Federal Compressor Function Question

[schmidlin]

I have been digesting this thing for 2 years now and have one haunting question:
What does C6 do??  (off the 6SQ7)
http://www.waltzingbear.com/Schematics/Misc/AM-864.htm
...just can't figure it out...everything else makes perfect sense.

Thanks in advance.

 

[Johan]

I am just here trying to learn, as you are. but as I see it...it decouples the output of the sidechain amp ( the triode part of 6sq7 ) from the rectifier ( diode parts of 6sq7, pin 4 and 5)
without it the strain on the triode would probably be to big and R7( anode resistor) might get too hot for comfort.
j

 

[gary o]

Hiya always interested in vari mu limiters inc the federal I dont know loads of eletcronics theory.....sadly, I would a thought it blocks DC going to input tubes grids as you only want control voltage going there......if thats right why have it there at all?.....I understand the 6SQ7 is doing 2 jobs amplifying then rectifling (half wave only) to make control voltage

Perhaps someone will come along and give us a much better explanation ......

 

[Johan]

......if thats right why have it there at all?.....

becouse the controll voltage is a negative voltage and the cap blocks the positive DC voltage of the triode output. the anode probably idles somewhere around 120 volt positive. shunting that to ground through the diodes would possibly fry the 27k resistor from anode to PSU. with the cap, you have the signal at ground level, then rectified halfwave to produce a negative controll voltage.
j

 

[gary o]

Cool this all sounds good...... I myself am having to learn my electronic terminology as I go, but thats cool this all helps....thanks

 

[PRR]

{sigh...} Didn't anyone work on tube radios?

The rectifier and cap (C6) make DC.

It looks funny because we want a negative voltage yet it is VERY convenient to use a grounded cathode. (In this specific case: a small solid reference voltage on R8.) So the parts are "reversed". The result is the same.

Drive V4 triode so plate rises. This pulls up on C6. However the other end of C6 can't rise much above ground (or R8 tap) because of the diode. Instead C6 charges-up.

Now drive V4 triode so plate falls. This pushes down on C6. The other end of C6 can fall as much below ground (or R8 tap) as it wants. You got your negative voltage nearly equal to your positive peak.

 

[emrr]

Thanks PRR.  No, we're all too young.  Or stupid.  I don't know which.  Probably both! 

 

[shabtek]

A little of both...no a lot of both compared to PRR
thanks again PRR

 

[Johan]

Thanks PRR.  No, we're all too young.   Or stupid.  I don't know which.  Probably both!   

+1
johan

 

[gary o]

I once saw a tube radio.........

 

[schmidlin]

As usual, many thanks PRR.  By the looks of it, many of us feel like noobs standing next to your insight.

Still trying to grasp it.  I assume you mean by "rising and falling" the plate current.  "Pushing" has me scratching my head.  I understand current flow etc, but all I see is C6 offering AC signal to the side chain 6SK7 bias voltage.

By your last sentence, it seems there is some sort of balancing act going on here.  I ask about this mainly for trouble-shooting purposes.  Perhaps an explanation about what would happen if you varied the value of C6 would help.

No reply necessary of course, it may just be over my (non old tube radio) head.

Thanks again.

 

[abbey road d enfer]

I understand current flow etc, but all I see is C6 offering AC signal to the side chain 6SK7 bias voltage.

C6 is part of a single-wave rectifier (see attached schemo). The implementation is different than usual, but it's still a rectifier. You could say that instead of passing the negative half-wave, it shunts to ground the positive. Very crude. It works (?) because the time-constants are quite long, so ripple is eliminated.

Perhaps an explanation about what would happen if you varied the value of C6 would help.

Not much would happen; decreasing its value would just high-pass the side-chain signal.

 

[schmidlin]

Thanks abbey: I can say that makes sense to me.  Its a way of stripping off the positive voltages in the chain.  Shunting was on my radar but I dismissed it b/c I was looking for the usual large PS filter cap to deal with it, and this has to first wrangle with the 27K.

Thanks again, all.