Blues Junior Power Up Crackle

[Ted]

I've got a Blues Junior here that makes a sharp cracking sound just as the tubes warm up - maybe twenty thirty seconds after power up, just before the amp starts to hiss. After that it works fine. The amp's a year old and the owner reckons it started doing this after a couple of months.

Powered it up in the dark, no visible arcing. Changed the tubes. Still does it.

Anyone seen anything like this before?

[Enzo]

COuld be anything. Might not be malignant.

Does it change at all if ALL the controls are at Zero? If the controls are up, including reverb control, does the pop reverberate? Do the tone controls have any effect on the tone of the crackle? Do ANY of the controls affect this sound in ANY way?

Turn the reverb down to zero, then ball up your fist and whack the top of the amp. Does that make any sounds? Shouldn;t.

Take the chassis out, connect a speaker, and using something insulated like a wooden chopstick, push each component on the circuit board and each wire connection. ANything respond with a crackle?

[Ted]

Pretty sure its not malignant as the amps been in use like this for 10 months.

Noise is still there with all controls at zero - it varies from a sharp crack to a short burst (< 1 second) of buzzing.

Nothing is obviously microphonic.

The exact sequence is something like this:

1 Power on amp.
2 After thirty seconds there's a soft hum which lasts maybe half a second.
3 The crackle happens.
4 The amp begins to make its normal hiss


The EL84s are powered up and drawing current a few seconds before the noise happens. Bias is normal at -10.7V

Pulling the phase inverter (V3) stops it.

[Enzo]

Well then, that means the ophase inverter itself or something before it is the source. Does pulling any other preamp tube stop it?

[charlo]

YES!

My Blues Jr does the exact same thing!!! Have you diagnosed the problem? I have noticed that one of the EL84 power tubes gets very hot very fast- Please impart any info you may have-

Thanks-

Charlie

[Kazooman]

No Noise Here

Just FYI, I have a Blues Junior and it does not make the noise you deescribe so it is not a generic "feature" of the amp design.