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Help me identify this homemade amp
Old 28th July 2019
  #1
Here for the gear
 

Help me identify this homemade amp


Back in 2008 a close friend built me this amp as a wedding present. I’m since divorced, and have lost touch with my friend—both unfortunate—but his amp still stands. He repurposed a sweet old radio cabinet for it, and it looks great on my shelf, but I want to return it to service.

At some point, he made a modification to the gain stage that created some hideous buzzing that was never solved. I would like a local repair shop to take a look at the amp, but they won’t do it without a schematic — which I don’t have.

Can anyone ID this build? Better yet, is there a readily available schematic?
Attached Thumbnails
Help me identify this homemade amp-3426aa50-d4eb-45ed-95ce-fcd41841f071.jpg   Help me identify this homemade amp-eae63ba3-9dcf-4627-a33d-16c06cfc1e68.jpg   Help me identify this homemade amp-bde02c18-058c-439d-b93b-2fa69b88a2da.jpg  
Old 28th July 2019
  #2

Looks like it's a Walz wedding amp....

Since it only has one preamp tube (and the output transformer is marked), you know it's a parallel single ended configuration. You'll have to trace the circuit, but it will be a lot like a Fender Champ or a Vox AC4 - but with a second output tube hooked up in parallel.



-tINY

Old 2nd August 2019
  #3
Gear Maniac
Take it to a different tech. A single-ended amp with a single preamp tube... you really can not get any simpler than that.

Turning it away without a schematic... sheesh.

If you can take a picture of the insides it might be easier to tell what it is.
Old 2nd August 2019
  #4
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enorbet2's Avatar
You can web search the tube pinouts and from that create a schematic. Someone experienced with amps and schematics could do that easily within an hour. A rank beginner who is careful and thorough should still be able to do that in a single afternoon. It might take a few tries to get it really well laid out and clean looking but so what?. Odd/interesting transformers, BTW. The PT looks very substantial for the amount of output I expect. The OT is a bit less substantial but then available options are not what they used to be by a long shot.
Old 2nd August 2019
  #5
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It's pretty clear from several not-so-subtle hints that it is a modified radio that hasn't strayed too far from its origins.

Westinghouse cabinet with that rectifier (rectifier alone is pretty much a dead giveaway that it is a consumer hifi amp modified to be a guitar amplifier).

Added the output transformer with custom send/return to the top of the box so that it didn't kill you.

My guess would be that if you found a Westinghouse radio that matched that cabinet that the schematic will be identical other than the components for reception/tuning removed, input added with necessary impedance adjustment, and addition of the output transformer.

Barring that - check out the filter caps as if they were re-used that's likely a good place to start and relatively cheap (and necessary regardless).

Option two - dropkick it into the ocean and go amp shopping.

Good luck!
Brock
Old 4th August 2019
  #6
Quote:
Originally Posted by wanghuskahn View Post

Back in 2008 a close friend built me this amp as a wedding present. I’m since divorced, and have lost touch with my friend—both unfortunate—but his amp still stands. He repurposed a sweet old radio cabinet for it, and it looks great on my shelf, but I want to return it to service.

At some point, he made a modification to the gain stage that created some hideous buzzing that was never solved. I would like a local repair shop to take a look at the amp, but they won’t do it without a schematic — which I don’t have.

Can anyone ID this build? Better yet, is there a readily available schematic?
No. We can't ID it because we didn't build it and we don't have it in front of us to trace the circuit.

That being said, this is a pretty simple amp and any COMPETENT tech should be able to trace out a schematic in half an hour or less - if he would even need a schematic to work on it - As I said, it's a really simple amp.
Old 4th August 2019
  #7
Quote:
Originally Posted by Bstapper View Post
It's pretty clear from several not-so-subtle hints that it is a modified radio that hasn't strayed too far from its origins.
How would you know that? Is your psychic ability working better than mine (or Enorbet's, or Tiny's) today?

Looks to me like a home build somebody stuck in a handy case. I would not make ANY assumptions based on the case.

