Help! Standby switch->THUMP

[capomonster]

Hi everyone,
new poster here, looking for help & wisdom.
I'll start with the simple things...
My dear Fender Bandmaster BF is behaving like follows:
since yesterday, switching the standby off (=amp full on) causes a thump, accompained by a blue glow on just ONE power tube. What can it be? The big resistors in there (470ohms if I remeber ok) were replaced just 12 or 15 months ago.

Same thing happened with my other amp, an Ampeg V4. I didn't use it for long (shame on me), then tried to play it a month ago, same thump (but waaaay louder... maybe because the amp itself has nearly 3 times power compared to my Fender?), then noticed a power tube had the plate going red, switched off and nearly cried.
Could it be those 470ohm resistors? HEEEEEEEEEEEEEEEEEEEEEEEEEELP

[MWJB]

"Could it be those 470ohm resistors?" it's more likely that a failing tube would blow the resistor, than the other way around.

Does the amp work properly irrespective of the thump? The blue glow is not necessarily indicative of a problem, neither is the lack of a blue glow.

If here are no other ill effects you may just need a new standby switch?

BF & SF amps often have an audible "dee-dump" sound that follows throwing the standby, this is normal. A very loud "THUMP" on the other hand isn't normal.

The red glowing plate in the Ampeg is a sign that the tube is drawing way too much current, this needs checking out by a tech.

[capomonster]

Hi,
thanks for your reply!
I thought about the resistors since a year ago or so I already had a problem with my Fender, strange noises coming out of it, that changed when moving thetubes around a bit (not with my bare hand of course!). The resistors were on their way to death, changed them and everything went fine since.
This "thump" is not sooo bad, but louder than it used to be. I was worried about the glow since it occurred on just one tube and not both; otherwise I would not have thought about that.
Should I try to replace the tubes then?
Regards,
Maurizio

[MWJB]

What does the amp sound like, apart from the thump? How many hours have the tubes had, if more than 2000 hours of hard use, then they could probably do with replacing.

Strange noises coming out of the amp, when moving the tubes around, points to loose tube sockets. Deal with these first. Power down the amp & unplug from the wall, put the standby switch in play mode (this drains the filter caps on your amp).

Remove the power tubes and using a small, strong implement (small watchmakers screwdriver, big safety pin) gently bend in the metal contacts in the tube sockets until you get a firmer grip on the tube. Apply a LITTLE isopropyl alcohol to the tube sockets and to the tube pins and gently install & remove the tube a few times to scrub the contacts clean.

Nine times out of ten this works a treat, sometimes the socket can't be retensioned satisfactorily and needs to be replaced.

When done, you should be able to turn the amp on, volumes & tone controls wide open (no instrument connected) and gently rock the tubes in their sockets with no unwanted noise.

Don't replace the power tubes without checking the bias.

[Bassman3]

Thump

Turn down your Gain & Master Volume to minimize the noise.