Quote from: aquataur on June 04, 2026, 03:20:35 AMYou create the impression of believing that discrete design is less demanding - it is not. In fact the contrary is the case, worsened by the fact that op-amp design is better understood today than discrete.

I don't have the impression that discrete design is less demanding. I'm interested in learning how amplifiers work and how the components interact with each other.

The only assumption I have is that opamps or IC's do the work of numerous discrete components. I would like to have a better understanding of basic electronic circuits.

Thanks for the response.

Yes, I would like to build a guitar amp. What inspired me was an old Univox SS amp I found recently, it has all of the attributes I mentioned except it has a "tone" control and I would like separate bass and treble. I suppose I could build a clone of it (I have the schematic) but would like some options.

From your words I deduce that you want to build a musical instrument amplifier, not a HiFi unit.
10 Watts is nothing for a MI amplifier. Although the power itself is not the restriction, it is the lack of headroom.

SS amps clipped sound awful. A 10W tube amp can create the appearance of being of much higher wattage due to the compression and benign clipping, which most of the time goes unnoticed.

Your description fits SS amplifiers from the past, such as early John Linsley-Hood class-A designs. He was designing for HiFi, and even he, at a later time, went to higher wattage push-pull designs.

You create the impression of believing that discrete design is less demanding - it is not. In fact the contrary is the case, worsened by the fact that op-amp design is better understood today than discrete.

Schematics are around, but be aware that this is only half the story. PCB and other layout is ultra critical with power amps. The components reach up into the MHz range, and they can and will oscillate vigorously if you are not experienced, most time leading to immediate self-destruction. This can hinge on simple things like two tracks running in parallel for a length. This reaches deeply into HF design, where other rules apply, and they are not forgiving.

For a newcomer into that field, this is a very ambitious endeavor.
If you are determined, be at least prepared for harsh set-backs.

I'm relatively new to electronics so would like to use this as a learning project. I've restored a number of early transistor & tube audio amps and one SS guitar amp. Have dmm, oscilloscope, sg, soldering station, etc.

Are there any amps out there I can clone or are there any tried and true schematics available that I can use?

My goals are:

all discrete circuit (no op-amps, IC's, etc.)

10-20 watts (+-)

treble & bass controls

capacitor coupled output

thanks in advance

If the hum is increasing as the outputs turn on, I suspect it is ripple on the power supplies rather than DC imbalance.
With speaker connected, measure both AC and DC volts at speaker with stock bias setting, then with bias setting that gives loud hum.
Also measure AC volts at + and - 42V supply rails under low and high hum conditions.

Thanks again for your reply.

Here are the results.

Combined resistance of R35 and R318 decreased to 790ohms

Q34 base to emitter -0.572 volts
3.0 millivolts across R321

Q36 base to emitter +0.566 volts
4.6 millivolts across R320

The hum is still there.
Might be a little louder.
If I reduce the resistance lower than 790 ohms the hum gets a lot louder.
As I sweep the resistance,using a pot,from 900 ohms down to 790 ohms the hum does not reduce at any point.

I had another SA1695 so I changed it.
All the voltages stayed the same.
I don't have another SC4468.
I had already replaced both of these before.
I'm just wondering if they are all unbalanced.

It seems like Q34 is not conducting at all, and Q36 just barely.

edit:  I think I went the wrong way with R35, to increase idle current of the outputs, you need to reduce R35. 
If you have a 10K pot you could wire it up as a rheostat and start with a 10K setting, then slowly reduce til you see a few millivolts across each of R320 and R321.  From there once you have a good setting, you can measure the pot resistance and install a resistor of that value for R35.
If you don't have a pot to use, you can do trial and error with values less than 10K for R35, or add a parallel resistor if that is easier than replacing it.
The way it is set up, R35 and R318 in parallel are making about 900 ohms.  You want to slowly decrease that to get the outputs conducting.

Quote from: g1 on May 02, 2022, 12:43:02 PMHere you go.

G1 - Thanks for sharing the schematic. I did not realize there was a version "ii" and Peavey dumped the folded PC board design for a single flat layout.  I guess the quick giveaway on this amp is the Power Switch and Standby switch right next to each other.  I also see that each output tube now has its own screen resistor- yeah!! So this version might be a good one to consider if you are looking for the Classic 30.

Thanks again.

R35 disconnected.

R320
With speaker 1.2 millivolts
Without speaker 0.0 millivolts

R321
With speaker 0.0 millivolts
Without speaker 0.0 millivolts

Try removing or disconnecting one end of R35 (10K), then repeat those last measurements.