Quote:
Westinghouse cabinet with that rectifier (rectifier alone is pretty much a dead giveaway that it is a consumer hifi amp modified to be a guitar amplifier).
Why would you make that assumption?

Quote:

Added the output transformer with custom send/return to the top of the box so that it didn't kill you.
WFT are you babbling about? That makes no sense.

Quote:

My guess would be that if you found a Westinghouse radio that matched that cabinet that the schematic will be identical other than the components for reception/tuning removed, input added with necessary impedance adjustment, and addition of the output transformer.
My guess - and it's just a guess, as is yours - would be that it's not. One tipoff is the output configuration, which is like nothing I've ever seen in an old table radio.

"Addition of the output transformer?" What? Why? Tube table radios have output transformers. If he was using the existing audio circuitry of of the radio why in hell would he go to the extra expense of changing the output transformer?

Quote:

Barring that - check out the filter caps as if they were re-used that's likely a good place to start and relatively cheap (and necessary regardless).
Always a good idea but not a likely cause in this case because the amp was not humming before the ill-advised gain mod.

Quote:

Option two - dropkick it into the ocean and go amp shopping.

Good luck!
Brock
That's REALLY helpful.

Last edited by John Eppstein; 4th August 2019 at 04:06 AM..
Old 4th August 2019
  #8
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Output transformer: quite common to cut costs by eliminating it. The addition to the underside of the top is what led me to that conclusion.

Meanwhile is it more common for guitar amplifiers or radios to use that rectifier?

Later,
Brock
Old 4th August 2019
  #9
Gear Maniac
Quote:
Originally Posted by Bstapper View Post
Output transformer: quite common to cut costs by eliminating it. The addition to the underside of the top is what led me to that conclusion.

Meanwhile is it more common for guitar amplifiers or radios to use that rectifier?

Later,
Brock
That's either a choke or an isolation transformer, definitely NOT an output transformer.
Old 4th August 2019
  #10
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enorbet2's Avatar
The OT is 99.9% certain to be the one with WALZ on the side in felt marker. Note the proximity of the Output Tubes and Speaker Jack as well as only 2 wire leads from the upside-down on top Choke..
Old 4th August 2019
  #11
Gear Maniac
Quote:
Originally Posted by enorbet2 View Post
The OT is 99.9% certain to be the one with WALZ on the side in felt marker. Note the proximity of the Output Tubes and Speaker Jack as well as only 2 wire leads from the upside-down on top Choke..
I'll raise you 0.1%.... 100% certain the WALZ is the OT... it's a Hammond 125ESE... a perfect match for a parallel 6V6 SE output section.

I'm leaning more towards isolation for the third one... note its proximity to the IEC socket.... between it and the PT. If it were a choke, I would think the hole in the chassis for it would be in a different spot. I could be wrong though. Only way to be 100% certain is to take a peek at the guts. It certainly does have the size and look of a choke though.

Well, after looking at it closer, it probably is a choke. An iso would have four wires... two for each side, this one definitely only has two.
Old 5th August 2019
  #12
Quote:
Originally Posted by dpsbb View Post
That's either a choke or an isolation transformer, definitely NOT an output transformer.
Why in the world would you think that?

What would an iso transformer be doing there (besides the fact that an output transformer is technically an isolation transformer. And it that's an interstage iso transformer (EXTREMELY unlikely) then where's the output transformer?

And no, it's not a choke, because chokes are not really necessary and output transformers are in tube amps.
Old 5th August 2019
  #13
Quote:
Originally Posted by Bstapper View Post
Output transformer: quite common to cut costs by eliminating it. The addition to the underside of the top is what led me to that conclusion.
You can't "cut costs" by eliminating the output transformer in a tube amp. The amp can't drive a speaker without it, unless you have a really weird transformerless design that uses cathode followers for the output, which this obviously is not. And a design with cathode follower output would not be "cheaper" as it would require at least double the number of output tubes.


Quote:
Meanwhile is it more common for guitar amplifiers or radios to use that rectifier?

Later,
Brock
Both guitar amps and radios use rectifiers - they're required to change the line AC into DC to run the amplifier.

