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Teisco/Audition amp schematic

Started by galaxiex, August 25, 2015, 12:05:33 AM

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I will add them as attachments at the bottom of each page.

So I "acquired" this amp...  <3)

It had some issues and I could not find a schematic....

So I traced the 2 circuit boards and created schematics from that.

Probably the only schematics for this amp on the whole interweb...  :loco

It has power supply and power amp on one board,
and pre-amp board separate.

All component values are original as found on the boards.

EDIT: redraw of pre-amp schematic to make it easier to "read"  ;)

On the power amp board there are 2 mystery components that look like capacitors but I can't get a reading with my cap tester.
They do give a resistance reading.

There are no markings at all on these parts.

They are marked on the power amp schematic as ? and 22R because they measure 22 ohms.
Right next to R3 and R4

Seems odd to me that they would put them in parallel with 20 ohm resistors.
Can anyone shed some light?

EDIT: Corrected pre-amp  and Power amp schematics.
Fixed reverb tank.
Added transistor types to pre-amp schem.
Added themistors to power amp schem.
If it ain't broke I'll fix it until it is.

J M Fahey

1) THANKS for taking the time to draw this beautiful example of time-travel  :dbtu:, a perfect example of Germanium Technology at its best  <3)

2) before I forget, the schematic is perfect, except that the Reverb tank goes the other way: the 4 ohms end is the driving one (we need to feed power into it)  while the 1600 ohms end is the output one, we want many turns of fine wire so output signal is highest.

3) please PUT BACK those mystery 22 ohms "black lentils" , those are NTC thermistors, voltage variable resistors, and being there they lower resistance, lowering bias voltage, when they heat up: thermal compensation.
Without them the amp will be heavily overbiased and may easily burn.
Pamper them because nowadays they are not popular, so are hard to find.

4) a capacitor is a capacitor, a resistor is a resistor, etc. so no big deal, I'm very interested in the *mechanical*  side of Electronic things, can you please post a couple pictures of the Reverb tank?
Specially the transducers.
Without seeing it, I guess it might be one of the infamous "Piezo" brand tanks, ubiquitous in all mid/low price Japanese amps, until one day Accutronics (or a forerunner)  got fed up and stopped them.

5) if at all possible, I'd also love to see the speakers.
No need to pull them out, just the magnet (which I guess is Alnico, although Japanese were early Ceramic users) and of course will also dee the frame.

6) please please please  :'( we NEED an MP3 or YT sample to hear it in all its glory ;)


Mister Fahey, Thanks so much for your interest in this amp!  <3)

Always glad to oblige with more pictures.

2. Reverb tank backward, I will correct that.

When this came to me the Tremolo did not function.
Replacement of the electro caps (C17, 18, 19, 20) in that section created a functioning Tremolo, with a speed range of fast to insane.  :o

Testing of the original C17, 18, 19, 20 showed they had reached the end of life, shorted, open, far off values....
so I went thru the amp and replaced all electros with new of the same uF values, perhaps some with higher voltage ratings.

Then I changed all ceramics to box film caps of the nearest values.
Testing the old ceramics showed some very far from even the 20% tolerance value.

I left all resistors original.

3. THANK YOU! for telling me what those mystery components are.
I never would have guessed, but your explanation makes perfect sense!
I never intended to leave them out, I only removed them for testing to see if I could figure out what they are.
They are already re-installed.  :)

5. Speakers, here they are.  :)

6. Ahhh, an empeethree or a YT vid.... hmmm...

ok I can work on that... but no promises yet, and anyway you might not like my lousy guitar playing...  ;)
Or.... I'm too embarrassed to show my lack of talent to the public.  :o

Bear in mind I have already begun to alter things about this amp so it won't sound the same as when I first got it home and plugged it in.
If that "original as found" sound is important to you I can easily reverse the changes I have made so-far and make a recording.

Otherwise I will carry on with some small minor changes, such as an input buffer etc.

I have already altered some of the values in the trem to get a more usable speed range. 
Oh, and I removed the "Death Cap" that was hooked to AC line and ground.

EDIT; I installed sockets for the transistors on the pre-amp board for testing/possible replacement purposes.

Already had Q3 fail, at least, it wont work in the circuit, but it tests "ok" on my transistor tester....????

I replaced it with a PNP marked D2060Z.
Not sure where I got this... it was in one of my parts cabinet drawers marked "Generic PNP" ?  :o

If it ain't broke I'll fix it until it is.


trem goes from fast to insane.  You changed C18,19,20, but if you increased their value, it will slow the trem, for example make then 1uf instead of 0.47uf, or increasing resistors R27 R28.  I often slow down trems.  I think they should slow down to a slow sexy throb, and even then, the fast end of the scale is still way too fast.


Thanks Enzo! I already did that.  :)

Changed C18, 19, 20 to 4.7uf. Trem is much better with a nice slow throb.

Also experimenting with R27 and the speed pot got a much better speed "spread" on the knob.

Using a 750R for R27 and a B10K for the pot. Can't go much lower on R27 or it kills the trem.

I never tried altering C17 or R28.

The way it is now, it could be faster on the top end but it will do.

To be honest, I don't really know what I am doing.
I have only enough knowledge to be dangerous.

When it comes to oscillators I'm pretty lost.
I write down what the original values are, and then throw different parts in to see what they do.

