are you saying you can simply add the circuit (the top 2) to an existing tube, like v1
Yes.
You just have to add the side chain circuit.
The top 2 circuits will work if the input signal is big enough to produce enough signal on the output to make a sufficient amount of control voltage. Probably won't work well as a first stage of a guitar amp. Also cathode resistor should be kept small or otherwise changing cathode voltage will counteract control voltage on the grid.
These are good points.
or will you have to add a gain stage?
You don't need a dedicated gain stage all you need is to tap a gain stages output for a sample ac signal and rectify it for a control -dcv.
It's all about the -dc control voltage. It's about cutting signal not increasing (gain) signal.
The way I understand it is;
The -dc control voltage is used to re-bias the tubes grid with respect to it's cathode but only on the peaks. The more the tube puts out the more -dcv is fed back the input, which changes the tubes bias and limits it's output gain. Because it's a dcv I don't think you have to worry about the phase. So you can take the sample acv from, say the 2nd gain stage and feed it back to the 1st (input) stages grid. You can take the sample signal from the output power amp and feed it back to the 1st stage which would give you a whole amp loop. Depending where you take the sample and where/what stage you feed it back to will change the R and pot values in the side chain. Also including more stages in the loop may make the amp unstable just like with a normal NFB loop and may not be needed to get a good compression effect ?
The threshold knob sets/controls when the diodes (SS or tube) turn on. Until they turn on NO -dcv is fed back to the input grid, so no compression.
My understanding of this is that it's a type of negative feed back loop (NFB) but with a control that you set for when it kicks in. That way you set it to only kick in on the peaks so it limits the output signal which is compression. You can also add an attack and release control.
A regular NFB loop is on constantly and is set to the amount (of looses) the designer chooses. Like on the output stage OT secondary taken back to the PI.
Got any ideas of what would be a good start on component values?
Look at the schemos in Lauri's 1st post and also at Altec 436C, it has threshold, "C" has attack and release controls.
I found this on adding an attack control to the Altec 436C.
my guess is that you make the resistor R12 (33k) right after 6AL5 tube variable. This controls the time how quickly the capacitor c4 charges - just like the pot P2 controls its discharge (release).Is this MOD even close to the compression of these?
I have not built this into an amp, but I do trust that Kevin knows what he's talking about. I also trust what PRR has to say and he sited some reservation about this side chain, so?
Is it close? Not sure it's quite the same thing (apples to oranges?) but I do think for a guitar amp it might/will work fine.
I do have 1 of his sustain kits (small pcb with parts) that one of these days I'll put in something.
There's more info on his London Power web site that's worth reading.
Kevin brought up in 1 of the TUT books on sustain/compression that a tube amp pushed into clipping
will sustain/compress by itself but sometimes you want/need a
clean tone to sustain/compress. He said this is a way to do that.
This type of side chain compression will work on a high bias amp like a SE Champ or Vox AC30 where you really can't get power supply sag because it's already sagged.
Brad
Topic: Simple compressor circuit for guitar amps (Read 14006 times)