The TYPE of rectifier is utterly beside the point. Very old amps from the'20s and '30s used selenium rectifiers, at least sometimes. Tube rectifiers haves been around since the '20s. Solid state (diode) rectifiers came around in the '50s.

Thje type doesn't matter - a rectifier is a rectifier.
Old 5th August 2019
  #14
Gear Maniac
Quote:
Originally Posted by John Eppstein View Post
Why in the world would you think that?

What would an iso transformer be doing there (besides the fact that an output transformer is technically an isolation transformer. And it that's an interstage iso transformer (EXTREMELY unlikely) then where's the output transformer?

And no, it's not a choke, because chokes are not really necessary and output transformers are in tube amps.

ummm... read my post directly above this one maybe???
Old 5th August 2019
  #15
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Quote:
Originally Posted by John Eppstein View Post
You can't "cut costs" by eliminating the output transformer in a tube amp. The amp can't drive a speaker without it, unless you have a really weird transformerless design that uses cathode followers for the output, which this obviously is not. And a design with cathode follower output would not be "cheaper" as it would require at least double the number of output tubes.
Yes. You can. And there are many cheap amplifier designs that did exactly that and a whole world of amp techs that have installed isolation xformers in order to render those designs safe.

In fact I have three such amplifiers here right now. Full disclosure, I'm no amplifier tech. I just play them and own them. But I sure as hell wouldn't be bringing any to you because this is pretty basic stuff and the fact you are not aware of this would be a huge red flag to me.

The amplifiers I have here that are in need of isolation transformers to make them safe due to the fact they are designed and built without output transformers:

Vintage Kent cheap tube practice amp - I'm sure they were proud of the money they saved.
A late 40's Danelectro
An early 60's Penncrest




Quote:
Originally Posted by John Eppstein View Post
Both guitar amps and radios use rectifiers - they're required to change the line AC into DC to run the amplifier.

The TYPE of rectifier is utterly beside the point. Very old amps from the'20s and '30s used selenium rectifiers, at least sometimes. Tube rectifiers haves been around since the '20s. Solid state (diode) rectifiers came around in the '50s.

Thje type doesn't matter - a rectifier is a rectifier.
Spouting the obvious doesn't change the fact that the particular tubes chosen for a particular circuit tell you a lot about the origins of the design. And in this case the selection of that particular rectifier points directly to the original amplifier that was in that original chassis that was in that original housing which was a tube radio. The fact that they did not change that out to a more typical rectifier used in guitar amplifier circuits suggests that it is likely not a whole lot changed in the power section of that amplifier.

Through with your abrasive and negative nature. I have work to do and you are a waste of time and energy.
Old 5th August 2019
  #16
Quote:
Originally Posted by dpsbb View Post
ummm... read my post directly above this one maybe???
Ah! I missed that somehow.
Old 5th August 2019
  #17
Gear Maniac
 

Quote:
Originally Posted by Bstapper View Post
Yes. You can. And there are many cheap amplifier designs that did exactly that and a whole world of amp techs that have installed isolation xformers in order to render those designs safe.

In fact I have three such amplifiers here right now. Full disclosure, I'm no amplifier tech. I just play them and own them. But I sure as hell wouldn't be bringing any to you because this is pretty basic stuff and the fact you are not aware of this would be a huge red flag to me.

The amplifiers I have here that are in need of isolation transformers to make them safe due to the fact they are designed and built without output transformers:

Vintage Kent cheap tube practice amp - I'm sure they were proud of the money they saved.
A late 40's Danelectro
An early 60's Penncrest






Spouting the obvious doesn't change the fact that the particular tubes chosen for a particular circuit tell you a lot about the origins of the design. And in this case the selection of that particular rectifier points directly to the original amplifier that was in that original chassis that was in that original housing which was a tube radio. The fact that they did not change that out to a more typical rectifier used in guitar amplifier circuits suggests that it is likely not a whole lot changed in the power section of that amplifier.

