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Anybody built this compressor?

5/4/2000 10:36 PM
Reid Kneeland
Anybody built this compressor?
Here's a schem for what looks like a cool little tube compressor. Has anyone here built this or one like it? My next big project is a bass amp with a tube preamp and MOSFET (hi, RG!) power amp. I'd like to put this in. Any comments? Also, what do you suppose would be a good plate voltage?  
Thanks. Here's the link:  
5/4/2000 10:56 PM
ken gilbert
you know, reid, i've looked at this compressor many times, and i'll be damned if i can figure out just how the hell it works. where's the first tube's second half's grid connected? is it shorted to the girst half's grid? it can't be left disconnected!  
it looks as though the second half of the second tube works as a variable shunt impedance to ground, which provides the compression/limiting. have you got a handle on what's going on here?  
i say build it so you can tell us how it works ;-)  
5/4/2000 11:45 PM
Reid Kneeland

I don't completely understand it either, but the second section of the first tube is being used as a diode (the grid is floating). It's rectifying the signal, which comes in on the plate, to supply the control (envelope) voltage for the second tube. Two of the four sections (6,7,8 in each tube) are just cathode follower buffers, and the remaining output section seems to be mixing the rectified signal with the input.  
I am indeed planning to cobble one together just to play with it and see what it does. I'm especially interested in what the two pots do - the both seem to be amplitude controls for the control voltage, but why two?  
5/4/2000 11:57 PM
I think the final stage is being used as a variable  
resistor to load the signal-- the current is set by  
varying the grid voltage, which goes with the  
'envelope' signal! maybe the reason there are  
two pots is that one of them (the one with a  
cap across it) sets the attack/decay time.  
Fascinating little circuit, I couldn't  
figure it out til you pointed out the  
'diode' action of one of the sections.  
All the other tube compressor circuits I've  
seen have had unobtainable LDR's or  
transformers; this thing is actually buildable!
5/5/2000 12:05 AM
Reid Kneeland

"maybe the reason there are  
two pots is that one of them (the one with a  
cap across it) sets the attack/decay time."
Either that or the ratio... as I said, I'm still scratching my head over this thing. It certainly is weird seeing a cathode cap on a CF!  
5/5/2000 12:13 AM
if that pot really is 1M (I can't read it too  
clearly) then 0.01uF times 1M gives 10 milliseconds,  
so I think you just have a DC level representing  
the 'envelope' at that point....
5/5/2000 12:13 AM
Chris Barrow
That's not really a cathode follower in this case. It's more akin to a power supply filter stage.  
Chris Barrow

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