Found this old schematic

[Peregrine]

Hello guys, I'm new to the forum, I'm a guitar tech and just generally interested in audio electronics, especially valves and FX pedals.

My dad was an electronic engineer and I found an old schematic of what looked like an amplifier by him.
It was on really old paper etc... so I couldn't decipher all of it.

I re-wrote most of it, but I'm not great with figuring schematics out.

Could anyone help me out ?

I'm just wondering where the ins/outs are.
If there's any way to figure out what the transformer voltages are etc..

Just general help figuring the schematic out, even improving it, or help figuring out if it even works.

I definitely want to give building it a go.

Thanks in advance for any help any of you guys can provide, I'd be over the moon if this worked.

[Jazz P Bass]

Nice find.

It appears to me to be simply a power amplifier.

The "IN" is off to the left, at the .01uf capacitor.
The 100k potentiometer would be a level control.

The output would be taken off of the transformer secondary, the primary being attached to the tube plate.

It is a cathode biased configuration.

I do not 'see' a phase inverter tube.

The whole shebang will need a preamp.

[Peregrine]

Ahh thanks man. That's already a tonne of help!

So this is just a power amplifier, that's good to know.

Sorry in advance if I ask any silly questions by the way.

So does there need to be a phase inverter tube?

[Enzo]

It is a single ended amp, so there is nothing for a phase inverter to do. Like a bicycle has no need for a differential rear axle.

The drawing is not complete. Parts are missing. At the volume control, I suspect the input goes to teh top end of hte pot, but the wiper probably alone connects to the rest rather than to the top.

Also I see the 2.2k resistor with a bypass cap, and a 220k resistor at top. Clearly there sould be a triode there. The 220k connects to its plate, the 2.2k and cap to its cathode, and the volume wiper to its grid.

Likewise immediately to the right is another 220k and 2.2k. Another triode went there. The cap from the first 220k, down to the 47k was a coupling cap between stages, the 47k connecting between the grid there and ground. These new 220k and 2.2k connect to respectively the plate and cathode of the second triode.

Two tries? Voila: 12AX7 not drawn in.

Also, you are missing a wire from the top end of the output transformer primary left to the joint of the 50k and 3.9k.

This schematic is super similar to a Fender Champ 5F1. Add that 12AX7 in there where it belongs and it is a complete guitar amp, no more preamp needed.

[Peregrine]

Wow man thank you!

You're a legend.

When you say volume wiper what are you referring to? Are you referring to a lug of one of the potentiometers?

Is there any way to know what the two transformers would be?

[Peregrine]

Also, what valve do you think is being used there?

[Enzo]

I have the idea a 12AX7 is what is not drawn in.

The wiper is the moving contact in the pot, the center leg, indicated by the arrow.

The output transformer is something appropriate, like a Champ output. A small amp like this is usually a 6V6 or an EL84. The power transformer must provide enough current at 6vAC for the heaters, and whatever high voltage you want. Say 300v.

[Dave H]

Check the value of the power tube's cathode resistor. 10k is much too high. It would be a few hundred ohms for an EL84 or 6V6. The same goes for the 10k resistor between the diodes and the 100M (100u) capacitor.

[Peregrine]

"The 220k connects to its plate, the 2.2k and cap to its cathode, and the volume wiper to its grid."

When you say this man, the 2.2k resistor is a fixed value, not a pot so what did you mean by wiper in that scenario?

I'm currently redrawing a rough version of the schematic with the 12 ax7s in but I can't figure out where you mean to connect it there.

[Dave H]

"The 220k connects to its plate, the 2.2k and cap to its cathode, and the volume wiper to its grid."

When you say this man, the 2.2k resistor is a fixed value, not a pot so what did you mean by wiper in that scenario?

I'm currently redrawing a rough version of the schematic with the 12 ax7s in but I can't figure out where you mean to connect it there.

Replace what you have drawn with a normal triode symbol with its grid, plate and cathode not connected together. The wiper (middle terminal) of the pot connects to the grid of the left triode. Disconnect the connection from the pot CW (top) terminal and wiper. Connect the 2k2 resistor and parallel capacitor between cathode and ground. Connect the 220k between the plate and B+ supply node (32u cap +ve). The right triode is similar. It is drawn a little better. I can see the dashed line of the grid.

[Rhodesplyr]

I agree with the responses, but my impression is that there's a lot in this schematic that simply doesn't make sense like the 10k cathode resistor or the 47k grid-leak resistor fed by a 0.01uF capacitor. We can recognize an overall topology, but I wouldn't call it a functional circuit as drawn unless it was for a very unusual application.

[52 Bill]

Perhaps you could post a photo of the original drawing to see if anyone here can better decipher the elements.

[Peregrine]

I'll find the original piece of paper but I just copied it like for like, if it's not a viable amplifier it's alright, just would have been really cool to build it.

Thanks for everyones help though guys I'm gonna keep trying to figure it out

[oc disorder]

This is a rough sketch of where the 2 halves of a 12AX7 would go.
The dashed line on the original below the second 220k suggests to me that it was a grid for a 12ax7.
You may have misinterpreted the symbol for a simple triode on the original sketch and joined what appeared to be
"broken connections" for want of a better description.
There's a goldmine of information on this sort of thing on the net if you have the passion and the urge !
Search for something like... "simple single ended low power amplifier schematic" or similar.

[Enzo]

Yes, that is it exactly, though you still have missing the connection of B+ to that 50k resistor.