Through with your abrasive and negative nature. I have work to do and you are a waste of time and energy.
I think you might be thinking of amps that don't have power transformers.
Old 5th August 2019
  #18
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Quote:
Originally Posted by untitled73 View Post
I think you might be thinking of amps that don't have power transformers.
Nope. I’m not.
Old 5th August 2019
  #19
Gear Maniac
 

Quote:
Originally Posted by Bstapper View Post
Nope. I’m not.
Ok, can you send me a model number of one of those amps that you mentioned?
Old 5th August 2019
  #20
Gear Head
 

I have read about guitar amps that were built by Garr Gilles (Garnet amps) that were basically re-purposed tube radios with no power trannies...lethal in the wrong hands. They were marketed through Canadian stores like Sears-Simpson and Montgomery-Wards. They had NO POWER TRANSFORMERS and took their power straight from the wall. But they did have an output tranny...
Old 5th August 2019
  #21
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Quote:
Originally Posted by untitled73 View Post
Ok, can you send me a model number of one of those amps that you mentioned?
Will do - to repeat I'm not a tech so I could very well be using the wrong terminology although I'm sure you get the gist of my comments.

I'm a bit busy and scattered in the brain department at the moment.
Old 5th August 2019
  #22
Gear Maniac
Quote:
Originally Posted by John Eppstein View Post
Ah! I missed that somehow.
Heh no worries. It happens to all of us sometimes.
Old 5th August 2019
  #23
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Quote:
Originally Posted by untitled73 View Post
Ok, can you send me a model number of one of those amps that you mentioned?
The first one that came to mind is the old Danelectro I've got (Leader):
https://reverb.com/item/3622829-dane...n-usa-tube-amp

It's just like this one in the link above (except not as pretty), where someone has mounted the iso xformer in there to render it safe. And obviously I was wrong and it is the power xformer that is missing in these designs.

The schematic for this one is in one of the pics of this listing, but as I've found out I don't think any of the schematics available for this amplifier are actually correct as to how the factory wired them (there's a few different ones available out there).

Thanks for setting me straight!
Brock
Old 5th August 2019
  #24
Gear Maniac
Quote:
Originally Posted by Bstapper View Post
Will do - to repeat I'm not a tech so I could very well be using the wrong terminology although I'm sure you get the gist of my comments.

I'm a bit busy and scattered in the brain department at the moment.
OK then, so let us help straighten you out here. Some of us replying here ARE techs and I've even designed a few of my own circuits, as I'm sure the others who have chimed in probably have as well.

There are typically three types of transformers in amps, a fourth not common at all anymore and even sometimes a fifth and sixth kind, which I won't bother bringing up because they are so rare and are off the point.

1) The power transformer. Required to convert the wall voltage to the voltages tubes need to see to function properly. Which leads to the fourth type... the isolation transformer. Some old (and potentially deadly) amps ran directly off the wall voltage without the use of a power transformer. Isolation transformers protect the user from the pitfalls of those designs. It has been established that this amp does NOT have an isolation transformer but it DOES have a power transformer. How do I know? Only two wires coming out of that little add-on transformer... an iso transformer will have four... two for the wall-voltage side, two for the isolated circuit side... not to mention the fact that there IS a power transformer sitting right there so it doesn't need one.

2) A choke. Not required, but sometimes they are used to help filter the DC voltage. They are part of the circuit that reduces the "ripple" caused by capacitors charging and discharging, which causes all kinds of nasty hum. Most modern designs do not use them and use a simple high-wattage resistor in its place instead, but some designers still choose to use them (one of those long-debatable "mojo" things.) Personal preference nowadays. The small "add-on" transformer here IS a choke. Two wires only is the identifying feature (and to some extent its size and location.)

3) The output transformer. This IS required for a tube amp. There were some extremely rare OTL designs (output transformer-less) but they do not work well... at all. Thus, they are still used in ALL tube power amps... radio, guitar, whatever... makes no difference. OTL designs have been a design "holy grail" ever since tube amplification came about, but as I said, it has to date never been achieved in a practical sense and thus the quest has pretty much been abandoned. Now, they are NOT required for solid-state power amps, but for tubes amps, yes you absolutely need one. In this amp, the transformer with WALZ written on it is the output transformer. How do I know? It is a Hammond 125ESE (right there on the label) and... well, see for yourself... http://www.hammondmfg.com/pdf/5C_125SE.pdf

4) already mentioned... the isolation transformer as an add-on safety device if the original design has no power transformer.

