1956 Gibson GA-6 driving me NUTS!

[Fred G.]

Restoring a Gibson GA-6. It was a rescue from a home demolition site, tossed out a 2nd story window as the customer's brother walked by, he took it and gave it to his brother. Sat in a closet for 15+ years.
I got it working, and actually thought I had it ready to return to the customer, but as I was doing some wrap up testing, it began acting badly (again).

The "mic" input works fine. The "instruments" circuit is the culprit. As the volume pot is turned up. I first get a quick "ssssst" and as I turn further up a very LOUD motorboating ensues.
When I try to test pin #4 of the 6SL7 for DC, it hums and the hum gets louder as I get near the pin. when I contact the pin, the ensuing pop is EXPLOSIVELY loud. Same thing when I approach the middle lug of the "instruments" (volume) pot.
I had this thing behaving earlier today, and thought I could be shed of it, but now it's back to it's evil ways. HELP!

[Jazz P Bass]

It would be helpful if you could label the inputs.

[DrGonz78]

The mic input is the top left one where it has no grid resistor and the old solitary coupling cap. The three inputs below that are the instrument input.

[Jon Snell]

I would replace the 4.7M and 100k resistors on the 12AY7 first.

[Leo_Gnardo]

"Got it working" ... well apparently not so much. Amps of this age - ancient - really need ALL caps replaced. Don't think twice about it, just do it. Also don't trust old resistors, they drift like crazy, and are often noisy. What's really necessary, a complete rebuild to make sure none of those crusty old components are doing you dirty. FWIW motorboating is usually a symptom of insufficient power supply filtering, so make sure you swap in new caps there too.

Heck of a find, an amp that was actually defenestrated! (That means, thrown out the window, so you don't have to look it up.) Some history it's got already, and I'm sure after you rebuild, a glorious future.

[Chuck H]

...really need ALL caps replaced. Don't think twice about it, just do it. Also don't trust old resistors, they drift like crazy, and are often noisy. What's really necessary, a complete rebuild to make sure none of those crusty old components are doing you dirty. FWIW motorboating is usually a symptom of insufficient power supply filtering, so make sure you swap in new caps there too.

THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

defenestrated! (That means, thrown out the window, so you don't have to look it up.)

Looked it up anyway. Awesome. Commonly used as a description for that particular method of homicide! We have a word just for that? I see that it also finds other analogous meanings in use too, but really? What sort of culture needs a word just for that?

[Fred G.]

OK, based on the posts above, I need to provide more info:

All the resistors in the amp have been replaced, with the exception of the cathode resistors on the 12AY7 and 6SL7;

All the filter caps have been replaced. I bumped up the 20uF to 30uF, as per a recommendation I read;

All of the capacitors have been replaced;

All of the pots have been replaced.

As another poster said, the "mic" input is at the top left; the "instrument" inputs are the bottom left. The topmost pot coming out of the 12AY7 is the "mic" control; below it are, in order, "instruments" and "tone".

And as I mentioned above, when I try to test the grid pin #4 for DC volts with a DMM, the hum gets louder as I even get close to it, and then pops explosively LOUD. Also, I had to replace the NEW instrument pot with ANOTHER New pot - apparently the first one was ruined by some bogey in the preamp circuit - I'm guessing DC volts on the pot (which I have been unable to test as yet because of the LOUD popping, which concerns me that I could destroy the old Jenson C12R speaker in it).

HELP!!! lol. This thing is taking up wayyyyy too much time.

Thanks again,

Fred G.

[Leo_Gnardo]

What sort of culture needs a word just for that?

Middle Ages, look up the Defenestration of Prague. Some party that must have been! Sometimes these sort of activities were blamed on people eating moldy rye bread. Stick to the beer guys, it's much better, especially in Prague.

These days, we hear a lot about "getting thrown under the bus" instead. I'm sure the sentiment is much the same.

Back to windows, of course there's musical accompaniment. Skip the first 28 seconds intro & get to the song. We can add a verse to this one I'm sure, to commemorate the subject at hand. Try this: "The wreckers were workin' on a house, went upstairs - it was rotten and damp, they found a '56 Gibson amp, and threw it out the window."

[Chuck H]

FWIW there's a rumor that in the early years Pete Traynor would occasionally test the durability of his product by tossing an amp off the roof of a two story building. For the purposes of this thread I'll postulate that this is a dramatization on the truth. Why carry an amp to the roof? So much easier to use the window.

[Leo_Gnardo]

FWIW there's a rumor that in the early years Pete Traynor would occasionally test the durability of his product by tossing an amp off the roof of a two story building. For the purposes of this thread I'll postulate that this is a dramatization on the truth. Why carry an amp to the roof? So much easier to use the window.

Indeed, the version I heard was Pete ... threw it out the window. "Look out belooooooooow.....!" KLANK!!! Call it the Defenestration of Mississauga.

The location being Canada, I s'pose the demonstration didn't count for much if the amp happened to land in a 10 foot pile of snow.

[Leo_Gnardo]

(which I have been unable to test as yet because of the LOUD popping, which concerns me that I could destroy the old Jenson C12R speaker in it)

OK looks like you've done all the right things so far and we have to dig deeper. But in the meantime please disconnect that fine old Jensen and run speaker output to a sturdier speaker that can take the abuse.

[Fred G.]

Is a dummy load ok? I have a couple of 8 ohm 50W resistors I bought for just that purpose. I was hoping to disconnect the Jensen and alligator clip the OT leads to one of those to test for DC on the pot and 6SL7 grid pin #4.

[Leo_Gnardo]

Is a dummy load ok? I have a couple of 8 ohm 50W resistors I bought for just that purpose. I was hoping to disconnect the Jensen and alligator clip the OT leads to one of those to test for DC on the pot and 6SL7 grid pin #4.

Sure thing, maybe it's best to avoid being distracted by loud annoying noises whilst you troubleshoot.

[Helmholtz]

You can still connect a speaker in series with a 100R resistor across the dummy load if you need to hear some sound.

[g1]

The schematic does not show which 6SL7 grid is pin4. Is it the one connected to the pots?
You don't get the same kind of pop off the mic pot wiper as the inst. pot?
And aside from that, pop when probing grids is not uncommon, so not sure if it's really revealing anything? But wondering if you are losing the ground at the inst. pot.