Bandmaster goes Thump in the Night

[GM Reszel]

I have a 65 Bandmaster head, 2 6L6's for power.

Problem: coming off standby I get a thump, thump in the speaker. The thump is intermittent and doesn't have any set rhythm, just comes and goes and during this there's no audio to speak of.

When observing the power tubes: while the thump occurs there is a deep blue plasma flash in the tube. I shouldn't say flash; it is not the bright, shorted tube 'lightning' spike but a quick burst of blueish glow. Switching the tubes and new power tubes doesn't fix the trouble - the problem is specific to that one socket only.

There were a couple resistors on the socket, 470 and 1500 ohm that I replaced, no change.

Letting the amp sit on for awhile might have the amp come back up and it plays fine, then suddenly it will do the thump thing and nearly all audio goes away (if you listen you can hear a faint, crackly audio at this point).

I know it's hard to analyze but does this sound like a familiar scenario to anyone? Thanks ahead for any help.

[gldtp99]

Sounds like motorboating which is a sign of old, failing filter caps------does this '65 Bandmaster still run the orig electrolytic capacitors ? If so, it's well past time to change them--- or it could just be a failing preamp tube-----use substitution (with known good preamp tube) to rule out this simple-to-fix problem..................gldtp99

[GM Reszel]

I hadn't considered the pres might be affecting the powers, I'll check that as well as the fil caps, Thank you.

[GM Reszel]

Update; swapping all tubes with known good ones and viewing the filter caps (all new, blue sprague). No dice, still thumpa thumpa. Any suggestions folks?

[donnyjaguar]

Check the values on those replacement capacitors. Most instabilities like this, assuming the amp hasn't been bastardized (modified) are caused by inadequate de-coupling between stages. Curiously enough, it can be caused by using *too much* capacitance too if the design was marginally stable to begin with. And let's face it, most vintage Fenders were marginally stable. :)

[Tonic Amps]

This is a common issue actually. When the amp is in standby, voltage increases in the power supply since the current draw is less and can cause enough voltage stress that when you take it off standby, you get an arc which causes the "pop" or "thump". I would wager that that longer you leave it in standby, the more apt it is to arc. More than likely the standby switch itself has developed the arc. You can replace the switch, or look for other places in the amp where the arc may be occurring.

[Mr. SD-1]

One thump coming off standby is normal in most blackface fenders, but repeated intermittent thumping is not. The quiet scratchy audio and blue glow in just one power tube suggests an intermittent misbalance in the push-pull. Try re-tensioning both power tube sockets. Make sure the filter caps are discharged first. Also check the power tube ground connections.

[Tonic Amps]

Ah, I didn't read closely enough. Yeah, there are many things that could cause this: bad preamp or power tubes, open plate of cathode resistor, bad coupling cap, loose tube socket, bad filter cap, cold solder joint somewhere, etc. There is also a possibility that the trem circuit is causing issues. Pull the 12AX7 that is used by the vibrato circuit and see if that cures the issue. Then try a known good tube there.

If you are not comfortable working with high voltages, have a tech repair it for you. But the initial pop coming off standby is very common. The drop outs of sound are not.

[GM Reszel]

To clarify - no, I didn't mean the thump coming off standby. Excellent suggestions and knowledge pool here thank you very much. I'm experienced in poking around tube amps, soldering, discharging the caps and things of safety and sense, but I'm not a tech and I appreciate and understand the cautions. I was foolishly hoping this would be a textbook problem with a pat answer and I should know better. I think it needs to go to a good tech but unfortunately I don't know of a good one in NE IN (suggestions?). Thank you again for the answers, I have reciprocated by offering my knowledge in the guitar technical threads.