5 & 6 (and possibly more)... like I said, not seen enough to even worry about... Interstage transformers and even rarer transformers used as phase inverters. I guess I should add that sometimes transformers are implemented in the "direct out" circuitry of tube amps... those are more common than the others in this category nowadays.


If you are really interested in reading more and gaining a basic understanding of how tube amps work, I highly recommend the series of books called "The Ultimate Tone" by Kevin O'Connor or Merlin Blencowe's (The Valve Wizard) excellent website http://valvewizard.co.uk/ Both of these men are legends in the amp design/mod world (although Mr. O'Connor can be a little controversial, he is great at explaining the basics.) Another great read is the "P1eX Theory Document" over at AX84.com which walks you through a working amp schematic (one that is probably VERY close to the amp we have here) and explains the function of every component in the circuit. A guy named Pete Millet (I think) has a website of all kinds of out-of-print textbooks on tube amp design and theory if you really want to stretch your brain even more on the subject.
Old 5th August 2019
  #25
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Quote:
Originally Posted by dpsbb View Post
OK then, so let us help straighten you out here. Some of us replying here ARE techs and I've even designed a few of my own circuits, as I'm sure the others who have chimed in probably have as well.

There are typically three types of transformers in amps, a fourth not common at all anymore and even sometimes a fifth and sixth kind, which I won't bother bringing up because they are so rare and are off the point.

1) The power transformer. Required to convert the wall voltage to the voltages tubes need to see to function properly. Which leads to the fourth type... the isolation transformer. Some old (and potentially deadly) amps ran directly off the wall voltage without the use of a power transformer. Isolation transformers protect the user from the pitfalls of those designs. It has been established that this amp does NOT have an isolation transformer but it DOES have a power transformer. How do I know? Only two wires coming out of that little add-on transformer... an iso transformer will have four... two for the wall-voltage side, two for the isolated circuit side... not to mention the fact that there IS a power transformer sitting right there so it doesn't need one.

2) A choke. Not required, but sometimes they are used to help filter the DC voltage. They are part of the circuit that reduces the "ripple" caused by capacitors charging and discharging, which causes all kinds of nasty hum. Most modern designs do not use them and use a simple high-wattage resistor in its place instead, but some designers still choose to use them (one of those long-debatable "mojo" things.) Personal preference nowadays. The small "add-on" transformer here IS a choke. Two wires only is the identifying feature (and to some extent its size and location.)

3) The output transformer. This IS required for a tube amp. There were some extremely rare OTL designs (output transformer-less) but they do not work well... at all. Thus, they are still used in ALL tube power amps... radio, guitar, whatever... makes no difference. OTL designs have been a design "holy grail" ever since tube amplification came about, but as I said, it has to date never been achieved in a practical sense and thus the quest has pretty much been abandoned. Now, they are NOT required for solid-state power amps, but for tubes amps, yes you absolutely need one. In this amp, the transformer with WALZ written on it is the output transformer. How do I know? It is a Hammond 125ESE (right there on the label) and... well, see for yourself... http://www.hammondmfg.com/pdf/5C_125SE.pdf

4) already mentioned... the isolation transformer as an add-on safety device if the original design has no power transformer.

5 & 6 (and possibly more)... like I said, not seen enough to even worry about... Interstage transformers and even rarer transformers used as phase inverters. I guess I should add that sometimes transformers are implemented in the "direct out" circuitry of tube amps... those are more common than the others in this category nowadays.


