Help with vintage Gibson GA-40 motor boating on ch1

[spark]

Hi, I'm working on a vintage AG-40 Les Paul amp. On ch1 when I get to ~the 2 o'clock mark on volume with or without guitar plugged in the amp starts to motor boat. I replaced both power supply caps and I have ordered a new rectifier tube as there is a lot of hum in the amp. Anyone have any suggestions as to the motor boating?

[Chuck H]

If you're familiar with this amp and you know this to be a new problem that didn't start only after something else was done to the amp I will speculate that it's a bad ground. Did the problem exist before you replaced the power supply caps?

[spark]

Not familiar with it. It was sitting in the back of the shop. I tested all the tubes except the tremolo, powered up ok. Ch2 is nice and clean. However the tremolo doesn't work. Ch1 sounds good but motor boats when turned up. Thought motor boat could be caused by weak caps on power supply. Replaced 20 uf with a 22 and replaced the 10 with a 22 (only had 22s on hand). Still getting the motor boating. I have a full set of tubes on order as the preamp are not the standard 12ax7.

[Chuck H]

Don't expect the motor boating to stop with new tubes. That problem is a function of phase and gain. Nothing about new tubes will change those things. Other than the possibility of more gain, which is more likely to exacerbate it.

Here's the schematic:

[Chuck H]

After looking up the schematic I see there is a third power supply cap. If you haven't replaced it yet you should. Incomplete decoupling is the most likely cause. I discounted it on faith before because you said you preplaced "both power supply caps". Which makes it sound like there are only two.

[spark]

Where is the cap you are referring to? I replaced the 20 and 10 by the 30th choke.

[Chuck H]

Ha! There are two more power supply caps that perform decoupling. I only have the schematic to reference so finding them in the actual layout is up to you. But it should be possible by backtracking the circuit from the tube socket pins. There's just not many electrolytic caps in there. The caps I interpret you changed are circled in blue. The two that are still hiding in there are circled in red.

[spark]

Thanks Chuck, I'll check that when I get back to the shop. Yes, my spell checker changed 3H to 30th, go figure. I'll let you know if that was the fix in a couple of days. It probably is the 10uF that feeds ch1 through the 100K resistor since ch2 gets fed via a 510K but it might be best to replace all 4 caps that are circled and see where it leads.

[Chuck H]

it might be best to replace all 4 caps that are circled and see where it leads.

[spark]

Just ordered 10 pieces of 10uF caps for replacements. It will be a week or more before they get here.

[Chuck H]

You can use 22uf's if you have them on hand. No one gets hurt and the tone shouldn't suffer. The most probably reason for the 10uf stock value is that when the amp was designed electrolytic cap technology was such that higher uf values were disproportionately expensive. But staying with the stock values for authenticity is cool too. It may make a detectable tonal difference so if you want the original sound 10uf is "correct".

[spark]

The replacement 10s were not that expensive and since everything else is authentic, including the speaker and even the mahogany block foot switch, I'll try to keep it close to original as practical. I'll replace the power cord with a 3 prong for safety.

[spark]

The 10s took care of it. Thanks for the help.