low freq. oscillation on startup only

[AndrewM]

Hey guys,

I Just finished a new SE EL84 build and it has a strange problem I'm hoping someone can help diagnose.

When the amp is first turned on and with the volume past half way the amp makes a strange low frequency rumble type oscillation. After about 30 seconds the sound dies off and the amp works perfectly....

Any ideas???

[Jazz P Bass]

Are you using a standby switch?
NOS tubes are quoted as having an 11 second warmup time.
Don't know about the newer tubes, but still, you have to let the heaters warm up.
The heaters "heat" the cathodes.
The heated cathodes emit electrons.
No electrons, no amplification.

[pdf64]

What circuit?

[AndrewM]

Its pretty much a modified valve junior schematic

3 x 47uF instead of 3 x 20uF filter caps
R6 and R7 deleted
Input wired with stock Fender values
Harvard tone circuit added




It was an impulse build so the layout was not really thought out..... I don't think its a lead dress issue because the oscillation disappears without intervention...

I'm baffled!