Brighten up old russian tube amp

Hey All,

hoping someone can help me with cap values to brighten this output stage up a bit, I tried fiddling with the feedback loop, removing C9, but didn't really change much. Also tried changing C10/C11 to .033uf which helped a little but seemed to change the bottom end in a way I didn't like.

Any other thoughts? Not informed enough to know what C12 and C13 might be doing to the circuit....  also the device coming off point "12" of the transformer and the associated resistor r23 is a Neon bulb/level indicator circuit.

The last suspect is it looks like this amp is designed for a 30ohm speaker.... currently driving an 8ohm speaker. Not sure how much that would affect the response.  Can a stick a fat resistor across  7 and 5 of the output transformer to correct for an 8 ohm load? 

I think R14 coming back from the OT is going to produce quite a bit of negative feedback which is going to negate / resist whatever changes you try to make in between.

You can use that feedback to make any number of very effective filters no problem. At least I think you can. I'm not a tube guy but it seems to me the principles are the same.

Hmmm, The thought had crossed my mind to implement a presence circuit similar to a 64 bassman.... not sure if that would be effective

TubeMonkey said:

The last suspect is it looks like this amp is designed for a 30ohm speaker.... currently driving an 8ohm speaker. Not sure how much that would affect the response. 

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Don't guitar players use this to their advantage....... Mis-matching impedances for different tones????

TubeMonkey said:

hoping someone can help me with cap values to brighten this output stage up a bit, I tried fiddling with the feedback loop, removing C9, but didn't really change much. Also tried changing C10/C11 to .033uf which helped a little but seemed to change the bottom end in a way I didn't like.

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C10/11 control the LF response; changing them won't do anything to the HF response.

Not informed enough to know what C12 and C13 might be doing to the circuit.... 

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C12boosts the HF response of the second half ot the Phase Inverter. No use in fiddling with it, it would only introduce distortion.
C13 is the decoupling cap for the output stage's cathodes. If it's an electrolytic, after all these years, it is probably completely dry, but if it's paper-in-oil, it may still be good. Just put a modern electrolytic 22uF 25V across it (negative towards ground) and you may or may not hear a difference. If yes, leave the new cap permanently connected.

The last suspect is it looks like this amp is designed for a 30ohm speaker.... currently driving an 8ohm speaker. Not sure how much that would affect the response. 

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Both LF and HF response would probably suffer. Can you hear the amp straining?

Can a stick a fat resistor across  7 and 5 of the output transformer to correct for an 8 ohm load?

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That would only decrease the load, which is the contrary of what you're aiming at. Putting a 22R resistor in series with the 8 ohms speaker would provide the correct load to the amp, but you would waste 75% of the power in the resistor.
Have you tried connecting the speaker between 5&6 or 6&7?

abbey road d enfer said:

Both LF and HF response would probably suffer. Can you hear the amp straining?

That would only decrease the load, which is the contrary of what you're aiming at. Putting a 22R resistor in series with the 8 ohms speaker would provide the correct load to the amp, but you would waste 75% of the power in the resistor.
Have you tried connecting the speaker between 5&6 or 6&7?

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That amp doesn't sound like it's straining at all... it actually sounds quite good, but needs about 4db up from 2khz up for it to sound "right" to me. 

Yes wasting power on the resistor is not ideal....

I haven't tried between 5&6 or 6&7 yet... I will do that shortly.

Maybe a better approach is to brighten up the preamp stage? 

I'll post a pic of that shortly as well.

Here's the front half of the amp...

I've removed anything before the the Grid of the preamp tube and am direct injecting the guitar signal into it.  do i need a resistor here to ground to match the pickup to tube impedance?

also i've disconnected the feedback/tone loop for now...  thinking it might be easy to implement a tone control a la 5e3?

I would just put a 1M to ground upstream from C1 and plug the guitar in there. Then load with a cheap 12" alnico 32 ohm speaker from eby (preferably something with a really crappy high frequency response). Add a presence control by adding a pot and cap to the net at R14 and grid of the PI. There are many ways to do this but you basically want to keep the 3k9 load on the xformer but adjust the level of feedback reaching the grid but using a cap so that DC doesn't go through the wiper (or it will make scratchy noises) and try smaller caps (like 100n maybe) so that you're only boosting high frequencies. You'll need to get rid of C9 though. Not sure what that cap is even doing honestly. It's probably balancing something perfectly because this is clearly a hi-fi amp. So for guitar you don't need it (and a presence control won't work with it).

CORRECTION: The 3k9 is not loading the transformer because it's connected directly to the grid. So you want to actually remove that (and the big cap) and use something much larger like 47k. Then a 10k from grid to ground with the wiper to ground through a 100n cap. Then also add 470p across the 47k to stablize things or the whole thing will probably howl.

Note: My personal feeling is that guitar amps are sort of designed to operate on the threshold of stability. So you want to push the OT a little and the power tubes a little and the PI a little and so on. Push it. Also the speaker is super important. Must be 12" and needs to be pushed a little. So compute wattage and find a speaker that you can push to the edge without damaging it.

Alright stuck the 5e3 tone knob in there and ditched c9 and it's just what the doctor ordered. I may still play with the presence a little but the 3.9k resistor seems alright.... I may stick a pot in there temporarily, dial it in how I like it and throw in a fixed resistor.

Only thing left is that there is a bit more ground hum than i'd like so need to chase that a little.

On the subject of "Edge of stability"  while i was originally playing with new caps I stuck in a 22uf cap right off the rectifier tube which raised the RMS and gave it an extra 100v dc...  giving 450v instead of 365v.  Needless to say most of the caps at the top of the supply were only rated for 400v as well as Max V for the tube at 400v so it crashed about three minutes into operation, but holy shit did it sound glorious! Super touch sensitive! Luckily 6l6's are drop in replacements for the russian tubes and are rated at 500v max so I may up those cap values and try 6l6's and see if I like it better.... I could even stick a fat switch in there which flips that cap in and out of the circuit so I can drop either set of tubes in at any given time.....

TubeMonkey said:

That amp doesn't sound like it's straining at all... it actually sounds quite good, but needs about 4db up from 2khz up for it to sound "right" to me. 

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Have you checked anode currents balance of output tubes?

what's at the connection 12 on the output transformer?  Is that a neon lamp?  I'm curious if that is some sort of limiting function?

Have you replaced the original electrolytic capacitors yet?  It sounds like the old ones are dragging the voltages down very significantly and inducing hum.  Unless you want serious problems like defective transformers replace these first... 

Is the 1M feedback adjustment on the input pentode adjustable from the outside? 

Can you post the entire schematic?  It is a very interesting piece!

bibi said:

what's at the connection 12 on the output transformer?  Is that a neon lamp?  I'm curious if that is some sort of limiting function?

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That has already been answered in the OP: "the device coming off point "12" of the transformer and the associated resistor r23 is a Neon bulb/level indicator circuit. "