amp hums on start then fades

[mikebarg]

I am working on one of my old amps, a Gibson GPA50(50 watt PA amp). I pulled it out and turned it on and it had quite a bit of hum(60Hz). So I replaced the power supply 2000uF/35V caps with new 2200uF/35V caps. Now when I turn it on you hear hum that lasts about 5 seconds then fades to a soft hum( you have to be 2ft or so close to the speaker) that never goes away. The hum is unaffect by the volume. Otherwise, the amp seems to work normally. I really want to clean up the hum as much as possible.
Any ideas? I have included the schematic.

[pinkmouse]

Replace all the other electrolytics as well, and see if that helps.

[HDTVman]

I agree with pinkmouse. C22, C23, C24 and C25 are in the power supply too and the most likely target. They provide the filtering for the low level stages at reduced voltage set by the 12 volt zenors.

Later BZ

[mikebarg]

I replaced these 4 50uF caps. However, the hum did not change.

[Netlist]

Can you measure the ripple on the PSU caps?

/Hugo

[mikebarg]

Unfortunately I don't have a scope or a true RMS meter.
Measuring accross the 2200uF caps with my VOM I see 33V DC and .06V AC.

[Netlist]

And what voltages do you get at the output?

/Hugo

[johnnyx]

I've had this in valve amps., but I think you have a different problem.

[mikebarg]

DC .1 V
AC .002V

[destroyer X]

Switching on, and having short time high current, will overcharge supply and the transformer hum can be heard during around half second...then bias goes reasonable and the amplifier goes silent.

I also think electrolitic condensers, the supply filters are the first and main suspect ones.

regards,

Carlos