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What Should Be The Cut Off For Calling a Circuit "Starved Plate"
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Rob Coates
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18th November 2011
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What Should Be The Cut Off For Calling a Circuit "Starved Plate"

What should be the cut off voltage for calling a tube circuit "starved plate?"
Most of the starved plate designs run at 40 to 50 volts I believe. However, some starved plates like in the ART Pro Channel run at 90 volts. I would still call that a starved plate but with a slightly higher voltage it may sound and behave more like a traditional tube circuit. The ART MPA Gold and MPA II on the other hand run the tubes at around 180 volts. Again, lower voltage than a traditional tube circuit but I've never heard anyone claim 180 volts would be staved plate. So, should we call anything below 100 volts starved plate? 120 volts? Another question is: does the behavior and sound quality of a tube circuit increase incrementally with an increase in voltage or is it an equal level of crap up to a certain point, like say 150 volts or so?
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18th November 2011
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I have wondered the same thing along with this one -

if you HAVE a starved plate design (like the Pro Channel that i think actually runs at 60 volts - tho I may be wrong), is there a type of tube to replace the standard 12AX7 that breaks up nicely at lower voltages so you can still get the "warm" tube sound?
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Quote:
Originally Posted by changeng View Post
is there a type of tube to replace the standard 12AX7 that breaks up nicely at lower voltages so you can still get the "warm" tube sound?
if it breaks up earlier, you will get more distortion and with low voltages, this distortion will be most likely uneven harmonics (3rd order), whereas the 'nice tube sound' comes from predominantly even order harmonics.
And of the 12A*7 family, I believe the 12AX7 is the one with the most gain, or at least in the top 3.
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on the topic of tube breakup and performance at different plate voltages, keep in mind that a good tube preamp is supposed to NOT push the tube into serious distortion levels.

from my experience with mpa golds I can say that on lower voltage the there is less gain before breakup and breakup is more suddenly moving into audible distortion. on high plate voltage there is more gain before breakup and breakup is a much more gradual, sheeny, magical kind of thing and it's quite hard to make the tube audible clip the waveform.

For me that makes high voltage drastically superior.

just some random thoughts about my pro channel experience (I meander and there is no conclusion, just thinking aloud) :

with my pro channels I can push into breakup but it isn't as easy as it is in the mpa gold with low plate voltage, probably due to something about the gain staging I choose in mpas (I run the 20db boost button all the time in mpa golds, as do most users).

but when the pro channel preamp stage distorts it can really distort if pushed hard enough, but there is still a magical point where it sounds just so darn good. however because it's such a narrow level range where it hits the "darn good" tube sheen thing it in't as easy to get great tone out of compared to mpa gold on high voltage setting.

In the comp stage I can also get some gain push as well out of opto mode with the right gain structure, or the tube compressor mode itself actually always is pushing the tube hard by design, so it definitely has it's own sound. the eq stage I haven't yet managed to push into any kind of distortion levels and am actually curious about messing with tube swaps in that stage to see if I can get it a little sheeny as well....

I don't know if this is relevant or helpful, just some observations.

now for what is starved plate level?

I found this quote from a peavey engineer on talkbass.com:
MI Amp Engineer: Peavey Electronics
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The "standard" plate voltage for a 12AX7 is 250V. If you look at a lot of amp designs, they are run higher and lower, but that's kind of the median. That's also the plate voltage on a lot of 12AX7 reference designs.

I use higher voltages for bass circuits, mainly for headroom. I won't go into a lot of detail, but the plate voltage range and loading of tube gain stages has a HUGE effect on harmonic content.


If you run a 12AX7 below 100V, the response is very much like a transistor. You will get a lot more odd order harmonics. The tube will clip differently as well--a lot more like a transistor. Chances are, if somebody puts out a product with a tube running at that low of a voltage, the tube is in there for marketing purposes only.
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18th November 2011
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note: I disagree about "marketing purposes only" for starved plate designs (From my previous post's quote). tubes still are pleasant sounding at that level in the right circuit (and art found the right circuit years ago and have kept using variants of it ever since).

