45 years of playing with tube amps I've never seen this direction and this level of mismatched impedance to be detrimental to the amplifier or the speaker.
Well, with nearly 30 years working as a service tech on musical instrument amps, I've seen a number of them, including a couple of Fenders. A few years ago Vintage Guitar magazine ran an article with photo evidence of the typical kind of damage in the output transformer of a vintage Vox amp, disassembling the windings to reveal the pinhole burns in the coil.

Most vintage Fenders are more or less immune, and Boogies even more so because Boogie and Fender (for the most part) used transformers that were well overbuilt for the requirements of the amp. Not all manufacturers do. Be thankful you have a Boogie. Old Super Reverbs also have a very over-built transformer, it's essentially the one used in the Bassman except for load impedance.

Quote:
Adding power resisters to the circuit is not recommended unless it's a tube amp you're trying to attenuate. Adding load resisters will actually make the output transformer work harder driving a non-reactive load than just playing the 16ohm speaker from the 8 ohm output of something like a Boogie Studio 22.
Tube amps LIKE to be loaded - it's when the load is too light (too high an impedance) that you get problems.
Old 1 week ago
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I said "The only tube amps I've ever seen smoked with improper impedance mismatches were also old amps from the 50's with barely adequate output transformer designs. "

Vintage Vox were always questionable builds in my experience.

I've been responsible for winding thousands of transformers for a variety of applications so I'm familiar with them. I ran the service department and had several techs that reported to me so I was always aware of trends. We specialized in odd tube amp repair throughout the 80's, Marshall, Hiwatt, Fender and Boogie were warranty repair products for us. We were also factory authorized warranty repair for Crate, GK, Fostex, Allen and Heath and many other products.

Speakers are not a static impedance, they're rarely 8 ohms or 16 ohms, it depends on the frequency coming across the coils and so many other factors.

A mismatch of 8 to 16 is nothing compared to a short or no load which causes quick death in a tube amp.

Good music to all!
Old 1 week ago
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It might be worthwhile to consider that in well-made amplifiers, like Fender, having overbuilt OTs, that load impedance is far more important to the mode of operation of the output tubes than the OTs.

For example, if you look in a tube manual for 6L6s, and many Pentode power tubes, you will see that the recommended load seen by a Push Pull set of 6L6s in Class AB is roughly 3800 Ohms. The same tubes in Class A prefer a load of ~6600 Ohms, which is only slightly lower than the average load (7600 Ohms) caused by connecting a 16 Ohm speaker to an 8 Ohm output.

I'd never place a purely resistive 16 ohm load in parallel with a speaker for anything but bench testing. For live playing, I would and have often connected a 16 Ohm speaker to an 8 Ohm output and simply dropped the bias voltage closer to Class A operation, reducing the output but increasing sensitivity Many players prefer the response and tonality of Class A operation.

Totally anecdotal but FWIW I played a Princeton Reverb for a couple years, averaging 4-6 shows a week, with the original output transformer (rated at 20 Watts) running 2 x EL34s in Class A (nearly 30 watts after a minor Power Transformer upgrade) though it was also fan cooled. About 8 months after the PT upgrade, I replaced the OT with one from a Leslie 147 (complete with the OC3 grid supply regulator and Cathode Bias) rated at over 40 watts and got a matching 16 Ohm speaker and breathed a sigh of relief.even though the original OT never gave me any sign of problems. It did sound better as well as being better headroom for reliability.

I've witnessed some crazy variations on this phenomenon. I saw a guy plug the output of one Silvertone tube amp into the input of another and it lasted several songs before the first amp fried. However I knew a guy who plugged a 50 watt Kustom SS head output into the guitar input jack of a Marshall Major which despite my horror, went on happily for years and oddly enough, sounded amazing..
Old 1 week ago
  #16
Quote:
Originally Posted by foldback View Post
I said "The only tube amps I've ever seen smoked with improper impedance mismatches were also old amps from the 50's with barely adequate output transformer designs. "
Not quite true - the "only" part, I mean.

