GA-45

[lowell]

Hi there,
Am troubleshooting this amp, it's also called a Saturn. I found a schematic here http://www.gibson.com/Files/schematics/GA-45RVT.jpg. However it's quite different from the amp that's in front of me. My amp here has a different tone stack wiring and most of the resistors throughout the amp are extremely different values. I did find one somewhere but can't find it now and in any case it was too blurry. Please help.

[Enzo]

Welcome to the world of servicing Gibson/Epiphone/Kalamazoo amps. I have one model in my files with three different Gibson published schematics, plus three different hand drawn version, all of the same model number. I generally expect the schematics not to match.

Many times I;d find my Gibson something was in fact an Epi something under the skin.

If the number lookup drawing doesn;t match the unit, I usually leaf through the big Gibson book looking for models with the same tube complement as my amp. The tube order is usually prominent on Gibson drawings.

[lowell]

So I'm SOL I guess. The one I have is similar but still it's a serious mind f when I start tracing the circuit. My brain hurts. :-/ This amp has way too much bass and we're thinking it's this damn Baxandall tone stack.

[Enzo]

Signal tracer. Buy one, make one, or make a tracer probe for an existing amp. COmpare the sound of a test music signal before and after the tone stack then.

Left to right across the rear, what is the tube complement?

[fredy2]

Try this site: GIBSON GA-45RVT SATURN SCHEMATIC Service Manual free download, schematics, eeprom, repair info for electronics
SCROLL down to "Get Manual" and click to download. IGNORE other download links.

[lowell]

Thanks Fredy2. Okay so that's the right schematic. The bass has been reduced to a good point now after disconnecting C28, the cap to ground from between the 2 270k resistors on the grid of V4A. Now there is a real bad distortion when the amp is turned up to 7'ish on the volume. Until that point the amp sounds good. It's that scratchy flubby distortion like when there's a bad filter cap solder joint, kind of a hiccup. I believe I've narrowed it down on my scope. It only appears on and after the plate of V5A - the last pre-PI stage. The signal on the grid of this tube is ok. This stage has what I believe to be a bootstrapped bias arrangement. Not sure if that's right. There also seems to be some negative feedback from V6A's grid back to here.

Anywho the voltages on V5A are:
4:80v
5:39v
6:230v

I checked all resistors in this circuit as well as caps they're good. The only thing is the 150k resistors in the LTP are actually 1meg inside the amp. Don't think this is a problem but could be wrong. I also suspected that maybe there wasn't sufficient PS decoupling for this stage but the 4.7k dropping resistors are good.

I've attached some pictures of V5A's plate when this nasty distortion is happening... One is of the signal right before the clipping, the other with it.