but my findings with the mpa gold certainly match that of the peavey engineering guy... low voltage is a buzzy distortion and it hits hard, like a transistor. high voltage is a sheeny pleasant sound that hits much more gently and magically.
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Hi
There will be no' cutoff' point for 'starved' or not as it is a continuous range and as mentioned above is critically affected by the source impedances and the 'load' impedances attached to the valve. It is 'simply' a matter of deciding how much distortion and what 'type' you want then jig around with valve types and circuit parameters to get it.
Some of the 'cheaper' brands use the fact that it has a valve in it as part of marketing and in order to make it 'stand out' often have to resort to fairly extreme distortion levels. This becomes an 'effect'.
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19th November 2011
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Yes, 12AX7 is the highest output tube in the 12A_7 family and is used to guage the output of other tubes within that family. Like, a 12AT7 has about 80% the output of as 12AX7; 12AY7 about 50%; 12AU7 has about 18%. HTH.
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Quote:
Originally Posted by Analok View Post
Yes, 12AX7 is the highest output tube in the 12A_7 family and is used to guage the output of other tubes within that family. Like, a 12AT7 has about 80% the output of as 12AX7; 12AY7 about 50%; 12AU7 has about 18%. HTH.
really? the AY and AU are that much lower? I didn't know the difference was so extreme. I assumed, based on how it feels when I swap them out in devices, is much more subtle. I'm not saying you're wrong - I'm just really surprised!
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19th November 2011
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Quote:
Originally Posted by Rob Coates View Post
What should be the cut off voltage for calling a tube circuit "starved plate?"
It depends entirely on the type of tube involved. There were tubes designed for lightweight, portable applications, e.g. hearing aids, such as the 6418 which has a maximum plate voltage of 30 volts and can be used at half that with little change in performance. Looking at its published specifications the 6418 doesn't really enter starved plate territory until its plate voltage drops below about 6 volts.

Quote:
Another question is: does the behavior and sound quality of a tube circuit increase incrementally with an increase in voltage or is it an equal level of crap up to a certain point, like say 150 volts or so?
It changes incrementally with changes in plate voltage but there's a point where performance starts dropping off more rapidly as the plate voltage decreases below a certain point.
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19th November 2011
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A.R.T.'s original Pro MPA is at a confirmed 48volts for the tube. I guess it's not just starved, it's dying for more volts!

It sounds decent to me though, definitely different compared to my favorite Eureka it doesn't have that sheen, but I like it for the flavor. My eureka is kinda magical, so the reason I have it instead of say the MPA gold is becasue I've got the nice sheen up-top already. So it's just another color on the palette. I do still really like it regardless of the specs.
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19th November 2011
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A 12AY7 might have 50% of the availible gain but remember that that's a 6dB difference, not as big as it would seem! Especialy when they're both being driven into clipping and probably being followed by another gain stage.

You also have to remember that the average length of "Musical memory" is 7 seconds, so your perception of which is "louder" is going to be skewed.
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Quote:
Originally Posted by Jazz Noise View Post
A 12AY7 might have 50% of the availible gain but remember that that's a 6dB difference, not as big as it would seem! Especialy when they're both being driven into clipping and probably being followed by another gain stage.

You also have to remember that the average length of "Musical memory" is 7 seconds, so your perception of which is "louder" is going to be skewed.
I think I'm unclear on the scale that you're using. you said the output (I think you said output) of he au is something like, what, 15% of the output of the ax. which tells me that if the ax is considered 0db unity output, then drop in an au7 and you will be down to almost inaudible output, not just a few db down. assuming a dynamic range of 100db for simplicity here then 15% of that is only 15db output level.

so I think maybe if we're talking gain then I was misunderstanding. if what you really mean is that an ax7 has 100% gain OVER unity compared to an au7 having only 15% gain OVER unity then I can start to see this being possible, where both of them amplify the sound but there's very little amplification from the au7 but 6.66 times as much amplification from the ax7. so if the au7 is adding 2db only in it's stage, then the ax7 is adding around 12 or 13db (off the top of my head).

I hate thinking in db when we're talking about relative voltage/current ratios though - it's really inaccurate.
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19th November 2011
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What I was reffering to was 50% relative to the 12AX7, which is what Analok was saying (unless I'm misreading this). So it'd be, by example, a gain of 2 vs a gain of 1. The difference is 6db V. The AU was something like 17% which would roughly 2.5 times smaller than the AX or about 15dB, which is more substantial. Still not as scary looking as a figure like 17% however!

Working with linear values and dB can be confusing, and even further when you want to translate it into actual loudness, but it's not a bad system! Alot of the issues are actually with linear number systems. We're born thinking in a logarithmic fashion, we learn to linearize our math and thought processes, and then we reintroduce logs!
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Hmmm...