There were a lot of amps made in the '50s with inadequate transformers, either because they were cheaply made or because the requirements of reliable transformers for audio amps were still evolving, but that's not the whole story. Amp design enters into it as well. Some amps are designed with tubes and transformers running extremely close to design maximum tolerances, in certain cases exceeding them. Such amps are often pretty intolerant of mismatches. I'm thinking particularly of certain British amps fr4om the late '60s through mid to late '70s, in particular many Marshalls (which, of course, had triple impedance selection available, so there's no real excuse for mismatching.) The Marshall Major in particular was notorious for popping output trannies, often taking out the power transformer as well, because, although it was equipped with very hefty transformers it was simply running the output tubes too hot. Majors would often blow OTs without mismatching, for no obvious reason. Mismatching a Major was an open invitation for the magic smoke to make an appearance. Old Marshalls also had a problem with the impedance selector, with the plug-in widget that would sometimes fall out and get lost when the socket contacts got loose. Although I've been out of the professional amp repair game for just shy of two decades I'm somewhat leery of certain modern boutique builders because the both bias their amps too hot and run the plate voltage higher than spec'd maximum. I understand why they do it - it's a particular sound that's quite popular in some circles - but it's dangerous.

Another problem with some '50s amps that can lead to transformer problems is that they were designed for earlier version of output tubes, which were much lower wattage than later, upgraded versions of the same tube. An amp designed to be used with the old metal shell 6L6 tubes (which were were spec'd at around 15 watts each) might not get along very well with modern 6L6GCs, which deliver around twice that. So in such cases it's not so much "inadequate transformers" - the transformers were perfectly adequate when the amp was made - so much as using tubes that the amp was never intended to use.

Quote:
Vintage Vox were always questionable builds in my experience.
If depends. According to the article in Vintage Guitar Vox went through several sources for transformers during their early days. The transformers from their original supplier were probably no more unreliable than most, but after a little time they switched suppliers for some reason. The new supplier did not use the same material to pot the transformer as the originals, substituting wax. Wax is not a good potting material for transformers in power handling applications as it tends to melt when it gets warm and run out of the coils. This exacerbates heat buildup and drastically shortens transformer life. Also, since the potting material has insulating properties when the wax goes away the insulation in the coil is reduced, making flashover between windings more likely. And without adequate potting the wires in the coil can and will move with the signal, which could cause wear on the enamel insulation of the wire itself. A lot of Voxes were made with the inferior transformers before Vox corrected the problem by changing suppliers again.

Actually I never thought about it much before I ran across that article that drew my attention to it, but quite a few of the dead transformers I've seen over the years showed signs of wax leakage.

Quote:
I've been responsible for winding thousands of transformers for a variety of applications so I'm familiar with them. I ran the service department and had several techs that reported to me so I was always aware of trends. We specialized in odd tube amp repair throughout the 80's, Marshall, Hiwatt, Fender and Boogie were warranty repair products for us. We were also factory authorized warranty repair for Crate, GK, Fostex, Allen and Heath and many other products.
Interesting. Although I was warranty repair for quite a few brands if amplifiers I never got into transformer winding, although I would have liked to. Did you wind transformers for mostly musical instrument amplifiers or did you also get into the more esoteric aspects of audio transformers, like the specialized designs used in tube mics, and the various transformers used in various tube processors like compressors, etc? Do you know anything about special winding techniques that some companies employed or about the metallurgy of core material?

Quote:
Speakers are not a static impedance, they're rarely 8 ohms or 16 ohms, it depends on the frequency coming across the coils and so many other factors.
Quite true. However since low frequencies comprise the greater portion of power in an audio amp, lower frequency performance would of greatest concern.

Quote:
A mismatch of 8 to 16 is nothing compared to a short or no load which causes quick death in a tube amp.
Unless there's something really wrong with the design of the amp a tube amp should be able to tolerate a dead short for a fair amount of time (certainly long enough for the player to notice there's no sound) without any more problem than a bit of additional stress and wear on the tubes - which most, if not all vintage Fender amplifiers employ a shorting jack on the main speaker out. That way the amplifier will never be operated in an unloaded condition (unless the switching jack goes bad which is pretty unlikely.) A number of other companies followed suit, and I've never understood why any tube amp company wouldn't incorporate this cheap insurance unless they employed some other means of output protection. ( Dynaco hi-fi amps, which provided the power amp circuit and transformers for all the original Sunn tube amps used a 5 watt resistor - 10 ohms IIRC?) across one of the secondary windings of the transformer. I'm not sure if all the Sunns retained this feature but I know the first models certainly dad. Old Ampegs which were not combo amps used an ingenious protection system with their 4 pin XLR speaker connector on the main output. The two "extra" pins were shorted on the cabinet side of the speaker connection. When the speaker was not connected this would lift the B+ from the power tubes, throwing the power amp into standby mode but allowing operation of the preamp, which was equipped with one of the first line outs for recording. This scheme was employed in a couple of variations up through the first generation of SVTs but was rather stupidly discontinued when St. Louis Music bought Ampeg in the early '70s.)

