Harsch oscillations in Vox AC15

[ricothetroll]

Hi,

My guitar player has an issue with his Vox AC15 : it sometimes starts to oscillate like crazy. The oscillations sounds dirty like a sawtooth or square and I would say somewhere between 1.5kHz and 2kHz.

So I took it home to take a look at it see if I can fix it. I first though of microphonic preamp tubes (12AX7 here) but those seem OK. I tried hitting gently the EL84 power tubes with a chopstick and I found I could initiate the oscillation this way : it starts with what sounds like a bad contact noise (or connecting a jack in an open amp) and then the oscillation starts. I switched the two EL84s to be sure it's the tube and not the socket and it's still the same tube that starts the oscillations. I must also precise : the oscillation is the same on any volume or master setting, so it's definitely a problem in the power section.

I'll change those EL84s. But I'm curious : what could be the phenomenon causing those oscillations ? When hitting the tube with the choptick it didn't sound microphonic.

Best regards.

Eric

[DavenPaget]

I bet the (failing)EL84 is starting to be sensitive to vibrations .

[petemoore]

  These amps are known for beating up tubes.
   Use of a pencil eraser or even a cotton gloved hand to lightly press against [dampen] or tap on [vibrate] the tubes in the amp can/may provide useful oscillation 'control' to indicate which of the tubes is the 'input'...they get to be microphonic or otherwise feed vibrationals into the SP...fedback through...produces 'oscillations... First check for bad tubes ! !
   New tubes makes it possible to check old tubes, old tubes can be checked in other ways if the equipment is available, but the amp often does all the tube-checking [in my case] by process of eliminating each tube [using 1 or more new ones].
   Having robust replacements is similar to having a matched spare tire for a 2 or more wheeled vehicle..if it blows you're stuck without a spare...might as well be something robust you can drive on if you're spending the time to change it...

[ricothetroll]

Hi,
I put new EL84s and now it's running OK ! We'll test it tomorrow night in rehearsal conditions.
Thanx for your help !
Best regards.