If you are really interested in reading more and gaining a basic understanding of how tube amps work, I highly recommend the series of books called "The Ultimate Tone" by Kevin O'Connor or Merlin Blencowe's (The Valve Wizard) excellent website http://valvewizard.co.uk/ Both of these men are legends in the amp design/mod world (although Mr. O'Connor can be a little controversial, he is great at explaining the basics.) Another great read is the "P1eX Theory Document" over at AX84.com which walks you through a working amp schematic (one that is probably VERY close to the amp we have here) and explains the function of every component in the circuit. A guy named Pete Millet (I think) has a website of all kinds of out-of-print textbooks on tube amp design and theory if you really want to stretch your brain even more on the subject.
Thank you!
Old 10th August 2019
  #26
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Thanks everyone for your help
Old 3 weeks ago
  #27
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kingofspain's Avatar
Quote:
Originally Posted by wanghuskahn View Post
Thanks everyone for your help

Hi,

A little late to the party, but did you ever find out what was wrong with the amp? Or draw a schematic?

Like Tiny said at the top of the thread, the valve complement points at a very simple circuit inside the box - most likely something very similar to a Fender Champ in a dual single-ended configuration (again, as Tiny said).
The attached schematic could be similar to what's inside your amp.

A couple of questions:

1. The valve (tube, if you like) marked 6LS7 - is that actually a 6SL7? I had to 'translate' all the codes into english, so they're all a little unfamiliar to me. The 6LS7 doesn't appear to exist, where the 6SL7 is a common dual triode.

2. What do the three knobs on the front do? I'm guessing volume, bass and treble?

Could you post some pics of the inside? I'd be happy to have a look and see if I can draw up a schematic. I'm probably a frightful pervert, but that sort of thing keeps me entertained Or out of trouble...

As others here have mentioned, I'm surprised your repair shop asked for a schematic at all. Unless the insides are considerably more complicated than the visible components suggest, any tech worth their salt should have been able to suss out what's going on. Like many here, I'm a happy tinkerer, rather than a qualified tech, but I'd be surprised if I can't make some sense of it.

KoS.
Attached Thumbnails
Help me identify this homemade amp-champ_circuit-1024x723.jpg  
Old 3 weeks ago
  #28
Here for the gear
 

Thanks for volunteering your time! At some point this week I'll try and dig into the box a bit more and post some detailed photos.

Quote:
Originally Posted by kingofspain View Post
Hi,

A little late to the party, but did you ever find out what was wrong with the amp? Or draw a schematic?

Like Tiny said at the top of the thread, the valve complement points at a very simple circuit inside the box - most likely something very similar to a Fender Champ in a dual single-ended configuration (again, as Tiny said).
The attached schematic could be similar to what's inside your amp.

A couple of questions:

1. The valve (tube, if you like) marked 6LS7 - is that actually a 6SL7? I had to 'translate' all the codes into english, so they're all a little unfamiliar to me. The 6LS7 doesn't appear to exist, where the 6SL7 is a common dual triode.

2. What do the three knobs on the front do? I'm guessing volume, bass and treble?

Could you post some pics of the inside? I'd be happy to have a look and see if I can draw up a schematic. I'm probably a frightful pervert, but that sort of thing keeps me entertained Or out of trouble...

As others here have mentioned, I'm surprised your repair shop asked for a schematic at all. Unless the insides are considerably more complicated than the visible components suggest, any tech worth their salt should have been able to suss out what's going on. Like many here, I'm a happy tinkerer, rather than a qualified tech, but I'd be surprised if I can't make some sense of it.

KoS.
Old 3 weeks ago
  #29
Quote:
Originally Posted by Bstapper View Post
Nope. I’m not.
Yes, you are. There's no such thing as a tube guitar amp without an output transformer. You have to be able to match the very high output impedance of the power tube to the very low input impedance of the speaker - otherwise the output tube sees what is essentially a dead short and the amp goes up in smoke really fast.

There are, however, many cheap old practice amps without power transformers. You can easily identify there amps because they have tubes in them with strange numbers like 35W4 and 50C5. On (US numbered) tubes the numbers before the letter denote heater voltage. An amp without a power transformer runs the heaters (and everything else) directly off the power line voltage, and an amp without a power transformer will have a heater string that adds up to something close to 120 volts, give or take a smidge.