Quote:
Good music to all!
And the same to you!

Last edited by John Eppstein; 1 week ago at 09:16 PM..
Old 1 week ago
  #17
Quote:
Originally Posted by enorbet2 View Post
It might be worthwhile to consider that in well-made amplifiers, like Fender, having overbuilt OTs, that load impedance is far more important to the mode of operation of the output tubes than the OTs.

For example, if you look in a tube manual for 6L6s, and many Pentode power tubes, you will see that the recommended load seen by a Push Pull set of 6L6s in Class AB is roughly 3800 Ohms. The same tubes in Class A prefer a load of ~6600 Ohms, which is only slightly lower than the average load (7600 Ohms) caused by connecting a 16 Ohm speaker to an 8 Ohm output.
Well, they want to see that higher impedance on the primary because Class runs the tubes a lot harder than Class AB. Something that makes very uneasy about some of the new boutique amps is they're biasing their what in my opinion is much too hot because they say it sounds better. Which is true but I'd much rather have an amp that doesn't sounf quite and works than an amp that sounds utterly amazing until it emits the magic smoke. Go ahead, call me a conservative old man! When what you're conserving is your output transformer a little conservatism MIGHT not be bad thing.

Quote:
I'd never place a purely resistive 16 ohm load in parallel with a speaker for anything but bench testing. For live playing, I would and have often connected a 16 Ohm speaker to an 8 Ohm output and simply dropped the bias voltage closer to Class A operation, reducing the output but increasing sensitivity Many players prefer the response and tonality of Class A operation.
Sure, if you know the amp is designed for it. However I've seen a lot of guys who bias their amps too close to class A and wind up costing themselves a nice chunk of change...

Quote:
I've witnessed some crazy variations on this phenomenon. I saw a guy plug the output of one Silvertone tube amp into the input of another and it lasted several songs before the first amp fried.
Amazing! Those old Danelectro built Silvertones don't exactly have the best quality transformers. On the other hand they also don't really have nearly the output they claimed - actually about half in most cases IIRC. The 2-6L6 version only puts out about 25 real watts with new tubes.

Quote:
However I knew a guy who plugged a 50 watt Kustom SS head output into the guitar input jack of a Marshall Major which despite my horror, went on happily for years and oddly enough, sounded amazing..
EEK!!!! That's one of the most extreme cases of sheer dumb luck I've heard of in the context. I wonder if the Major was modded at all for reliability, or at least a reduced inclination to go up in smoke?

Of course the Kustom was putting out very little power into the 1 meg input impedance of the Major, but still...... I'm really surprised that it sounded OK....
Old 1 week ago
  #18
Gear Addict
 

It's so nice to read an intelligent discussion among intelligent and qualified techs. I had nearly forgotten things like this happened around here. Seriously. Nice work
Old 1 week ago
  #19
Lives for gear
 
foldback's Avatar
Ampeg was acquired by Magnavox in the early 70's. SLM did not acquire the name until the mid 1980's, all they bought was the name. There was very little IP that came with the purchase because the company had been essentially dead for five years.

Many of my friends in St Louis worked at SLM. A young repair customer of mine grew up, got his electrical engineering degree and became their chief tube amp designer, he is now with the son (of the former SLM owner) who is trying to get Magnatone off the ground.

All the SLM Ampegs were clean sheet designs or created based of schematic info they collected. The SVT amps built by SLM always sounded cold and weak compared to the early 70's models.

While the XLR 4-pin was a clever idea it was a bad connector to implement this with, it's very expensive at an OEM level and difficult to wire on a production basis.

