Federal AM-864 - 6SQ7 Questions

[GussyLoveridge]

Hi Folks -

I'm trying to figure out the gain reduction circuit in the Federal AM-864. I have thoroughly read through the manual (which was quite impressive). I get the broad strokes about how the thing operates - but I don't fully understand how the duplex diode section works exactly and only partly understand the attack and release times. I have tried searching a few times through the forum - but haven't been able to find exactly what I'm looking for. Can anyone point me in the right direction or suggest a resource?

Thanks so very much,

Gus.

 

[GussyLoveridge]

So when the duplex diodes begin conducting - R11 and C1 set the attack time - R11 is a current limiting resistor and sets the charging time in conjunction with C1's c capacity?

And then when the diodes stop conducting - R1 provide a discharge path for C1 which set the release time? And the voltage across C1 is what alters the bias of V1 & 2?

The stuff in quotes is from the circuit description in the manual.

"A small portion of the signal from V2 is fed to the triode section of V4, where it is amplified." - Yes, I understand.

The amplified single from the output of the triode section of V4 is capacitance coupled to the self-contained diodes of V4. - This is the part I don't really understand. Signal goes in at pins 4 & 5 on the diodes, but I don't follow how it comes off the cathode? and makes it back through R11 to C1 and R1?

 

[Piedwagtail]

V1 and V2 are remote cutoff valves with a gradual fall off in transconductance determined by the negative bias of the grid.
The usual bias is -1.5V or so.
When we drive that bias lower into the weeds of the plate characteristic chart, to say -10V, current through our valves becomes pathetic and voltage gain seen across the plate resistors R3/4 is reduced.

The design sloshes the bias around by playing with the diode plates relative to the cathode in V4. When above the 2.5V set up by R8, they conduct and pull the ungrounded end of C1 negative. See excerpt.

......
Slightly funny in retrospect, that I now see how quirky the V4 set up is. Trimming R8 and not getting out of hand with R10 - all detailed in the manual.
I was sure it wasn't working and abandoned the build a decade ago, especially when it sucked out gain with R10.

R