Homebrew Amp I.D. Help?!

[simplejones]

Howdy,
So here are 3 pics of a homebrew tube amp i picked up. Here's the thing tho: I know nothing about it! It has a little transformer with no markings and 2 12Ax7 tubes. 1 output jack (which I can't figure out if it is a Speaker(if so, the Ohms??) or if it's Line). I've included pictures here of the front, back, and underside in hopes that some of you might be able to look at how it's wired and offer your suggestions on what I have here.







Many thanks. (I can get more pics too if anybody wants).

[flateric]

I think you can assume with a single output jack this is for plugging a speaker into, and it's typically gonna be an 8 ohm 12" guitar cab speaker.
I would also assume that the power rating with 2 x 12AX7 tubes is going to be between 5 - 10W max.
I am a little worried whoever you bought it from was not able to provide this basic information - have you ehard it work yet?

The following link gives you a load of DIY amp sites where you could browse for soimething similar looking to help identify the model and specs.
www.diycustomamps.com/diyamplinks.htm

[simplejones]

hey thanks for the response!

I actually found it at a Thrift Store for 8 bux. (Which is why I know nothing about it) I have plugged it into a line-in on a mixing board and listened to it through headphones for a minute or two, and it sounds fine. But I haven't wanted to risk plugging it into anything else until I know more about it so as not to damage anything by using an 8$-thrift-store-amp on it. (I'm hoping it wasn't totally wrong to plug it into that mixer).

Anyway, I'll check the site you suggested. If I can get this thing running, and if it sounds any good I'll post a sound clip.

[ChrisK]

Well, although we're awfully dang smart here, we're going to have some difficulty in determining exactly whose home in which this was built.


Some clues:

This has "Gain", "Bass", Middle", "Treble", and "Master" so it has a preamp section with both pre and post volume controls.

It has a (as in one) little transformer and, since this is a tube amp and it's near the mains cord and fuse, it's most likely a power (input) transformer.

The jack labeled ".....t" is connected to the the "Gain" panel control potentiometer . This is likely the "Input".

The "Master" panel control potentiometer has its wiper connected via a white wire to the jack labeled "Output".

There is no output transformer.

The 12AX7 is a weak sister regarding plate current compared to a 12AU7, which is sometimes used as a low power output amp stage of perhaps 1/2 watt.


This is likely a preamp.

It seems to have a lot of tube stages (4) for a Fender tone stack (guitar'ish) or Bauxandall structure (Hi-Fi'ish), both of which are cut-only structures, but it may also just have a cathode follower or two for tone stack drive isolation and relatively low (for a preamp) output impedance.

A key to understanding this (sans schematic tracing) would be to know the resistance value of the "Master" pot.


Since the controls are labeled "Gain", "Bass", Middle", "Treble", and "Master", it is likely Fender'ish in structure.

www.duncanamps.com/technical/baxandall.html

www.duncanamps.com/technical/tonestack.html

www.duncanamps.com/tsc/


(These are cut-only in that a passive network can only cut. While there is a gain stage before the structure as well as after the structure and the overall effect may well result in gain, since the passive structure IS NOT used as a feedback structure around a gain stage (as in that of an OP-AMP), it has the undesirable noise susceptibility of such.

A Proper Active Tone Structure

[simplejones]

Okay ChrisK,

Thanks for the assesment. So I have some (possibly dumb) questions for you:

1. if it is just a preamp, can i just run the output into another amp's input or a line-in on a mixer?

2. what should I look for to see if it has a cathode follower?

3. to find the resistance value of the "Master volume" pot, I just need to use my multimeter right?

Thanks again! This is becoming a very educational experience.

[ChrisK]

1. if it is just a preamp, can i just run the output into another amp's input or a line-in on a mixer?

That depends on the output level, impedance, and the input requirements of the "another" amp. Since the preamp has a master volume as the last element in its structure (which seems to imply that the output can be turned down to zero), likely the answer is yes.

2. what should I look for to see if it has a cathode follower?

Uh, well, a cathode follower section. This is a tube stage where the output is taken from the cathode rather than the plate (essentially across the cathode resistor which is usually grounded on one end). Its main characteristics are a non-inverting gain of one (the cathode is a'followin' the grid) and a low output impedance (that low valued cathode resistor) which means that the "stuff" downstream doesn't affect its output signal nearly as much.

I would suggest (see your last sentence in your last post), as part of this "very educational experience", that you trace the circuit. Then you could post it and we all could see things.