In the 80's my lead tech built very high end output transformers for expensive audiophile tube equipment. These transformer models produced a lot of profit because of all the smoke and mirrors included in the high fi industry. Segmented windings, esoteric insulation material and secret winding technologies made this a good business until people stopped caring about what audio sounded like. In the 90's I left transformer and manufacturing technology to pursue repairing emergency sound systems on cruise ships which paid a lot more and took me around the world several times.

On the cruise ships I used custom made isolation transformers to decouple the 140 volt outputs of our solid state power amps so we could cut crosstalk down to unhearable levels on the Macrotech 10K amplifiers.

Several tube amp gurus who specialize in great distorted sound have incorporated impedance mismatches into their basket of tone shaping techniques, it shortens tube life but enhances tone according to some. I only used my two Boogie combo heads with 8:16 ohm mismatches because Randall Smith did not include 16 ohm outputs on the Studio 22 models. I recommend matched impedances when possible.

In years past many tube amplifier manufacturers (Marshall comes to mind) voided their warranty if you used a fixed resister load on the output of the amp, this type of load is non-reactive and puts a bigger strain on the output transformer because of the constant current dissipation. Marshall included a strange stepped autotransformer in the design of their output attenuator (Power Brake) in an attempt to keep the load reactive. The Power Brake sounded like crap IMO, it destroyed the breath in the sound from my 100 watt Marshall head.

Many tube amp manufacturers have stepped up their game and increased the overbuild of their transformers to accommodate all the weird ways modern players use their tools. I have a Bugera 1960 infinium which has massive oversize transformers in it. I run mine with 6V6 tubes (instead of EL34) and the middle pair of the four output tubes pulled so I get around 20 watts out of this 100 watt chassis. In this tube configuration you use the next lower impedance tap for the next higher impedance (8 ohm output for 16 ohm load, 4 ohm output for 8 ohm load, 4 ohm load is not recommended for optimum tube life when running on two power tubes).

I think we've beaten this as far as it can go with meaningful information.

Good luck, good music to all and I'm outta here.
Old 1 week ago
  #20
Lives for gear
 
enorbet2's Avatar
Quote:
Originally Posted by John Eppstein View Post
Well, they want to see that higher impedance on the primary because Class runs the tubes a lot harder than Class AB. Something that makes very uneasy about some of the new boutique amps is they're biasing their what in my opinion is much too hot because they say it sounds better. Which is true but I'd much rather have an amp that doesn't sounf quite and works than an amp that sounds utterly amazing until it emits the magic smoke. Go ahead, call me a conservative old man! When what you're conserving is your output transformer a little conservatism MIGHT not be bad thing.

Sure, if you know the amp is designed for it. However I've seen a lot of guys who bias their amps too close to class A and wind up costing themselves a nice chunk of change..
I am decidedly not conservative and possibly radical when it comes to respect for designers as The Final Authority, at least on my own personal equipment though never with a client's. Taking liberties with my own was part of the process of understanding limitations.. As long as one knows the fundamentals and pays attention to ALL of the variables almost anything can be properly re-purposed.

Just as in Hot-Rod cars the work isn't over when you drop a more powerful engine in. It has just begun. One needs to know the axle, transmission and radiator can handle the increased load, or be willing to pay the cost if they can't .

In amplifiers a great deal of reliability headroom comes just with decent cooling since heat is the main enemy of electronics, and one might say tube amps which create heat on purpose as well as incidentally, are prime candidates for cooling. Certainly electrical issues are of concern as well but heat even affects that. That's why cheap electrolytic caps are rated at 85C, junk at 65C, and good ones over 105C. Just for global references, 65C is well above the human threshold of pain and 105C above the boiling point of water. The delta T is already so great to ambient, even a heat sink or proper fan goes a very long way to mitigating temperatures.

Additionally, unless a player always plays loud with extreme distortion, guitar amps are "bursty" in that full output on larger amps is a rarity, only lasting for seconds out of a 3 minute song.. Dissipating heat returns the components to a state that can better handle the next burst if the increase is within reason. Most people would be shocked at what operating temperatures are possible, especially at very fine points like a voice coil or cap internals. Mitigating those temps can have a major effect on MTBF.

