Two channel class A amplifier

[Banjan73]

Hello!
Its been some years since i last visited this forum. In the meantime, I have worked with two amps on my own.
So I`m thinking, lets present the last one I`ve made first. This one is actually just finished, and it is not tested properly yet. It might need some tone tweaks, but that is minor changes in the circuit, so I guess it`s ok to present this one now.
The concept here is based on the Fender champ 5F1 amp, but with an EL84 tube instead of the 6V6. The transformers are from an old radio.
This amp is made as a collaboration with a colleague of mine, which share my interest of making tube amps. However, he has never made a guitar amp befor, only hifi-amps. So this one was a new experience for him.
Anyway, my colleague did all the basics on this amp, woodwork, chassis etc, and he constructed basically a straight forward "champ"-type amplifier based one the 5F1 schematics (with some adjustments off course due to different transformers and tube).
Then it was my turn to hotrod this thing.
This time I though the concept as follows: two channels ("clean" and high gain) with just a amp switch (no footswitch), effects loop, no tone pots /equalization.
In other words, quite straight forward... Ehh.. Not.
I was struggling a lot with hum, white noise and oscillation in this one. But after a while, I have now ended with a result that seems, after a few low volume tests, quite nice.
Well.. Enough talking. Lets see the construction:

[Banjan73]

Some pictures

[Banjan73]

and more

[Banjan73]

Now. A couple of comments.
First: on the schematics you will see R20 10ohms. That one is not going to be there probably. Its just a survivor from a unsuccessful low pass filter that I was trying out there. But since its only 10ohms, I have just left the resistor there.
You will also see that the build is quite messy. This amplifier has not been built to look nice inside, but to sound good. It also suffers from a lot of experimentation, so that also explains a little bit of the mess inside..

[Banjan73]

Oh. I forgot.
At the bottom of the schematics, you will see the recovery stage after the effects loop. This one is now not connected because it created a lot of white noise. So I removed it. I will test the amp today at a band rehearsal with my effects board. Hopefully it will handle all sorts of effects, but we will se.

[dogburn]

Interesting project! Is it loud enough to play with a band? And it looks like there is no negative feedback loop - I would have expected one for the "clean" channel.

[Banjan73]

Thanks Dogburn.
Well.. Loud enough is relative. Yesterday we played without drums, and quite laidback. Actually, I missed some more volume in the band to get a bit better distortion sound. I got the impression that it sounds a little bit thin at low volume. I might put on some anode decoupling capacitor or larger grid resistors to cut some treble in this one. Or maybe a better constructed low pass filter than the last one I made.
Sadly I realized that the effects loop didn`t answer to my expectations. When I use my full pedal board (8-10 pedals?), the volume drops significantly vs if I do not use effects at all. It also seems that the return side of the effect loop have some impedance issues. I get more distortion than I want with effects (and as I said low volume).
So I guess I got to try again (now is the third time) to get that recovery stage working properly. Any tips regarding this to avoid white noise? I already use shielded cables and DC for the filament.
Oh, btw. I considered negative feedback, but I dropped it to get as much sound as possible out of this tiny thing.
The loudspeaker is also extremly efficient. It is an old Norwegian SEAS symfoni 10" from the same radio I mentioned earlier. I think the efficiency might be at about 105 dB/W or something. It absolutely screams!

[Banjan73]

Under is my thoughts regarding the recovery stage. Are there any serious design flaws here that I don`t see?

[dogburn]

I usually put a switch on the negative feedback circuit, so it can be switched off to give more gain. As for the effects loop, that's beyond my level of useful understanding - if I were doing that, I'd look through the forum for successful implementations.

As for volume, I can say the AA764 Champ based amp I built ended up being surprisingly loud, and I expect it could do well with a quiet drummer or other subdued percussion.