Unknown circuit picked up at estate sale

[davohilts]

Hello,
I picked up a guitar amp project at an estate sale and am getting around to looking it over. It has one input and uses what I expect are 2- 12ax7 tubes and 2-el84 tubes. It is wired up with power switch and standby switch on front, what I assume are vol, treb, mid, base potent. and powers up when I installed the above tubes.
There are some unknown to me wiring concepts. The OT has 5 wires. The typical red and blue, green and black, but also a black w/red stripe wire that is run to the standby switch. There is also a switch on the back which has a cap across it. I was going to just remove the stuff and start again but now i wonder what is going on and if I might learn something new here.
I'll attach photos as a start to save a million thousand words.
Thanks for any help.
Dave

[J M Fahey]

Why not "reverse engineer" (AKA "copy") and post it ?
It might already be something interesting .
Worst case, a good idea on what to do next.
Wire colors mean little, draw where they'r attached to.

[wallyo]

Push-pull amps should have five wires on the o/p xfrmr. Power is fed to the power tube plates via a centre tap on the transformer and in your case it seems to be controlled by the standby switch.
The other wires are two for the speakers and two for the o/p plates. Makes sense to me.

[davohilts]

Okay, the reverse engineering idea is a good one. I will work on a layout drawing and post it here and see if anyone recognizes the circuit.
Thanks for the help.
Dave

[redelephant]

Nice! Some very nice components in there.

The extra two wires from the OT could also be for ultra-linear operation?

How does it sound?

[davohilts]

amp has potential, the loud hum however makes it hard to really tell for sure. I leave tomorrow for a few days but I will get the layout drawn and up for review. I dont know a lot about drawing up a schematic but I will try to draw what is there in a layout form.

[redelephant]

From the pics it looks like the cathode resistor for the output stage is unbypassed. (Pin 3 of both EL84s going to the big brown resistor). That resistor should be bypassed, 20-150uF.
About the hum. Check if the heater is connected to gnd. If not, try connecting on end to gnd. If it is connected to gnd, or when connecting it to gnd and you still have hum, try lifting it off gnd and place one end to the positive side of the cathode resistor.

[jmaf]

Connecting one of the heater leads directly to ground will maximize hum during half the heater cycle.

Two matched 100 ohm resistors are a better solution. Solder a resistor to each lead of the heater and the other ends together to ground.

[NorCalTuna]

It looks like a princeton kinda thing with different pre bias and k-bias el-84s? Looks cool, more like an AC15 possibly? +++ drawing a schematic, it's such an education!

Anyhow, redelephant nailed it- since it's cathode biased, do all of the above! Put the 100 ohm resistors on, and then tie them not to ground, but the cathode resistor's "hot" side. That will bias up your heaters with a dc voltage and cut hum even more! Then, once the hum is under control, put the cap over the cathode bias resistor, and to be tricky, put it on a switch with a 10m resistor in parallel with the terminals. That will give you a huge power and gain boost, on a switch, and the 10m resistor will keep the cap charged so it doesn''t POP! when you switch it.

awesome find!

[wallyo]

Good advice re the heaters - some amps have centre-tapped heater windings for just this purpose and are usually connected c.t. to output cathodes as described here. The idea is to keep the a.c. from the heater supply from modulating the low-level circuits. Some amps have used d.c. heaters but that's a bit extreme.
Regarding the cathode resistor bypass cap, keep in mind that if the cathode resistor is NOT bypassed it provides negative feedback which will reduce the gain (as he says) but is done deliberately to reduce distortion. That's not to say that more gain and a bit more distortion is necessarily a bad thing - it's your amp after all. Another thing you can try is to replace the cathode resistor with a pot with the "wiper" connected to the bypass cap so you can continuously vary the feedback. Watch the power rating of the pot. Or you could use a rotary switch and a bunch of resistors to create different settings.
Note that the VALUE of the bypass cap is also significant in that it affects the frequency response (percentage feedback increases as frequency decreases) so effectively a low-end roll-off. Now having said THAT, you could get really creative with the feedback and do some serious response shaping. Or you could just put it back to what the designer originally did and enjoy it.
I'm just jealous 'cause I rarely find cool stuff like this at estate sales anymore. Bought a tube tester for $2 once though.

[davohilts]

Okay I've had a little bit of time to quickly take some voltages and draw up a layout drawing showing how the amp is laid out. There is a switch on the back which has a cap going to ground that is an on/on type switch and I'm not sure what the cap is doing, or the switch for that matter.
Anyway, I hope this is clear enough to get an idea of the circuit. I will attach the drawings I did and the voltages measurements for everyone who is interested.
Thanks,
I'll be back on Monday
Dave

[jmaf]

Discussion about that switch and capacitor you describe: http://music-electronics-forum.com/t14768/

[redelephant]

Regarding the cathode resistor bypass cap, keep in mind that if the cathode resistor is NOT bypassed it provides negative feedback which will reduce the gain (as he says) but is done deliberately to reduce distortion. That's not to say that more gain and a bit more distortion is necessarily a bad thing -

Hmmm. Isn't that resistor shared by the push pull pair? In that case when unbypassed the resistor act as the 'tail' in a 'long tail pair' and biases the stage in class-A. However as soon as the tubes try to conduct any more current than the total current it will act as an unbypassed resistor in a common cathode stage. Only at that point will the capacitor do it's part.
So the gain will no increase with the cap, only it will allow the output to enter class-B when the signal becomes large enuff.

[Gtr_tech]

I'd redo the gnd scheme for the front end. That old style "gnd bus to the backs of the pots" thing is a potential gnd loop magnet. Its not needed, the pot casings are grounded via the chassis connection. Just run the bus wire along the pot lugs (seperate them from the casing) and pick a quiet gnd point for it.

[davohilts]

Well I had a day set aside today to get back to work on this unknown amp and to finish up a cap job on a Magnatone M10A. I'll post another thread on the magnatone.
I started out by removing the unknown switch between the power entry cable and the first fuse. I rewired the power entry cable with the hot going to the fuse and the neutral going to the black wire of the power transformer.
I then rewired the center tap from the filament winding and connected it to the positive side of the cathode resistor and installed a 25mfd 25v cap across the resistor. I tested my work and the hum is still there so I went back and separated the grounding bus from the backs of the pots and tested my work and still have the hum.
Such is life.
Any ideas as to what to try next??