As soon as the Music Biz became BIG Biz and excruciating touring became commonplace and volume became massive until PAs grew even more massive, serious manufacturers were forced to either overbuild or increase tolerances to earn a deserved image of "RoadWorthy". .


Quote:
Originally Posted by John Eppstein View Post
Amazing! Those old Danelectro built Silvertones don't exactly have the best quality transformers. On the other hand they also don't really have nearly the output they claimed - actually about half in most cases IIRC. The 2-6L6 version only puts out about 25 real watts with new tubes.
True. Corporations cared little about differentiating between Peak Power and RMS and even invented new (and bogus) terms like Peak-to-Peak Power to inflate "bullet points". Those old Silvertone amps could sound really great, though, with even the slightest bit of tweaking.


Quote:
Originally Posted by John Eppstein View Post
EEK!!!! That's one of the most extreme cases of sheer dumb luck I've heard of in the context. I wonder if the Major was modded at all for reliability, or at least a reduced inclination to go up in smoke?

Of course the Kustom was putting out very little power into the 1 meg input impedance of the Major, but still...... I'm really surprised that it sounded OK....
Yes my initial instinctive horror was reduced a bit after I thought about it for a minute since the 1 Meg load effectively stopped all production of power and turned the Kustom into a voltage amplifier. The Major was not modded and he ran the Kustom on "5" and just barely cracked the Major's Volume control so all the overdrive was just on that first stage. Then I laughed since I finally realized he could have achieved roughly the same results with an EH LPB1 at the time. But then the guy was a crazy, spoiled rich kid who owned a 59 Les Paul (this was in 1968 when they were expensive but not in the stratosphere) that he played out with and many years later he became the only person I ever knew who actually bought a DeLorean, though I imagine he was crestfallen that flux capacitors were not an option
Old 1 week ago
  #21
Lives for gear
 
kennybro's Avatar
OP. Lots of really nice 8 ohm speakers available. Use one of those.
Old 1 week ago
  #22
Lives for gear
 
norfolk martin's Avatar
 

Quote:
Originally Posted by John Eppstein View Post
Your electronics are correct but your terminology is backwards.

The lower the impedance, the greater the load (the amp has to do more work.) Short circuit is infinite load.

The higher the impedance, the less the load. Open circuit is no load.

I know it's counterintuitive at first, but when you think about it, it makes sense,
This is correct, although it is quite awkward to get one's head around because it is a current-based model of loading. Hence, a "higher" load impedance results in less current and a "lower" load from the perspective of the amplifier.
Old 1 week ago
  #23
Quote:
Originally Posted by enorbet2 View Post
I am decidedly not conservative and possibly radical when it comes to respect for designers as The Final Authority, at least on my own personal equipment though never with a client's. Taking liberties with my own was part of the process of understanding limitations.. As long as one knows the fundamentals and pays attention to ALL of the variables almost anything can be properly re-purposed.

Just as in Hot-Rod cars the work isn't over when you drop a more powerful engine in. It has just begun. One needs to know the axle, transmission and radiator can handle the increased load, or be willing to pay the cost if they can't .

In amplifiers a great deal of reliability headroom comes just with decent cooling since heat is the main enemy of electronics, and one might say tube amps which create heat on purpose as well as incidentally, are prime candidates for cooling. Certainly electrical issues are of concern as well but heat even affects that. That's why cheap electrolytic caps are rated at 85C, junk at 65C, and good ones over 105C. Just for global references, 65C is well above the human threshold of pain and 105C above the boiling point of water. The delta T is already so great to ambient, even a heat sink or proper fan goes a very long way to mitigating temperatures.

Additionally, unless a player always plays loud with extreme distortion, guitar amps are "bursty" in that full output on larger amps is a rarity, only lasting for seconds out of a 3 minute song.. Dissipating heat returns the components to a state that can better handle the next burst if the increase is within reason. Most people would be shocked at what operating temperatures are possible, especially at very fine points like a voice coil or cap internals. Mitigating those temps can have a major effect on MTBF.

