GM 28 blocking distortion

[Leevi]

I'm trying to damp the volume by adding a resistor across (before) the power tube grid resistors.
It looks like a cross line master volume but with fixed value. If I select a smaller resistor like 22K or 33K
I get blocking distortion and wonder what is special in this particular case causing that?


See the schematic attached.


/Leevi

[Tony Bones]

I don't have an answer, but I'm curious, where are you putting the shunt resistor relative to the 0.1 caps? On the 6SN7 side or the 6L6 side?

I'm going to guess between the 6L6 grids. What happens if you put it between the 6SN7 plates?

[Leevi]

Between 6L6s. I tried to put it between 6SN7 grids as well but the sound became very
distorted.
/Leevi

[Tony Bones]

Between the 6SN7 driver grids? Or did you mean to say plates. I was thinking just move it to the other side of the 0.1uF coupling caps. Those 6SN7's can deliver a lot more current than a 12AX7.

Probably a crazy idea anyway. I have a lot more of those than good ideas.

I have to say, that's an interesting amp. The 6L6 screen supply has a tuned choke. It looks like what I think is called a Meisner circuit. Can't remember where I've seen it discussed. Maybe an ancient book called Radio Physics Course by Girardi, or something like that. I'm too lazy to look right now. Any tech-heads that like reading old electronics text books should get a copy though. It's a truly interesting approach to electronics, before a 'standard method' became the norm. Get a copy. Probably only a few bucks from Abe Books. Thank me later.

[Leevi]

Between the 6SN7 driver grids?

Between grids.

I have to say, that's an interesting amp.

Yes it is, and the implementation is very well done, a quality amp that
has been used among others in cinema.


/Leevi

[PRR]

> a resistor across (before) the power tube grid resistors.

That is inside the (unconventional) NFB loop. Gain won't drop much; instead the power stage strains.

I don't have a good way to "damp" this design.

[Tony Bones]

PRR, that's a very good point, as usual.

[Leevi]

I don't have a good way to "damp" this design.

I did it in preamp. Thanks for replies.


/Leevi

[DummyLoad]

there is a .02uF coupling cap on 6SN7 from tone shaping recovery amp and concertina. that value is much too high and likely the cause of the blocking distortion of the concertina. install a 330k-470k grid stopper to the concertina grid and replace that .02uF with a 1-2nF cap. the .1 coupling caps are also suspect - 22-33nF there would be more than adequate.



--pete

[Leevi]

there is a .02uF coupling cap on 6SN7 from tone shaping recovery amp and concertina. that value is much too high and likely the cause of the blocking distortion of the concertina. install a 330k-470k grid stopper to the concertina grid and replace that .02uF with a 1-2nF cap. the .1 coupling caps are also suspect - 22-33nF there would be more than adequate.

Might have been a good idea but unfortunately I cannot test it since the amp is no longer at my place.


/Leevi