Bogen AP-35 Static and Pop from one channel

[eelsid]

I have an old Bogen AP-35 amp that has popping from the left channel. It starts out quiet, starts to get static, the static continues to rise in volume for about 10 minutes, then a loud pop and the speaker is quiet again. This is happening with no audio being played through the amp.

Here is the schematic for a similar model:

http://ampslab.com/SCHEMATICS/BogenAP30.jpg

Could this be a bad input cap that is discharging? It's definitely tied to only the left channel, but I'm having trouble tracing that channel to a specific capacitor (my electronics knowledge is limited). The amp has two 20/20/20/20 450V can type capacitors in it, the closest I could find were these 475V:

Capacitor, electrolytic, 20/20/20/20 µF @ 475 VDC | Amplified Parts

I figure at this point the caps should be replaced anyway (they're probably original), would the above cans work with in this application?

Which capacitor would be tied to the left channel? I've tried switching the power and preamp tubes and it stays on the left side. I haven't tried switching the cans around yet as that's obviously a bigger job but I can do that as well.

Any help would be appreciated, it's not a great amp but I love the way it sounds with my vinyl

[HollowState]

Ok, lets use a little trouble shooting logic here. You've switched the tubes channel to channel, so it's not them. The electrolytic can capacitors are common to both channels so it's probably not them either. It's something in the left channel. That leaves resistors, coupling capacitors and a lot of connections. Your problem sounds thermal.

Start by looking very closly at all the solder connections in the left channel. This includes tube sockets, tie points and wires from controls, switches and B+ lines. Your looking for a cold or poorly soldered connection. I like to use a wooden chopstick as a probe to gently move wires and connections to test their integrity. Do this with the amplifier on and listening. Another area to be checked is the tube sockets themselves. Make sure all the pin connections are tight and clean.

Sometimes looking at the wiring in a darkened room will show a tiny spark from a loose connection that may otherwise be missed.

Old resistors can become very noisy and cause this problem. It will help to narrow down which stage is causing the noise. Try to localize it by grounding out the signal at each tube grid progressively. Us a jumper wire with alligator clips. IMPORTANT, do not ground and grids on the output tubes! They need their voltages to maintain a safe operating level.

A coupling capacitor with a poor internal connection could also cause this noise. But this is rare. Determining this is sometimes difficult. Shocking them with a freeze mist spray can help root them out.

Regarding the replacement capacitor you listed, yes, it's a good choice if you really want to replace it (them). The voltage rating is the maximum it can withstand, not what it needs to operate. Higher is ok, lower is not.