As soon as the Music Biz became BIG Biz and excruciating touring became commonplace and volume became massive until PAs grew even more massive, serious manufacturers were forced to either overbuild or increase tolerances to earn a deserved image of "RoadWorthy". .
This is largely nonsense. When a tube amp is biased too hot (insufficient bias) the tubes are conducting to some degree even when no signal is present. Taken to an extreme this results in "red plating", which, even when barely present, is REALLY BAD for the tubes. When red plating becomes extreme (plates glowing bright red to orange) tube life can generally be measured in minutes, if not seconds, followed by a smoking output transformer, possible burned tube sockets, and in extreme cases a burned out power supply. That's because the tubes are conducting to the point where thermal runaway sets in.

What's unfortunate is that when this starts happening the amp starts sounding REALLY GOOD for a few minutes, right before it gets to the point where damage starts occurring.

Any experienced tube amp tech will tell that it's quite common for a customer to tell you "Man, the amp was sounding REALLY GOOD! It was sounding better than it ever had before. Then it started blowing fuses!"

Some idiot boutique designers have taken note of this sonic change and bias their amps right to the point of the onset of visible red plating. This is stupid - it drastically shortens tube life and greatly increases the probability of catastrophic amp failure and consequent expensive repairs.

Not all designers are created equal, and in these days of a burgeoning DIY community some "designers" lack either the theoretical knowledge or the experience to understand when they're doing something really stupid.

Some hot rod designers do a similar thing and their engines blow up before they can even finish a 1/4 mile race. And they have to do a complete overhaul of the engine after every event anyway. Do you want to have to replace your power tubes after every gig?

BTW, the amps I'm talking about are not big touring amps - they're small boutique combo amps, usually with 1-12" and often running EL84s. Under biasing is just as deadly to little amps as it is to big ones.

Last edited by John Eppstein; 6 days ago at 09:41 PM..
Old 6 days ago
  #24
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enorbet2's Avatar
For cryin' out loud, John... "largely nonsense" ? Just because a few so-called boutique amp designers take too much of a risk doesn't mean Class A never works, is entirely unreliable or that cooling isn't an important issue with electronics. I didn't suggest liquid nitrogen like Crays and a few other SuperComputers use, just proper layout, heatsinks, and a simple whisper fan.. LOL. Seriously what manner of idiot would bias to the point of red plating? I've already stated that my heavily modded Princeton Reverb ran 2 x EL-34s in Class A for thousands of hours of live use. How nonsensical is that? I think you might want to restate that.
Old 3 days ago
  #25
Quote:
Originally Posted by enorbet2 View Post
For cryin' out loud, John... "largely nonsense" ? Just because a few so-called boutique amp designers take too much of a risk doesn't mean Class A never works, is entirely unreliable or that cooling isn't an important issue with electronics. I didn't suggest liquid nitrogen like Crays and a few other SuperComputers use, just proper layout, heatsinks, and a simple whisper fan.. LOL. Seriously what manner of idiot would bias to the point of red plating? I've already stated that my heavily modded Princeton Reverb ran 2 x EL-34s in Class A for thousands of hours of live use. How nonsensical is that? I think you might want to restate that.
I think you're not quite understanding me. First off, I'm not talking about a properly designed, true class A amp. I'm talking Class AB amps that are biased too hot - dangerously close to Class A, but probably still using Class AB transformers. This results in a tendency for the output tubes to "red plate" and you don't want to see that, ever. A properly designed Class A amp doesn't red plate the tubes.

Second, of course cooling isimportant, but if the amp is biased too hot and is starting to red plate cooling isn't going to protect the tubes unless you can figure out some way to get little heat exchangers inside the glass envelopes of the tubes. Vacuum tubes tend to have vacuum inside them between the plate and the external envelope and vacuum isn't a very good conductor of heat.

What idiot would bias to the point of red plating? Good question. I don't remember which company - it's been several years - but I distinctly recall the manufacturers of some fairly expensive and relatively well known boutique brand(s) stating that a little red glow on the plates was normal for at least some of their models. IIRC it was in a thread here maybe 4 or 5 years ago, maybe a bit less. As far as I'm concerned it should not be possible to adjust the bias on an amp to the point of red plating - if you can it's a design flaw because if it's physically possible there's always some monkey who will do it - I've run into more than a few people who advocate setting bias by ear. Remember, we're dealing with guitar players and that there are quite a few guitar players who are technical idiots. Hopefully nobody here, but........
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