3. to find the resistance value of the "Master volume" pot, I just need to use my multimeter right?

Yeppers! Make sure that the amp has been turned off for at least an hour (you don't want me to start calling you "Sparky").

Remember, something (or someone) is always the fuse.

This is becoming a very educational experience.

Heh, heh, another victim, er, uh, ok, yeah victim caught "in the web", eh Boris? ;D ;D ;D ;D

[simplejones]

Alrighty... So I've been staring at the innards of this amp for quite some time now and I've got more info. I have to warn you that I am an amp electronics novice (if you haven't already noticed) so I'm going try to trace the circuit for you without really knowing how to draw a circuit diagram (hopefully it makes some sense tho).

NOTE: I'm referring to Pot Terminals as 1, 2, 3; Looking at the back of the pot with terminals facing down.
O
123
ALSO: I'm leaving out all the Caps and their values in this layout. And I'm referring to the Tube socket Terminals as 1thru 9 as they appear to be numbered.

First:
The resistance reading I got on the master volume pot was: 1.095 from term1 to term2, 1.096 from term1 to term3, & 1.4 from term2 to term3.

NOW FOR THE CIRCUIT:

• TRANSFORMER
  
• Green & Brown Wire->OnLight->Circuit Board->Tube2(Terminals 4/5&9)->Tube1(Terminals 4/5&9)
  
• Black Wire->Power Switch
  
• Black Wire->Power Cord
  
• Yellow->Ground
  
• 2 Yellow Wires->Blue Cap->Standby->Ground
  
  
• FUSE 0.75ASB->Power Cable& Power Switch
  
  
• INPUT JACK
  
• Hot->Tube1 (Terminal7)
  
• Ground->Tube1 (Terminal8)->Ground
  
  
• GAIN POT
  
• Terms1+2->Tube1(Terminal1)->Ground
  
• Term3->Tube1(Terminal8)->Ground
  
  
• BASS, MID, TREBLE POTS
  
• Term1->Tube1(Term1)->Grnd
  
• Term2->Tube2(Term2)->Grnd
  
• Term3->Tube2(Term1)->Grnd
  
  
• MASTER VOL POT
  
• Term1->Tube2(Term1)->Grnd
  
• Term2->Out Jack(Hot)
  
• Term3->Tube2(Term2)-Grnd
  
  
• OUTPUT JACK
  
• Hot->Mast Vol Pot (Term2)
  
• Ground->Grnd on Chassis
  

OTAY, so that's what I got for now. Let me know if I missed (or misunderstood) anything crucial... but hopefully this info is interrrpretablllle (hmm... that may not be a word...)
Thanks!

[ChrisK]

Hmmm, I think that we've miscommunicated. I don't want you to describe the wiring, I want you to trace the circuit and post a schematic of the wiring.

Including the caps and resistors and their values is critical to understanding the amp.

Here's an example that will show the symbology for most of the components that you will find.

ax84.com/static/p1/AX84_P1_080708.pdf

Also, in your description, there seems to be a lot of grounding going on. I don't understand the precedence of your descriptions.

[simplejones]

Blasted! Okay I'll reassess. Thanks for the schematic link. I'm going to see this through...

[ChrisK]

The duncanamps links and the ax84 links will go a long way to showing how to do this.

"Schematic" is only a different language.....

[simplejones]

Hi ChrisK,

I've got my schematic in the works. (The AX84 links have been very helpful.) I'm learning this new "language" I go so it's slow goin'. I just wanted to clarify one thing real quick:

A key to understanding this (sans schematic tracing) would be to know the resistance value of the "Master" pot.

To do this, am I just needing to connect my multimeter leads to the 1 & 3 lugs of the the MV pot or somewhere completely different? Cause it's a 1M pot and it reads just about that. But now I'm questioning whether I measured the right places.

Thanks

[ChrisK]

Yep.

The two outside terminals are connected to the ends of the resistive element which can be within +/- 20% to +/- 30% of the nominal value. This value is the rated resistance value for a pot. The middle terminal is the wiper and the sum of the readings from it to each end terminal will be very close to what is read across the end terminals.

[simplejones]

Howdy Friends,

Longtime no talk. But I'm back to this thread with a schematic of the amp we've been discussing. It's my first one, so those of you who are pros at this will probably find flaws in my execution. but the info is there. so take a look and by all means, tell me what you think!

here's the link: farm4.static.flickr.com/3212/3309850832_3c8a8b8ec7_b.jpg