GA-40 RC network - what's the point??

[LooseChange]

I've seen this type of thing before... It appears to me as a tone circuit but can't this be done with less parts? And why did Gibson do this??

[Fresh_Start]

Just a guess:  is it an adjustable bandpass filter?

Cheers,

Chip

P.S.  Are you all scrounging up every schematic ever drawn which uses a 5879? 

[LooseChange]

It's not adjustable tho'.
Just a coinscidence on the 5879.  One of my friends heard this was David Grissom's favorite amp and being a fan, wants one.

[bigdaddy]

Are you sure that's not part of the tremolo circuit?

[Geezer]

One of my friends heard this was David Grissom's favorite amp and being a fan, wants one.

Ken Fischer said the GA40 was his favorite production amp (best sounding, other than his own TW circuits, of course......)

[Fresh_Start]

It's not adjustable tho'.
Just a coinscidence on the 5879.  One of my friends heard this was David Grissom's favorite amp and being a fan, wants one.

Doah - didn't recognize the tremolo circuit.

Those backwards volume pots are interesting.  Anybody know if the tone changes as you roll down the volume?  I'm guessing (again) that the backwards volume pot on channel 1 raises the cutoff frequency as you lower the volume.  May be the same with Channel 2.

Chip

[LooseChange]

I don't think it's part of the trem circuit. The oscillator circuit is drawn below the tube. I think I've seen this same arrangement in a AC15 or 30. ??

[bigdaddy]

It looks very familiar and if I remember it was part of the tremolo circuit or only on the trem channel.

[PRR]

The tremolo oscillator injects a BIG 5Hz signal to the cathode of the preamp.

The preamp tube is slammed OFF and ON. That's how gain is varied.

The preamp tube plate has ~~1V of guitar signal and 100V of 5Hz wobble.

The problem comes in the stage you snipped away. The next stage can handle 1V of guitar, but not 100V of 5V wobble. In fact when the 5879 is "on" and passing signal, its plate is most-negative which drives the next stage grid negative and turns it off. The result is an OFF-and-OFF tremolo, which is no good at all.

We need to pass 80Hz up but reduce 10Hz down. One 0.005u against 1meg is -3dB at 32Hz bass-cut. At full-up we have five such sections. Ignoring the heavy interactions, it is -5dB at 64Hz, -2.5dB at 128Hz, -100dB at 3hz and -40dB at 10Hz. Our 100V 10Hz wobble is cut to 1V, similar to desired guitar signal, and does not swamp the next stage.

There's another complication. The trem oscillator injects at both cathode and plate of 5879. Kinda shorts it out. But the plate connection is via more Rs and a C.

Why don't we see this more often? It may be Grissom's fav amp, but nearly everybody uses Fender's output-stage tremolo. The push-pull audio connection with single-ended tremolo makes the large throb cancel-out of the signal. (Or the later LDR cheat.) The other major single-ended tremolo throbs the driver before the output tube. Here signal levels are higher. Still you need some bass-cut to keep from slamming the output tube into OFF-OFF operation. Even so this is reserved for low-price beginner's amps.

[PRR]

The backward-pot is used in some Deluxe also.

If you have a low-Z source and a hi-Z load, the normal pot is appropriate.

If you have a hi-Z source and a lo-Z load, the flipped pot is appropriate.

This pentode stage output Z is quite high.

The next part of the plan (not shown) is kinda hi-Z. The pots could be flipped, and all vaules re-optimized. Does not matter which way you skin the cat.