Loud bang through speaker when powering off.

[tompagan123]

Colleagues,
I have a build similar in architecture to a AB763 deluxe reverb.  It's been working fine for a long time with no lingering issues or recent changes.   I was playing it loud (like I usually do) and suddenly the output went  to nothing - I powered it off and it a loud bang through the speaker.  There was also a faint burning smell, but the 1A slo blo didn't open.  After a couple minutes I turned it back on again and it  sounded clean and strong as usual, and shortly after the same thing happened along with the loud bang when powering off.  After a while again I took the chassis out of the cab, measured voltages, poked around with my chopstick, looked for burned components and found nothing. The PT was running really cool to the touch.    Also couldn't quite isolate the faint burning smell.   I'm going to beat on it some and see if i can reproduce it.
I'm guessing that since the output went to nothing but the fuse didn't blow then possibly something opened in the 5AR4, choke or input / resevoir cap?   Maybe the burning smell was arcing in the on/off switch (e.g. not the root cause)?

Just wondering if that loud pop symptom rings a bell with anyone.

Appreciate it!

tom

[eleventeen]

Not directly familiar with the symptom you describe, but a funky tube socket contact or a bad 5AR4 sound like reasonable culprits, definitely  valid first guesses. If you have one, I like to substitute in a 5Y3 whilst checking things out. Much cheaper tube in case something blows up the rectifier tube, lowers the B+ maybe 25-35 volts vs a 5AR4.


When you power off, depending upon exactly how you do it (eg; the sequence of switch or switches flipped) you are suddenly removing the load from the power supply. This can cause the B+ as seen on the rectifier cathode to pop up. Perhaps that is causing an arc within the 5AR4...those tubes are capable of some odd failure modes, with their indirectly heated cathodes.  You may want to take note of how you are powering down. Under some conditions if there is such an arc, it could get dumped through the OT and possibly cause your bang. Yeah, kind of goofy but not impossible.


It could also be produced by the particular way you have the filter caps wired, eg; some (or one) before/after the stby switch. If you have 2  series caps for the first HV cap, do you have the "balancing" resistors on them?

[tompagan123]

So the audible bang only occurs when it's gone into the no output failure mode.  I haven't yet recreated that - HOWEVER, I have noticed the following three changes from prior to the incident

1) idle plate current measured through 1 ohm cathode resistors went from the previous 23MA down to 17MA
2) B+ is down from about 427 to 422 (even though the plate current is lower, not higher than before)
3) The PT is indeed running warm to the touch, after awhile it gets pretty hot.  This PT normally runs really cool to the touch.
4) After adjusting the bias for 23MA plate current the B+ dropped to 419 (a normal drop, but the overall voltage is about 15V too low, and the preamp tubes are about 6V too low (were 180, now 174).
The PT being warm despite lower plate current makes me think there is some other current being drawn through the PT.   

But again, what made me notice this initially is that the sound went away altogether, I turned it off, it made a loud pop when I flipped the switch off.  I reproduced once again, and since then I've noticed the symptoms above.   The Hot PT and lower currents and voltages make me wonder if there could be a partial short somewhere in the PT which is causing it to run hot and resulting in the lower voltages.

Time to learn about transformers :-).    Meanwhile I'm going to swap out the 5AR4 with another if I have one (definately don't have a 5Y3) and see if things come back to normal.

Thanks again!