Favorite Ways to Control Compression

[roberto]

Larry Novosibir used a rheostat to supply the screen grids of his amps, letting everyone set his own preference at the volume he is playing at that precise moment.
See here for more infos: https://ampgarage.com/forum/viewtopic.p ... 865#p64865

[pdf64]

How about grid leak biasing?
As signal level increases and grid rectification begins to come into play, the grid should bias itself more negative, ie inherent compression?
The grid leak value could be adjustable / switchable, to control sensitivity.
It probably needs to be used on an input or fx return stage, rather than a stage that may get heavily overdriven (and so could then block out).

[Tobyk]

How about grid leak biasing?
As signal level increases and grid rectification begins to come into play, the grid should bias itself more negative, ie inherent compression?
The grid leak value could be adjustable / switchable, to control sensitivity.
It probably needs to be used on an input or fx return stage, rather than a stage that may get heavily overdriven (and so could then block out).

Good addition! And how do one promote this to happen more easily – a bigger grid leak resistor, like 10-15M?

[pdf64]

It’s just an idea that came to when reading thread, I’ve not experimented with or verified the behaviour yet.
Also there don’t seem to be many valve guitar amp examples of 12AX7 in grid leak bias.
I guess that lowish value grid leaks with smallish input coupling caps would bring the behaviour into play more quickly at lower signal levels, eg 10nF 470k.
Whereas higher values would keep it at bay until higher signal levels, eg 0.1uF 10M.

[roberto]

How about grid leak biasing?

It can be tested in early stages, but it is not recommended for output tubes, because it usually causes thermal runuway and tube failure.
This is the reason why output tubes have strict maximum values for grid leak resistors, and their value is lower if the idle power (current multiplied by voltage across them) is above a certain limit.

[pdf64]

Yikes, no, I wasn’t proposing it for output valves, apologies for not making that clear.
See https://el34world.com/charts/Schematics ... _schem.pdf

[Tobyk]

It’s just an idea that came to when reading thread, I’ve not experimented with or verified the behaviour yet.
Also there don’t seem to be many valve guitar amp examples of 12AX7 in grid leak bias.
I guess that lowish value grid leaks with smallish input coupling caps would bring the behaviour into play more quickly at lower signal levels, eg 10nF 470k.
Whereas higher values would keep it at bay until higher signal levels, eg 0.1uF 10M.

I’d like to try that setup for compression. Would you say 470K would be enough for a negative input grid voltage? If so, why use a 10M at all?
Most of Valco’s smaller, early 60’s amps (model 6606 , 6150, etc) used grid leak bias with a 12ax7. They went back to grid leak when everyone else moved to cathode bias..

[pdf64]

Valco seem to like a 6M8 grid leak with 12AX7 https://el34world.com/charts/Schematics ... _s6606.pdf
My guess is that the higher the value, the larger the max input signal before compression bias shift kicks in.
I noticed the Gibson GA20 uses 470k with its instrument inputs with 12AY7.

https://el34world.com/charts/Schematics ... 0_1959.png

So yeah, try that, and maybe use a high value input cap to provide a long release time, to make it noticeable, eg film 10uF.

[dorrisant]

Valco seem to like a 6M8 grid leak with 12AX7 https://el34world.com/charts/Schematics ... _s6606.pdf
My guess is that the higher the value, the larger the max input signal before compression bias shift kicks in.
I noticed the Gibson GA20 uses 470k with its instrument inputs with 12AY7.

https://el34world.com/charts/Schematics ... 0_1959.png

So yeah, try that, and maybe use a high value input cap to provide a long release time, to make it noticeable, eg film 10uF.

I assume this is an RC time thing... What values are you suggesting for the input cap and grid leak?

[Tobyk]

He wrote that above: 10nF and 470K

[Tobyk]

Valco seem to like a 6M8 grid leak with 12AX7 https://el34world.com/charts/Schematics ... _s6606.pdf
My guess is that the higher the value, the larger the max input signal before compression bias shift kicks in.
I noticed the Gibson GA20 uses 470k with its instrument inputs with 12AY7.

https://el34world.com/charts/Schematics ... 0_1959.png

So yeah, try that, and maybe use a high value input cap to provide a long release time, to make it noticeable, eg film 10uF.

Would’t a 10 uF cap make for blocking distortion at the input stage?

[pdf64]

Yes, the idea utilises bias shift, I suppose any RC coupling to a grid will block if overdriven sufficiently.
And with a long time constant, it’ll stay blocked for a few seconds.
The design challenge is to get an effect that’s pronounced enough to be noticeable, but without going too far and the signal blocking.
And the same circuit may not accommodate both low output single coils and high output humbuckers equally well.
And kicking in a fuzz pedal with a booster after it may be a problem.

[dorrisant]

He wrote that above: 10nF and 470K

He also wrote 6M8... I was just wondering if there were any legitimate target values that he had already played with.

[roberto]

When I played with output tubes' screens (EL84 and EL34), I used a RC time of 200 ms IIRC.
The R was dependant on the current drawn by the screens: lower for EL34, higher for EL84.
I'd do the same with preamp tubes, but take care that it will affect also the gain vs frequency of the stage.

There's a trick I use in some of my Hi-Fi amps that can be adapted to get compression in guitar amps.
In Hi-Fi amps I have a small (47-150 Ohm) resistor in series with the screens dedicated to screen current sensing.
A LDR is set across it (with a current limiter resistor) and it lowers its resistance on the R side when screen current is high.
This is used as a limiter of the signal that goes into the amp to avoid the amp going into huge distortion.
The effect is that when you push the amp you hear compression instead of higher order harsh harmonics.

Here we can use an RC to modify the response of the LDR and adapt it to the attack-release time we want.

I underlined that I use it in some of my Hi-Fi amps, because my main target is always to use two stages amps with local-feedback only, and this implicitly creates low order harmonics when pushed.

[Tobyk]

When I played with output tubes' screens (EL84 and EL34), I used a RC time of 200 ms IIRC.
The R was dependant on the current drawn by the screens: lower for EL34, higher for EL84.
I'd do the same with preamp tubes, but take care that it will affect also the gain vs frequency of the stage.

There's a trick I use in some of my Hi-Fi amps that can be adapted to get compression in guitar amps.
In Hi-Fi amps I have a small (47-150 Ohm) resistor in series with the screens dedicated to screen current sensing.
A LDR is set across it (with a current limiter resistor) and it lowers its resistance on the R side when screen current is high.
This is used as a limiter of the signal that goes into the amp to avoid the amp going into huge distortion.
The effect is that when you push the amp you hear compression instead of higher order harsh harmonics.

Here we can use an RC to modify the response of the LDR and adapt it to the attack-release time we want.

I underlined that I use it in some of my Hi-Fi amps, because my main target is always to use two stages amps with local-feedback only, and this implicitly creates low order harmonics when pushed.

Interesting. What does LDR stands for please? Do you have a schematic for this?
Did anyone try, say a 470 ohm resistor between rectifier and first filter cap?