Russian Amp

[J Rindt]

It has been a year or more since i worked on an amp. Trying to remember how things work...looking at Merlin's book.
My friends complaint is too much "brightness".
C8 is your typical "bright cap" isn't it.? If i remove that, it should help a bit.
I am thinking C3 is similar, but i am trying to figure out how it plays with C4 and its pot RV1.

1. Will removing C8 give some bright relief.?
2. Is there a link/explanation for what is going on with C3-C4-RV1.?
Thank You

https://www.yerasov.ru/sites/default/files/schemas/hammer_head_0.pdf

[shooter]

I would try removing C8, I would probably change C2 to something like 2.2uF, or maybe 4.7, also c13 to .1uF

I think the C3/4 is a treble bypass similar to Marshall , might try pulling C3.
 I've found during this part,(amp tuning), having the musician with guitar n you with soldering pencil saves lots of gas n trips.

[2deaf]

C4 is the only thing that is a bright cap.  Everything else is there to achieve a "Marshall sound".  If you remove or change anything other than C4, you will lose that sound. 

R11/C8/R12 is the classic figure that is the hallmark of the Marshall sound.  R5/C3/RV1 is a variation on that theme.  C4 is a brightness cap that comes into play when RV1 isn't turned clear up.  C2 is matched to R5/C3/RV1 to complete the Marshall sound.  This amp is intended to be played at or near full blast.

You can shear some highs off by putting a capacitor(s) across one or more of the plate resistors or from plate to ground.

[pompeiisneaks]

Also a simple mod would be to leave the amp as is, and put in a switch for that cap C8, so you can remove it with the switch... just put a switch before or after it in the loop and you're golden.

~Phil

[J Rindt]

Oh Right.....OK.
I am real rusty (not that i was so good anyway) at this. C8 is part of a static Voltage Divider.....which YES, could be made switchable, not a bad idea.
C4 would be the classic "bright cap" around a Pot, and loses its effect as the "gain" is increased.?
Thanks Again

[2deaf]

C4 would be the classic "bright cap" around a Pot, and loses its effect as the "gain" is increased.?

Correct.  If you set RV1 so that there were 500K above the wiper and 500K below the wiper, there would be about a 9dB boost at high frequencies.  The R11/C8/R12 figure gives about a 3dB boost at high frequencies.  So that bright cap. setup is way more potent at boosting highs.  If you put a switch anywhere, I would put it on the bright cap.

[J Rindt]

10-4.....Thanks.
I will present him with these options. :)

[J Rindt]

Actually...let me ask another question.
V1b, or V1.2  i guess they call it.
That is just straight voltage amplification right.?
They bias it it kind if "high" with a 10k at R7.....Why is there no bypass cap around R7.?
I know there must be a reason, but it is beyond my knowledge.
Thank You

[PRR]

> They bias it it kind if "high" with a 10k at R7

Exact copy of many Marshall second stages. Not a lot of gain. Sharp clipping.

Same for the 0.68U on the first stage. Stolen from Marshall.

3rd/4th stages Marshall stole from Fender Bassman.

There's nothing exotic here. There are a lot of bright-up caps you can reduce or eliminate.

[J Rindt]

OK...so not right or wrong. Just a matter of the sound they were going for.....or the circuit(s) they were Cloning/Using.
Thanks

[2deaf]

They bias it it kind if "high" with a 10k at R7.....Why is there no bypass cap around R7.?

A bypass cap. on a large cathode resistor will charge more positive with a large signal due to the radical asymmetry of the signal.  It will take longer for the cap. to discharge with a larger cathode resistor when the input signal decreases.  The tube will be pushed closer to cut-off while the cap. is more positively charged.

[J Rindt]

Its "funny' ...this stuff all seems  obvious when you guys explain it to me, but i do not see it myself when looking at the schem.
Anyway.....that certainly makes sense.
Thank You

[HotBluePlates]

... Exact copy of many Marshall second stages. ... first stage. Stolen from Marshall.

3rd/4th stages Marshall stole from Fender Bassman. ...

... the sound they were going for.....or the circuit(s) they were Cloning/Using. ...

From a quick glance this appears to be a copy of the Marshall JCM800, right down to the use of the two input jacks to select whether there is an additional preamp gain stage.

... My friends complaint is too much "brightness". ...

You really ought to see/listen to your friend playing the amp and showing you what he doesn't like.

Blackface Fender amps have a pleasing full-frequency sound which is warm and full at low-ish volumes.  But they can get flubby or farty when you dime the volume.

Marshall stole the tweed Bassman circuit, but didn't stay there.  Over time and as players pushed for more distortion, they sheared away bass in the preamp to keep the distorted sound from flubbing out.

If your friend is complaining about brightness, but mainly when turning the gain control low for a clean sound, then 2deaf is spot-on pointing at C4.  Bright caps like this sound best (for my taste) between min/max volume and closer to the latter for just a "little more bright".  They have no effect when that pot is dimed, and will be excessively bright when the pot is near minimum.

But you also have to consider that this amp was built almost exclusively for a distortion sound; everything is tilted to optimize the amp for that purpose.  He may need an EQ pedal or a different amp for a really good clean sound...

[J Rindt]

HBP -
Good points...i will run that strategy past him, and get a better feel for how he is Using/Pushing this amp.
I also have no idea what speaker(s) he is using. The guy is quite a ways, a way from me, so there is a lot of Back-And-Forth between us.
Thank You