1996 AC30 re-issue model. Excessive hum.



I'm looking at the above AC30 that needs to be used on a session next week.

It's quite seriously 'hummy'.

I took some measurements at the speakers and got the following readings.

Tone controls all centred. Normal input with a shorting jack inserted - gain
setting .
@ min 4.8mV (-67.2dB) hum
@ '5' 5.5 mV (-66dB) sounds mainly hissy, the hum has mostly cancelled out.

@ max 21mV (-54dB) loud hum

A spectrum analysis show the hum @ min to have its largest component @100Hz (HT
ripple) and @ max to have its largest component @ 150Hz (pickup @ 3rd harmonic
of the AC line frequency from the power transformer).

The 100Hz ripple suggests to me that the output stage has poor balance although
the guitar tech claims it was recently re-tubed. I'd like to put a new set in
just to check. Anyone care to make any recommendations for a reliable brand of
EL84s ?

The 150Hz pickup is more serious though. There's litle one can do about
transformer pickup. I'm sure I heard somewhere that the latest AC30 reissue
models had some corners that were cut and this might be one of them ?

Comments?

Graham




.



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