Example: I know increasing a cap value causes it to run slower, cuz it takes longer to charge and discharge a larger cap.
Same with resistors, a larger value can cause a slower charge, or discharge depending on where it is in the circuit....
but that's the thing, I don't know how to analyze the circuit to know which way the change will happen...
Hence, I throw parts at it until it does what I want, or I get tired of mucking with it. Either go back to the original values or get it "good enough" and call it a day.  :duh
If it ain't broke I'll fix it until it is.


I got the amp back together enough for testing and tweaking.

I'm just playing around here....  :duh

The signal thru the pre-amp is clean, no clipping, but when it gets to Q1 in the power amp schematic and my scope probe on Q1 collector, the signal clips the positive half on peaks. (Reverb and trem at "0")
The negative half is fine.

I tried playing about with bias resistors R10 & 11 to no avail.... put them back to original.

Then noticed the NFB at R12 200ohm and R9 20ohm.
Installed a 1K pot instead of R12 and tweaking that to ~ 33ohm got a balance of still good volume, and less distortion.

High Resistance at R12 = Louder amp but lots of distortion and very harsh "screechy" treble response.
(this seem to be how some "Presence" controls in tube amps work)

Lower or less resistance at R12 = quieter amp but tone gets darker, less "sparkle".

I understand that R12 and R9 form a voltage divider and are a direct path to ground for the output signal, so of course reducing R12 makes it quieter.

Next gonna play with R9 and see what happens...

Any knowledgeable folks got some insight for me on this area of the circuit?  ;)

If it ain't broke I'll fix it until it is.


Modified pre-amp for better control of the Tremolo.

Tremolo now has much better control "spread" on the Speed pot and the Speed range is "slow to medium slow".
No more fast to insane.  8)

The Depth control never was a problem. At Max Depth it has a nice strong throb throughout the speed range.

It's not as slow as some on the low end... and not as fast on the top end...

Such are the restrictions of a phase shift oscillator with only one adjustable component. (shrug)

Has a nice sine wave pulse, very smooth and dare I say, sexy sound.
Not choppy at all, no ticking.
Probably much better than how it left the factory. Very usable.  :)

New pre-amp schematic with the modified tremolo parts values.
Compare to the original pre-amp schematic in post #1.

This took me several hours of messing around trying different values.
Not to mention searching the intergoogle for information on Phase Shift Oscillators.  :o

In the end the only new parts are
R28 old = 10k  new =200k 
P5 old = A50k  new = B100k

I learned LOTS doing this. Very happy with how it turned out.  8)
If it ain't broke I'll fix it until it is.


Still tinkering with this amp...

I didn't like the tone control response.
The original circuit seemed to have both Bass roll-off and Treble cut. (see schem in post #1)
Presumably flat response at "5"...

Trouble was, at 5 there was not enough treble, so increasing the control got more treble, but it rolled off the Bass too much.
The tone became screechy and harsh. Lacked punch.

What to do....

So I started looking into a simple treble cut control to preserve the Bass response as the control was turned towards 10.

Found the "Stupidly Wonderful Tone Control" at AMZ.  http://www.muzique.com/lab/swtc.htm

I used the last circuit on the page linked above.

WOW what a difference!   :)
Much better Bass response and now I can dial in as much, or as little, high end as desired. 
A side effect is the amp is much louder as well.
I suspect that R9 250k in the original circuit was attenuating the signal somewhat...  ::)

Here is the schematic with the new tone control.
The only difference between this, and the Stupidly Wonderful Tone Control,
this amplifiers original Volume pot is not an audio taper as per the SWTC circuit.

Note also the modified R values of R5 R6 and R29.
These were changed to get the collector voltages closer to half the rail voltage.
These stages were clipping very easily before.
R6 changed to get a bit more voltage to the input stage.
With this change the amp is much cleaner and seems more dynamic.
I did play it for sometime with these new R values, before adding the Stupidly Wonderful Tone Control.

Next up... Input buffer...  :)
If it ain't broke I'll fix it until it is.


Ok, I think I'm finally done tinkering with this amp.

Here's the input buffer board I made for it.

Here is the schematic for the input buffer.

Jack's 1 & 2 are somewhat Fender like and Q1 has a tiny bit of gain.

Jack 3 is a FET buffer.

Changed the pre-amp again with the addition of R35 250k.
This is a resistor that was in the original circuit, I removed it when I modded the tone control.
Added it back in because it seemed to be over-driving Q5.
I suppose it could be replaced with a pot to act as, sort-of a gain control....
Or, could reduce the value to ~ 150k or so to get a bit more volume out of the amp.
The amp is a bit quiet...

Put the input buffer in place of the 3 jacks and R1 and C1.
C2 remains... it doesn't hurt anything.

More pics...

Added a modern power entry socket and cleaned up the AC wiring.

Ok, that's it, I think it's done... for now...  :)

If it ain't broke I'll fix it until it is.



Drew up a new pre-amp schematic incorporating the input buffer board.

... and to go with it... here again is the power amp board schem.

If it ain't broke I'll fix it until it is.


Hello, thanks for this thread, I was delighted to find it but then I saw that your drawings had been taken down. I have a Zenta amp which as far as I can tell is just a rebadged Teisco audition and am trying to fix the tremolo. Do you still have your drawings that you could share? Many thanks, Adam


Quote from: tysonad on July 19, 2023, 03:47:22 AMHello, thanks for this thread, I was delighted to find it but then I saw that your drawings had been taken down.
The missing pictures have been added at the bottom of each post (as attachments).  I don't know how you could actually miss them.