occasional feedback on tube amp

Hi guys

i have a very slight problem with my valve amp it is a bugera 333xl

Every now and then i get feedback/hum that stars as usual feedback caused by standing too close to amp but continues after guitar volume is turned all the way down and i have to switch to standby to stop it.
it seems to happen more often when the amp is first turned on and most of the time i go through a full 4 hour band practice without an occurrence.

i still have the original bugera tubes and i have not touched the bias.

other than this the amp works well and sounds great

any ideas what may be the cause of this?

Sounds like its probably a preamp tube.

Yes, it sounds like a preamp tube. When the amp is running, gently tap the preamp tubes with your fingernail and see if you hear something that resembles the feedback. If so, switch the tube that's making the noise out.

I have a 1961 Ampeg Reverberocket, and it often feeds back when I first turn it on. It clears up if I turn the reverb all the way down, then I can turn it back up and I'm fine.

I have a 1961 Ampeg Reverberocket, and it often feeds back when I first turn it on. It clears up if I turn the reverb all the way down, then I can turn it back up and I'm fine.

ah yes i did think it may be caused by the on board reverb. that sounds a similar issue i think the amp would benefit from a retube as its a year old and i suspect the stock tubes arent very good. perhaps a combination of dodgy tube and too much reverb

Thanks guys