THESE AMPS ARE EXTREMELY DANGEROUS AND SHOULD BE AVOIDED because the chassis of the amp is tied to one side of the power line. Asa long as the chassis is connected to the neutral side things are fairly safe, buty most of these amps use two conductor power cords and rarely have polarized plugs (which are not a 100% safe solution anyway.) If the chassis happens to get connected to Hot (50% chance of that) you have a major shock hazard that will put the 120VAC line voltage in the chassis of the amp and all the grounded metal parts of your guitar! If you happen to touch any grounded metal object while playing you'll get zapped with the full line voltage, which can be lethal if the current flows across your heart, and at best is painful as hell. PEOPLE HAVE DIED USING THESE AMPS!

There have been designs proposed for guitar amps that use a cathode follower circuit to achieve a low impedance output and those actually don't have output transformers but you'll never see one in the field - to my knowledge they've never made it past the "demo at NAMM" stage, because they're really expensive (they need an extra set of output tubes for the cathode follower output) and they don't sound right to most guitar players, who like the sound imparted by an output transformer. To my knowledge such an amp has never made it into production. Every 10 or 15 years you might see an announcement to a "revolutionary output transformerless guitar amp" but that's about as far as it's ever gone. And that technology is DEFINITELY not suited to small practice amps such as what you were talking about.

It should also be noted that there are some old amps that did not mount the output transformer on the amp chassis but instead had it on the speaker frame. These amps are NOT output transformerless, the manufacturer just opted to put the transformer in a funny place.
Old 3 weeks ago
  #30
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Quote:
Originally Posted by John Eppstein View Post
Yes, you are. There's no such thing as a tube guitar amp without an output transformer. You have to be able to match the very high output impedance of the power tube to the very low input impedance of the speaker - otherwise the output tube sees what is essentially a dead short and the amp goes up in smoke really fast.

There are, however, many cheap old practice amps without power transformers. You can easily identify there amps because they have tubes in them with strange numbers like 35W4 and 50C5. On (US numbered) tubes the numbers before the letter denote heater voltage. An amp without a power transformer runs the heaters (and everything else) directly off the power line voltage, and an amp without a power transformer will have a heater string that adds up to something close to 120 volts, give or take a smidge.

THESE AMPS ARE EXTREMELY DANGEROUS AND SHOULD BE AVOIDED because the chassis of the amp is tied to one side of the power line. Asa long as the chassis is connected to the neutral side things are fairly safe, buty most of these amps use two conductor power cords and rarely have polarized plugs (which are not a 100% safe solution anyway.) If the chassis happens to get connected to Hot (50% chance of that) you have a major shock hazard that will put the 120VAC line voltage in the chassis of the amp and all the grounded metal parts of your guitar! If you happen to touch any grounded metal object while playing you'll get zapped with the full line voltage, which can be lethal if the current flows across your heart, and at best is painful as hell. PEOPLE HAVE DIED USING THESE AMPS!

There have been designs proposed for guitar amps that use a cathode follower circuit to achieve a low impedance output and those actually don't have output transformers but you'll never see one in the field - to my knowledge they've never made it past the "demo at NAMM" stage, because they're really expensive (they need an extra set of output tubes for the cathode follower output) and they don't sound right to most guitar players, who like the sound imparted by an output transformer. To my knowledge such an amp has never made it into production. Every 10 or 15 years you might see an announcement to a "revolutionary output transformerless guitar amp" but that's about as far as it's ever gone. And that technology is DEFINITELY not suited to small practice amps such as what you were talking about.

It should also be noted that there are some old amps that did not mount the output transformer on the amp chassis but instead had it on the speaker frame. These amps are NOT output transformerless, the manufacturer just opted to put the transformer in a funny place.
Yes, I'm familiar with field coil designs and most of the information provided has been redundant information for me - despite my lapse of memory and obvious holes in knowledge resulting in using the incorrect terminology.

The resources provided by dpsbb and his patience in his thoughtful response are however greatly appreciated.

Cheers,
Brock
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