Tube circuit

[Deactivated-1]

can you explain why are there multiple capacitors on the cathode for the left side of the 6N2P tube

[T3sl4co1l]

Negative feedback, the 150p is compensation.

Tim

[001]

it is not more when some sort of fixed equaliser, isn`t it?
 

[T3sl4co1l]

Weird... they put the tone control, inside the feedback loop?  But most of the feedback is also bypassed away by the 15.0 cap.  So it really shouldn't do much of anything.  But then why connect it at all?

Maybe they just needed to use up more capacitors...

Anyway, cutting out the two extra(?) pots and the tone circuit, so the 6N2P left plate is cap-coupled to the right grid, is enough to do that.  Then probably cutting the 15u cap and adjusting feedback values, would get a pretty ordinary amp with flat response and modest overall gain.

Tim

[T3sl4co1l]

Short out the 150p, and there should be a, 470k or so from right grid to GND.  Otherwise yep, that's it.

Note that without feedback, it will have quite a bit of distortion at full output.  Gain may be pretty high as well.  Better perhaps for a guitar amp than general audio, but you'll figure things out as you use it I suppose.

Russian logic sometimes can't be explained. for example, this radio has 2x8inch speakers and 1 2x4inch one but it has only 1 6p14p tube. I think they were really optimistic about the power output of this circuit!

Yuo see Ivan, more speakers make moar sound, puny valve into make big!

Tim

[floobydust]

There's two feedback loops - one for tone controls/EQ and another for the amplifier.
I would keep the original's overall loop feedback, from loudspeaker output to cathode of 6N2. That 1k, 150pF, 20k, 351R - or some network like it.

The tone control/EQ feedback is a second 150pF cap at terminal "16" which I would delete. Just as Tim says, 6N2 pin 1 to cap to pin 7 with 470k to GND.

Russian circuits are very strange to figure out.
What's with the tweeter return being the bypassed cathode of the output tube? Ahhh so you don't need a non-polarized electrolytic cap in the crossover network.

[001]

Russian logic sometimes can't be explained. for example, this radio has 2x8inch speakers and 1 2x4inch one but it has only 1 6p14p tube. I think they were really optimistic about the power output of this circuit!

Oh boy
It is not for power reason 
In my childhood it was a sort of "surrounding" for "premium" audio

[floobydust]

Is this a capacitor?? In the schematic this is labelled as a cap

Yes, looks like 150pF +/-10% tubular capacitor

You can also add a potentiometer between the two 6N2 stages, if you want to overdrive the input tube for some crunch. That would look like a 250-500k pot where you have the 300k at pin 7, wiper goes to pin 7. Most amplifiers bypass the 2k cathode resistor on the input tube to keep gain high.

Older amps used several high efficiency, low power loudspeakers as an alternative to the cost of a higher power amplifier, to get the same SPL. If you play a gig in a big room, you will see it works just as well.

[floobydust]

The amp might need less feedback or the presence control is a bit too much.

Moving the volume control to after the first tube would upset the feedback loop here.
You'd have to change how it's done - make the feedback come into the second tube's cathode and flip the output transformer's secondary wires, to keep it the correct phase.

This is commonly how it's done on Fender, Vox, Marshall etc. where the guitar goes right into the first tube- instead of after a volume control, because you already have it in the guitar. So you can overdrive the first tube but keep the amp's volume low.