What's wrong here? - Gibson GA5 Crestline

[Tone Meister]

This amp was gone through a few years back and was supposedly working great. However, the volume pot recently developed a bad spot and was changed out and now it isn't working right. The amp will pass signal, but the VOLUME pot doesn't have any control in attenuating the signal and it makes a scratchy noise when rotated, the type of noise you get when there's DC on the pot. The VOLUME pot behaves like it's mis-wired, and I need a little help sorting it out. Supposedly, all that was done recently was the volume pot replaced, no other wiring or work done.

At the VOLUME pot I'm able to measure 14.3V DC from the wiper to ground and can measure a whopping 132.4V DC from the CW lug to ground. On the CCW lug there is 0.00V DC and there is no measurable DC on the body of the pot

The amp appears to be a GA-5 Skylark (Crestline) with a single volume, no tone control, and the following tube complement:

V1 - 6EU7
V2 - 6C4
V3, V4 - 6AQ5
V5 - 6X4

Here is a link to all the pictures I have: Gibson GA-5 Crest Photos by ResidentToneMeister | Photobucket


And here are a few shots of the affected area:

[eschertron]

If it is in fact a Gibson GA-5 (of any variety) then it doesn't match the schematics that I found (below).
http://www.paleoelectronics.com/blog...02c84-1024.png
http://www.gibson.com/Files/schematics/GA-5.pdf
http://www.gibson.com/Files/schematics/GA-5T.jpg

The schem from paleo shows a filter network in front of the volume control, but otherwise they are all about the same, and none show the extra components that your pictures show tagged onto the volume control. You may need to sketch out what you have and we can take it from there.

[Chuck H]

Some better shots of V1 would help. It's very hard to track the circuit in the photos. Someone went all shenanigans in there fo' shizzle! My guess looking at the photo's is a miswire but it could still be a bad tube (rare, but it happens) or coupling cap (not likely as the caps look to have been replaced with modern dry films). The circuit doesn't look to be stock right now. Was this amp ever working as it is wired now?!?

EDIT: That tone module follows a coupling cap according to the schem so it shouldn't be responsible for any DC bleed. That doesn't mean it shouldn't come out It might be easiest to just strip out the preamp and rewire it as a stock circuit (the one without the tone module.

EDIT 2: The schematic in the first post doesn't appear to have the tone module. So, if it's the accurate schematic, how did the tone module get in THAT amp?

[drewl]

There should he a coupling cap from the plate of the tube to block dc to the pot.

The orange filter should measure 470k and has a cap in it.
I removed one from a recent repair.

Gibson amps from this era often have filter networks like this which kill the tone IM0.

[Tone Meister]

That square orange component wired to the volume control is the infamous Sprague 102C84 filter. The schematic attached to my first post appears to be the correct schematic, but I'd stand corrected on that. I'll try to sketch out what I have and get it posted. Thanks for the reply.

[Tone Meister]

Was this amp ever working as it is wired now?!?

Yes, according to the owner it worked fine for a couple of years until the pot developed a problem causing a dropout. Once the pot was replaced, supposedly one-to-one, that's when things quit working right.

[Tone Meister]

...That tone module follows a coupling cap according to the schem so it shouldn't be responsible for any DC bleed. That doesn't mean it shouldn't come out It might be easiest to just strip out the preamp and rewire it as a stock circuit (the one without the tone module.

EDIT 2: The schematic in the first post doesn't appear to have the tone module. So, if it's the accurate schematic, how did the tone module get in THAT amp?

I stand corrected on the schematic, but that was the closest I'd found this morning with that tube complement. This amp is essentially a one owner, passed down from father to son, so I am going to presume that filter is stock to the amp, at least for now.

Gibson amps from this era often have filter networks like this which kill the tone IM0.

I am certainly open to stripping that section and rewiring it without that filter.