Perhaps 'strength' would be a better choice for words. Of course, the 'output' of any tube will vary based on its' supporting circuitry I still have a hard time wrapping my head around the 'less (voltage to plate) is more (distorted output)' phenominon. Think as if a higher plate voltage will keep the flowing electrons in a more uniform path to their preferred destination (a screen grid, control grid, etc). Dropping the plate voltage results in electrons going awry to other surrounding components, thus increasing distortion. Drop voltage too low and the output gets constipated & mugly; too high and component life suffers. This would be why hi-fi circuits typically present higher voltages within their power supplies - in order to produce cleaner output. Higher voltage requires heftier components (transformers, filtering caps, etc) which naturally spike up the end cost of the high fidelity amp. Fender, for example, was all about keeping costs down, so they'd opt out from buying choice components but rather ones that would 'satisfy the need'. The end results being a sound that we've all grown to love in the guitar world. Today, aftermarket manufacturers strive to get those characteristics by building components that back in the day were considered 'sub par', but are now 'custom designed'. Who'd da thought?!?
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20th November 2011
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Quote:
Originally Posted by Rob Coates View Post
What should be the cut off voltage for calling a tube circuit "starved plate?"
Most of the starved plate designs run at 40 to 50 volts I believe. However, some starved plates like in the ART Pro Channel run at 90 volts. I would still call that a starved plate but with a slightly higher voltage it may sound and behave more like a traditional tube circuit. The ART MPA Gold and MPA II on the other hand run the tubes at around 180 volts. Again, lower voltage than a traditional tube circuit but I've never heard anyone claim 180 volts would be staved plate. So, should we call anything below 100 volts starved plate? 120 volts? Another question is: does the behavior and sound quality of a tube circuit increase incrementally with an increase in voltage or is it an equal level of crap up to a certain point, like say 150 volts or so?
depends on the tube
look at the specs and curves

it wont be a sharp point
a modest range wuold be more accurate way to describe it
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20th November 2011
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Quote:
Originally Posted by dkelley View Post

I found this quote from a peavey engineer on talkbass.com:[/B]MI Amp Engineer: Peavey Electronics
Join Date: Oct 2005
Location: Mississippi



The "standard" plate voltage for a 12AX7 is 250V. If you look at a lot of amp designs, they are run higher and lower, but that's kind of the median. That's also the plate voltage on a lot of 12AX7 reference designs.

I use higher voltages for bass circuits, mainly for headroom. I won't go into a lot of detail, but the plate voltage range and loading of tube gain stages has a HUGE effect on harmonic content.


If you run a 12AX7 below 100V, the response is very much like a transistor. You will get a lot more odd order harmonics. The tube will clip differently as well--a lot more like a transistor. Chances are, if somebody puts out a product with a tube running at that low of a voltage, the tube is in there for marketing purposes only.
This is interesting information that pretty much answers my original questions. From what this guy says, anything below 100 volts would be considered "starved plate." Also confirms what I and many others have noticed: that the distortion from low volt tubes is kind of nasty sounding
odd order harmonics Vs. even order harmonics from a saturated tube run at higher voltages.
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Hi
With the discussions above about 'gain' you have to remember that the 'VOLTAGE' gain is not the same as the 'POWER' gain and this should be borne in mind as the various valves are not strictly interchangeable. As the types mentioned are the same pinout they will plug into the same circuit but the resistance / impedance of the anode / cathode circuitry should be changed for 'optimum' performance. They are usually biassed to give the highest output signal without excessive distortion. An 'AX' typically is used for high voltage gain, whereas the others can handle greater current and stand a chance of 'driving' a meaningful signal elsewhere. Putting low octane fuel in your car will run, but not to it's best performance. Circuit conditions are optimised for a given purpose and simply bunging a different valve in will upset the designed operating conditions.
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20th November 2011
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Whoa! a tube hearing aid. Where can I buy one so that when my drummer finally breaks my ear drum I can live happily ever after in harmonic distortion heaven?

Quote:
Originally Posted by lostintime View Post
It depends entirely on the type of tube involved. There were tubes designed for lightweight, portable applications, e.g. hearing aids, such as the 6418 which has a maximum plate voltage of 30 volts and can be used at half that with little change in performance. Looking at its published specifications the 6418 doesn't really enter starved plate territory until its plate voltage drops below about 6 volts.



It changes incrementally with changes in plate voltage but there's a point where performance starts dropping off more rapidly as the plate voltage decreases below a certain point.
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20th November 2011
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sorry if this is a little off topic but since we're talking about plate voltage and starved plates I thought this would be a good place to ask.

