6BA6 compressor (this is totally a proof of concept only)

[Blue Jinn]

OK,

So I threw this idea up a few places. I found this side chain in an old ARRL handbook. Seemed rather intriguing. Uses a 6AL5, but wants the control voltage on the suppressor grid of a pentode. It's for ham use, so intended for a limited dynamic range. So I sketched something out, whose flaws were immediatly identified.l OK so another crack at it. Here is the second draft schematic. The resistor values on the pentode are all placeholders, no calculations done. The side chain is from the book except larger value pots replace fixed resistors. The triode values are from Zugsters triode calc site.

It's just a sketch of an idea from a tube novice like myself. But it is also a good learning tool for me, and things like this force me to learn the math! (even if filled with design flaws...)

 

[XploZiveToyz]

I might as well be looking at a road map! LOL! We're all wired differently I guess. I'll be glad to test it for you though! LOL!

 

[drtechno]

well, yes while you can generate an agc and feed it into a supressor gid. The range of control would be small. however, you can get a little bit more out of it with gain at the detector (like your example in the drawing. even going 1:4 on the detector could help). A varimu works on varing the gain characteristic. For audio use, it is nice to have a big AGC range. This is why the bias is varied. noise floor characteristics would be the second consideration, but the use of fully balanced line stage helps reject the common mode noise.

varing the bias can be done at the grid (common. varing grid bias). another way would be using a constant current source in the cathode circuit, then varing the constant current source's bias.

 

[ranjam]

I'm not that good, but it looks like a lot of gain, and some noise will come along for the ride. You go through a 1:2 transformer on to a triode gain stage with a gain of 100K/1K5 (depending on the mu of the tube). Now again, I'm only smart enough to be dangerous, but the 6BA6 is a remote cut-off pentode, which should be quite desirable for a compressor. Oh, now I see; it's a 12AU7. Handles more current than a 12AX7, less gain, and should match an easy to find transformer impedance 'better'. But there's still a lot of gain, and some noise would be inevitable.

 

[drtechno]

yea, but the detector circuit is not quite there imho. it needs to be at a higher impedance than the tube for loading... I'll get back to this one with a revised detector circuit.

 

[Blue Jinn]

THanks. I had some mistakes in that draft, and some suggestions from diyAudio as well. The 6GY6 was suggested as a better tube choice. (That is a sharp cutoff pentode, but apparenlty has greater range of control with g3.)

The side chain RC network is misdrawn. Here is a revision. AFAIK, keeping the B+ on the low side and the screen close to B+ (but constant) should lower the gain ? I haven't calculated the screen resistors yet.

Also the 1:2 in the makeup gain stage was just a swag although I am not sure how much gain will be needed there. If this is good enough to prototype (!) I'd probably go for the 1:1 cheapy auto isolation transformers first there and at the input with an ersatz center tap like in PRR's. I wouldn't put teh B+ on the pentode to triode interstage I left the side chain interstage in the drawing at 1:2, but a 1:4 isn't any more expensive. So I'll redraw it with that in mind, and try to do some calc on the screen resistors. I haven't checked the specs on the tubes, (or got my head around what exactly is happening there) but with a 1:4 interstage, wouldn't that be sufficient impedance for that 12AU7 in the side chain?

 

[drtechno]

THanks. I had some mistakes in that draft, and some suggestions from diyAudio as well. The 6GY6 was suggested as a better tube choice. (That is a sharp cutoff pentode, but apparenlty has greater range of control with g3.)

The side chain RC network is misdrawn. Here is a revision. AFAIK, keeping the B+ on the low side and the screen close to B+ (but constant) should lower the gain ? I haven't calculated the screen resistors yet.

Also the 1:2 in the makeup gain stage was just a swag although I am not sure how much gain will be needed there. If this is good enough to prototype (!) I'd probably go for the 1:1 cheapy auto isolation transformers first there and at the input with an ersatz center tap like in PRR's. I wouldn't put teh B+ on the pentode to triode interstage I left the side chain interstage in the drawing at 1:2, but a 1:4 isn't any more expensive. So I'll redraw it with that in mind, and try to do some calc on the screen resistors. I haven't checked the specs on the tubes, (or got my head around what exactly is happening there) but with a 1:4 interstage, wouldn't that be sufficient impedance for that 12AU7 in the side chain?

((Rp of detector || Rp of detector + series res) || LVL Adj res value ) + Z of the transformer.

 

[drtechno]

but the rectification voltage would have to be large to move the master screen voltage circuit quickly.

that is why its more common to see the grid bias or cathode current manupulation

but this circuit does remind me of another circuit called a gates level devil

 

[Blue Jinn]

but the rectification voltage would have to be large to move the master screen voltage circuit quickly.

that is why its more common to see the grid bias or cathode current manupulation

but this circuit does remind me of another circuit called a gates level devil

I'll do some of that math when I get a chance. As to the Gates' the devil seems really complex, the sta-level uses the control grid and appears to have a different control voltage scheme, although the general same idea.

I have seen a couple of circuits that use the suppressor, and the DIYaudio guys suggest it would work. I don[t know if that would be better or worse or about the same from a DC bias point of view. If the voltage change is slower, wouldn't that give it a more exaggerated "soft knee" feel? OTOH, if I use e.g. a 1:4 in the side chain to increase the CV wouldn't that mitigate?