Automatic Gain Control or Compressor Limiter

[jewishjay]

I'm having a hard time finding much info about incorperating a simple comp control or AGC in an all tube P2P design. I found one design using triodes and one idea using a pentode. This would be for a bass amp. I just want some pre amp compression with a knob that goes from "no squish" to "too much squish."

Any links, schematics, suggestions?

[HotBluePlates]

I'm having a hard time finding much info about incorperating a simple comp control or AGC in an all tube P2P design. ... This would be for a bass amp. I just want some pre amp compression with a knob that goes from "no squish" to "too much squish."

There's a whole lotta good compressor pedals.

I mainly say that because the Rs and Cs in the compressor circuit affect the "attack" and "release" times of the compressor circuit, of which you may want control.  Additionally, the gain-control mechanism used imparts some "flavor" to the compression.

[jewishjay]

Yes, pedals exist... but how would one impliment AGC in a remote cutoff pentode stage? I basically want local NFB that varies with input signal. How?

[HotBluePlates]

... how would one impliment AGC in a remote cutoff pentode stage? ...

You posted 2 examples already:  replace a regular preamp gain stage with one of the two example circuits.

A challenge is getting the circuit to be appropriate for the applied signal-level.


Your pentode example uses a subminiature 5636 tube, rectifies the screen-voltage change (??, was it supposed to plate-voltage-change?), and applied the resulting negative voltage to the suppressor to control tube-current.  5636 are uncommon, but you can find them.

I see the 6SL7 example was swiped here.  The article they linked from that page shows how to implement it in a circuit (in Figure 2).

   - V2 (6SC7, though it could just be another 6SL7) is a push-pull output, but you could see it as a "phase inverter."
   - Both the "driven side" (attached to R5) and the "non-driven side" (attached to R10, R11) have plate-outputs connected to the interstage transformer.  C6 and C10 are coupling caps connecting V2's output to the interstage transformer.
   - R16-R18 is a pad on the primary of the interstage transformer.
   - V3 is the single-6SL7 you showed in your original post, but some of the particulars are changed.
      - Here, V3 is more-or-less the rectifier diodes that (along with C7-C9 and R10-R15) alter the bias & amplification of V2.


Stuff gets complicated fast... Sure you want to build your own amp-AGC instead of buy a pedal?

[kagliostro]

Don't know if this can be of your interest

In my archive I,'ve this

It can act as compressor or expander, inverting diode D (using a switch to have both functions ?)

ECH84 = 6JX8

Franco

BTW: I remembered that the circuit was planned for a Bass amp (and it comes from east Europe)

[jewishjay]

I was really just looking for information.  Education.  Has it been done in the past? How? Any schematics or examples? Links? Books? Previous threads?

[shooter]

yep
and the market reacts, driven by cash from guitarist the world over;

There's a whole lotta good compressor pedals.

[Willabe]

I was really just looking for information.  Education.  Has it been done in the past? How? Any schematics or examples? Links? Books? Previous threads?

Lab series amps had a built in compressor but it was solid state. Ronnie Montrose had an instrumental hit with a remake of Town Without Pity. He used the amps compressor on the song for long sustained soaring notes.

Guitar players use the sag from the amps power supply as a built in tube compressor. 

[separateness]

I don't know much but I have looked into old tube compressor circuits and most of them which are useful for audio compression (the way we use them in music) use differential pair signal topology, often splitting the single-ended input signal with a tapped transformer and making it single ended again with another at the output. I think this is because a compressor which acts fast enough to be useful for audio can cause audible thumps if the control signal isn't phase cancelled. One exception is the optical LA-2A compressor.  Tube audio compressors are big expensive pieces of equipment but I wouldn't discourage any one from exploring.

[Lauri]

Gibson had a bass amp GA100 that had a varimu compressor circuit. In RDH4 there's a simple single ended expander circuit that's easy to convert to a compressor. But if you want local NFB that varies with input signal using an optocoupler would be the easiest way to do it.

[kagliostro]

Here a well documented project

https://www.audiovalvole.it/progetto_preamplificatore_compressore_chitarra_basso_elettrico.html

Franco

[Merlin]

I'm having a hard time finding much info about incorperating a simple comp control or AGC in an all tube P2P design. I found one design using triodes and one idea using a pentode. This would be for a bass amp. I just want some pre amp compression with a knob that goes from "no squish" to "too much squish."

Any links, schematics, suggestions?

AGC works the same way as bias-wiggler tremolo; the functional principles are the same, and so are the problems/solutions to thump. If you understand bias-wiggler tremolo, all you really have to do is substitute the trem oscillator with an envelope detector instead (e.g. diodes + smoothing cap). There's a lot of possible way you might implement this depending on the rest of the circuit you're building.
The other option is the old light-bulb-in-series-with-speaker method.