[audiomagnate]

My 48-86 sounds great when I turn it on, but I noticed that after about 15 minutes or so it starts to sound distorted. This isn't caused by drift, as tweaking the tuning knob can't get rid of it. Any guesses as to which part could be causing distortion after warmup?

[Arran]

Has the set been electronically restored yet or is it as is? If it hasn't been electrically restored, works fine when it starts up, but starts distorting after 15 minutes that's a sign of a bad resistor, probably a bias resistor in the audio stages. Bad capacitors can do that as well but it usually shows up after a much shorter time frame. It could also be a defective tube, one with gas or secondary emissions.
Regards
Arran

[Mondial]

I would first check the coupling capacitor between the plate of the first audio stage and the grid of the output tube. If this cap has not been replaced, change it immediately.

Any leakage through this cap will upset the bias on the output tube, causing excessive plate current and eventual damage to it and the output and power transformers.

If this cap has already been replaced and is good, then as Arran has said it could be a bad resistor, other cap or gassy tube in the audio section.

[Ron Ramirez]

There is no such model number as 48-86. Do you mean 48-1286?

[audiomagnate]

Correct Ron, 48-1286. I'll figure out if it's in the audio section tonight and go from there. Thanks for the help.

[morzh]

Here's a possibility: some sets, including my 38-15, simply do not like 120V. They start OK, and then start sounding distorted etc.
I found this by lowering the voltage back to 110V - the set worked normally after that.

Try to eliminate that before you engage into a more elaborate troubleshooting. Unless you run it off of a bucking xfmr or such already.

[audiomagnate]

I would need a variac to step it down, which I don't have. Yet.

[morzh]

You should. A variac and an isolation transformer (or a combination of the two) is a basic necessity.

[Arran]

Well if it has not been restored correctly then the audio coupling capacitors in the audio section are a good place to start along with the grid bias resistor on the second audio amp tube or tubes, or the cathode resistor on the same if it has one. A gassy second audio amp can also cause distortion after it gets hot.
Regards
Arran