GA 200 Compression - How, What & Why

[Glennjeff]

Forum member Dreams has been working on a Gibson GA 200 recently, with success.

Dreams raised the observation that the GA 200 has a peculiar "compression" switch / facility back in February.

Could someone explain what this does and how it works please.

It appears the switch puts an identical "triode / dual diode"" stage in parallel with the original "triode / dual diode" stage (which then drives a PI).

How does paralleling these tubes stages create "compression".?
Is it really compression?
What are these diodes doing?
How are they not creating distortion, (probably biased on hard DC wise)?

Thanks in advance and All the best.

[PRR]

It's unclearly drawn.

I don't see any paralleling.

The V3 V4 stage is push-pull with variable grid bias. Just like bias-vary tremolo. Drive the grids negative, gain falls off.

Where does this bias come from? V5 V6 get the same signal, boost it a bit, and drive diodes. They liked vacuum diodes because they came free with a cheap triode that was going out of style. We can render it as crystal diodes. Works the same.

The amplified signal goes to a thing you may recognize as a Voltage-Doubler rectifier. Works on audio as good as on power. Doing it twice (not shown my sketch) reduces some flaws.

So as the input gets bigger, the bias goes more negative, cuts-off V3 V4, their gain falls, output does not get so much bigger.

There's various other parts, to get things to work, or in some cases for no clear reason until you try to build it.

There are FAR better limiters today, often in $59 pedals.

[Glennjeff]

Thanks heaps PRR. That will give me a bit think about.

Just want to have a small compression section in my amp for the sake of "doing it with valves". Good learning exercise I'm sure.

Have got some more of your work on file from a decade ago I believe.

[Dreams]

It's actually not that bad sounding, obviously depending on the guitar part you play. Not something I personally have any interest in replicating somewhere else, but if it's already there....