VOX AC100cp

Is this the output stage in question, on page 5 (assuming the head and the combo are constructed the same)?

It looks like there are a few bridge rectifiers in the circuit, one for the solid state preamp stuff, one for the output tube bias, and one for the preamp heaters. Which one failed?

It would seem that a loss of bias on the output tubes might account for some of the problems, but I'm guessing that you've already considered that and that the failure mode may actually be more subtle.

"Bitrex"

There is 50Hz ripple on the HT, of course.

Since my post I discovered that R107 ( 22 ohms 7W) is open - this is is series with the high voltage secondary.

All other supplies are OK.

... Phil

Well my uneducated guess is that it had to have started with the tube with the lost vacuum. Don't know what the normal subsequent failuire mode is, but I suspect either the tube would short Anode to ground via heater cirduit perhaps, or would just send the bias way off the planet. Huge DC through the OPT primary partly via the scorched grid resistors on the other side, and a nasty expensive smell. Poo.

Gareth.

Or maybe,

duff tube shorted some windings thorugh DC.. The amp was being played at the time, so the "good" tubes shorted their windings too but via AC.

Hey, this is a good game.

Gareth.

"Gareth Magennis"

I suggest you read the list of failures again.

The thing is to work out what part failed FIRST and how that led to each of the others.

... Phil

"Bitrex"

Recent model VOX schems and service manuals are hard to find.

... Phil

A Google search for "vox ac100cph service manual filetype:pdf" brings it up. Since most service manuals online are in PDF format, adding the filetype qualifier helps sift through the cruft.

I don't really know enough to achieve that, Phil.

I would guess again though that it started with the duff valve, which put excessive DC through its side of the transformer, maybe not enough to blow the fuse at first.. This would saturate the core and overheat the OPT. I guess the owner kept playing the amp in this sorry state, leading to the demise of the other 2 valves and their windings. He may even have disconnected the speaker, or had an intermittent speaker connection, or even swapped output valves around., causing even more damage.

Gareth.

"Gareth Magennis"

... Phil

Coca-cola spilt through the grille?! Might also have cracked the valve.

(I'm kind of an Occam's Razor / the customer is a Numpty kind of guy)

Gareth.

"Gareth Magennis"

... Phil

Well, one last try before I give up then.

The "lost vacuum" tube probably just went out of circuit, leaving one valve vs. two. I think I'm right in saying that this will produce a net DC field in the OPT primary that would push it towards saturation and overheating.

The amp was kept in use at high power with this fault.

So, the "two valve" primary winding got very hot, shorted some turns, then went open circuit. This meant that its Screen Grids became the Anodes, which got overloaded until the resistors burnt or fell off the PCB. Half of the Bridge rectifier may well have burnt out in the process of burning out the screen grid resistors - at some point the HT fuse would finally blow, halting the whole process. (Or the customer could have replaced the blown HT fuse with a higher rated one, then put the original back in before handing it in for repair).

There might be other problems due to disconnection of speaker, or arcing inside the OPT connecting secondary and primary, that might force HT back to the output circuitry, I don't see any "flyback protection diodes" or whatever they are properly called, in the schematic.

That's just about the limit of my knowledge of failure modes of valve amps I'm afraid.

Gareth.

"Gareth Magennis"

... Phil

Well, are you going to tell us then?

Final answer - some problem with the speaker connection fed back 2 x HT voltage all the way back to the mains transformer.

The customer DID replace the HT fuse with one of higher rating, ran the amp again, and caused lots more damage. He put the original blown fuse back in place before handing it in, in the hope you wouldn't realise what a Numpty he is.

:)

Gareth.

"Gareth Magennis"

Arcing damage will produce shorted turns and sometimes open windings - both happened here.

Once half the primary went open, the two EL34 screen resistors on that side began to severely overheat, the screens would have been glowing white.

The two EL34s on the other side began operating under great stress as the tranny now had shorted turns and a much lower impedance than normal. Dissipation goes well beyond the max 25 watts, the plates glow red and one EL34 quickly became gassy and out of control of the grid.

Then the 1 amp fuse blew, another maybe larger fuse was tried and that resulted in the 22 ohm resistor failing.

... Phil

Thanks. Didn't realise tubes could go gassy from their internal components.

I should Google more, its quite interesting.

Gareth.