What is the "standard" plate voltage for a 12at7 tube? Also, does anyone know what the voltage is for the tubes on a ART Pro VLA II is? I like my VLA II's but have always wondered if they are starved plates or not.
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20th November 2011
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or if there is a way I can test and find this out my self anyone care to share how?
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The 12A_7 family are pretty much all the same - they change mainly in the gain factor. They're all something like a high max plate voltage of 300v.
12AX7/ECC83 High-Mu Dual Triode Designs

and the other 12A_7s are there too, see Max Ratings tab.
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Quote:
Originally Posted by PhilDW View Post
The 12A_7 family are pretty much all the same - they change mainly in the gain factor. They're all something like a high max plate voltage of 300v.
12AX7/ECC83 High-Mu Dual Triode Designs

and the other 12A_7s are there too, see Max Ratings tab.
thanks good info there on 12A_7 tubes. anyone know what the plate voltage is in a VLA II is. to see how far the vla II is from the max plate voltage.
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Quote:
Originally Posted by PhilDW View Post
The 12A_7 family are pretty much all the same - they change mainly in the gain factor. They're all something like a high max plate voltage of 300v.
12AX7/ECC83 High-Mu Dual Triode Designs

and the other 12A_7s are there too, see Max Ratings tab.
Actually, here are the facts:

The heater cathode voltages are dramatically different between, for example, the 12ax7 and 12ay7, and the output impedance in a circuit changes dramatically between those two tubes if the rest of the circuit isn't modified when the tube type is swapped out. The output impedance should always be very high no matter what, but in some situations a change of doubling even a very high output impedance is enough to alter the high frequency balance of the audio circuit.

The end result is not only a change in the gain, but also potentially a change in the tonal balance or eq of the circuit.

This is why sometimes the change in tube type is done for tone improvements rather than to change the clipping point or similar.

Damn - sometimes my brain just turns on like a frickin light bulb. Other times it's like my memory is full of holes. This is a good day LoL :-)

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I've scoured the ART website and the only unit that mentions its plate voltage is the MPA Gold.
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Quote:
Originally Posted by ivmike View Post
I've scoured the ART website and the only unit that mentions its plate voltage is the MPA Gold.
because it's the only item they sell that has a plate voltage that isn't embarassing to them :-)

even though I enjoy their lower plate voltage items and use them for real paid work (shocking, I know), certainly I can't deny that the mpa gold's high voltage setting is superior sounding in every way.
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23rd November 2011
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Quote:
Originally Posted by gumby1220 View Post
sorry if this is a little off topic but since we're talking about plate voltage and starved plates I thought this would be a good place to ask.

What is the "standard" plate voltage for a 12at7 tube? Also, does anyone know what the voltage is for the tubes on a ART Pro VLA II is? I like my VLA II's but have always wondered if they are starved plates or not.
The ART VLAs are starved plate. 50 or 60 volts I think.
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My take on a lot of the recent "starved plate", low end audio gear with tubes in it is that if it doesn't have a transformer, it's automatically suspect. As it was explained to me, a large part of what we like about tube gear is a product of the transformers being used as much as the tube characteristics themselves. Anybody care to confirm this?
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Quote:
Originally Posted by electricity17 View Post
My take on a lot of the recent "starved plate", low end audio gear with tubes in it is that if it doesn't have a transformer, it's automatically suspect.
Not necessarily. The Electro-Harmonix 12AY7 preamp sounds good but doesn't contain any transformers in its signal path. It isn't starved plate either. It's the only low-end tube preamp worth owning in my opinion.

Quote:
As it was explained to me, a large part of what we like about tube gear is a product of the transformers being used as much as the tube characteristics themselves. Anybody care to confirm this?
Yes, but this is true for both tube and transistor designs a lot of the time.
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23rd November 2011
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Quote:
Originally Posted by dkelley View Post
because it's the only item they sell that has a plate voltage that isn't embarassing to them :-)

even though I enjoy their lower plate voltage items and use them for real paid work (shocking, I know), certainly I can't deny that the mpa gold's high voltage setting is superior sounding in every way.
Yes, however they don't mention the plate voltage of the MPA II. And as for the MPA Gold, they mention it as " the equivalent of a 300V power supply on the high voltage setting".
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23rd November 2011
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I swapped the stock tube in my cheap Studio MP preamp for a Sovtek 12AX7WA. I picked this tube because it provided more gain.

I do notice a difference. I use this preamp for an effect, when overdriving vocals, drums, synths, etc.

Or just to add that that extra punch (minus overdrive).

Does anyone know what kinda voltage this little bad boy is pushing? Not that it matters, just curios.
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