[Chuck H]

Ok... It looks like someone cascaded the channels so the two input triodes are in series. AND the "tone module" was retained in the process. Since the amp worked before and the owner is no doubt familiar with it's tone I guess this is a repair to the circuit it has rather than a correction of the circuit. In this case I guess it could be a bad tube or a leaky coupling cap. Pull the 6EU7. Is there still voltage on the pot? If not then it may be a leaky cap. This assumes there wasn't voltage on the pot once upon a time. It IS hard to trace a circuit like this. It should be easier for you with this thing on the bench. At this point you can crumple up any schematic you have and throw it at a passing car.

[J M Fahey]

That midrange killing network or fixed equalization is typical of Gibson amps and an essential part of their sound.

It might be discrete or, since they used so many of them, they ordered custom RC modules , but the schematic and net effect is the same.

And pulling it may be what's needed in some cases ... but then it's not "a Gibson" any more.

[Tone Meister]

To clarify things a bit from the first few posts, the amp was worked on a couple years back and has worked fine until recently, according to the owner. A new volume pot was fitted, with no wiring changes, but now the amp doesn't behave correctly.

[eschertron]

To clarify things a bit from the first few posts, the amp was worked on a couple years back and has worked fine until recently, according to the owner. A new volume pot was fitted, with no wiring changes, but now the amp doesn't behave correctly.

...and there lies the rub.

Since the amp has been 'worked on' or 'worked over' and no longer matches any of the schematics that we've looked at, there is simply no telling *if* the volume pot was indeed fitted with no wiring changes. If that assertion is true then we can troubleshoot other things like the tube, or capacitors.

But since we can't know if the assertion is true (unless you have some 'before' photos to share) then we may have to do a bit of reverse-engineering to see what the circuit you have really is; what may need to be done to it to make it what it really should be. And maybe the "infamous" filter module might help identify the circuit. Can you find amp schems with it as part of stock? Or maybe it's a Gibson secret that simply doesn't appear on any drawings... that would be a drag.

edit: found the link here: http://www.paleoelectronics.com/2007/09/102c84/ which indicates that the tone module needs to go elsewhere in the circuit. See the schem from paleo in my earlier post that shows how it's hooked up.

[52 Bill]

To clarify things a bit from the first few posts, the amp was worked on a couple years back and has worked fine until recently, according to the owner. A new volume pot was fitted, with no wiring changes, but now the amp doesn't behave correctly.

Well obviously there was a wiring change when the new pot was installed or it would still be working.

After reviewing your photos, the 100K resistor that is going to the CW end of the pot should be moved to the ground (CCW) pot terminal. That resistor is part of the voltage divider for the driver tube cathode circuit. This should also remove the voltage from the pot.

[Chuck H]

Well obviously there was a wiring change when the new pot was installed or it would still be working.

After reviewing your photos, the 100K resistor that is going to the CW end of the pot should be moved to the ground (CCW) pot terminal. That resistor is part of the voltage divider for the driver tube cathode circuit. This should also remove the voltage from the pot.

Nice, diligent circuit tracing. I hate tracing circuits from layouts or photos and get lost too easily.

TM, What Bill said. The pot is wired cattywhompus and now the circuit that's actually in the amp (which isn't the stock circuit) must be assessed to figure out how to rewire it correctly.

[Tone Meister]

...But since we can't know if the assertion is true (unless you have some 'before' photos to share) ...

Don't I wish, but no such luck. I'll back to the bench soon and will move that 100K resistor to the CCW lug per Bill's suggestion and see how it shakes out.

[eschertron]

Don't I wish, but no such luck. I'll back to the bench soon and will move that 100K resistor to the CCW lug per Bill's suggestion and see how it shakes out.

take a look at the paleo website's schem of the sprague filter module's innards. That should be enough to compare with the few amp schematics that have that filter module either before or after the volume control, but not wired across it. There's a wire to be moved to get the sprague filter thingy right. See the edit I made in post #11.