Flames in the power section - HELP!

[slidincharlie (Carlo P)]

The amp is my '79 Vibrolux Reverb, export version.

I put the chassis on my bench because I noted that the power tranny was very hot 30 minutes after switching off. This tranny had never got hot before.

Turned it on.
After about 20 min my children cried "Fire"!
There was a burning flame on the PT wires where they join the VAC selector.
Further, the two 100 ohm resistors on the heaters had gone in smoke too, a few minutes earlier I suppose...
From visual inspection I suspect that the flame started at the power tube (V8) end of the heater wires... is it possible?

Although the mains voltage is around 230-235 VAC here, I use this amp with the VAC selector on 220 VAC to keep the B+ around 440 VDC. If I put the selector at 240 VAC the B+ decreases maybe a bit too much.

What happened? Is the PT shorted?
Can you guys help me in the troubleshooting process?

Many thanks in advance.

[Alex/Tubewonder]

You most probably got a short between plate and heater supply pins on that tube socket. As there's no fuse on the plate supply the HV winding was effectively shorted drawing enough current to heavily overload the power transformer. It is toasted if you isolation on the fly out lead burst into flames.
Transformers are usually rated for max 130C winding temperature - that is well below combustion point of cotton or PVC isolation, temperature inside your PT was much higher. I wouldn't fool around with it. Get a new one.

[slidincharlie (Carlo P)]

Jesus!
The amp sounded absolutely good only a few minutes earlier...
Only trouble: that tube's plate was glowing (with loud hum) as soon as I turned the amp on. Pushed it in its socket and everything went ok. Then I played and everything was okay.
Soon after I retensioned pin 5 on that socket (this had already been an issue some time ago), turned the amp on, and there was fire.

[Alex/Tubewonder]

Carlo, it is primarily a safety issue. Even if that transformer after having cooled down seems to be working the overheating it was exposed to has made the isolation on the winding wire at least weakened if not melted. There'd be shorts between individual turns of windings already or just waiting to happen which will cause overheating and eventually another fire. Breakdown in isolation in transformers is difficult to diagnose without proper instrumentation (unless it's a full open circuit or full short).

I am surprised that main fuse didn't blow. Or was there a fuse of too high value in the amp?

This is why Fender and other manufacturers put thermal fuses inside the transformers set to blow at 130C. Once they blow they can't be replaced.

Better play it safe.

[Amp Kat]

Fire as in red plating or fire as in bright yellow light. ? Bright yellow fire is either a tube short or a tube in wrong. If the key breaks off it's very easy to be a pin off causing this. Red plate fire is either a bias issue or coupling cap gone south on you.

[slidincharlie (Carlo P)]

Alex,
the fuse did blow.

Fire as in red plating or fire as in bright yellow light. ? Bright yellow fire is either a tube short or a tube in wrong. If the key breaks off it's very easy to be a pin off causing this. Red plate fire is either a bias issue or coupling cap gone south on you.

I don't remember the colour of the flame.
I can only say that the flame was as big as two or three matches lit together.

[Alex/Tubewonder]

Alex,
the fuse did blow.

..too late... too high value.

I don't remember the colour of the flame.
I can only say that the flame was as big as two or three matches lit together.

That question was about color of tube structure when "red plating". somehow irrelevant when PT leads burst in flames...

What is your level of electronics/amp circuits knowledge? There are some tests/measurements that can be done by an experienced tech to diagnose the transformer beyond the simple short/open circuit test.

[slidincharlie (Carlo P)]

Alex,
the '79 (pull-boost) SFVR original schematic calls for a 2A slo-blo fuse in the export as well as in the US version.
Probably 1A would have been safer, considering that modern Fender schematics (e.g., Blues Deville) call for "x" value (120VAC) and "x/2" value (for 240VAC).

I am no tech, or else I wouldn't be asking for help here .
I have been tweaking inside amps quite a bit in the last 4 or 5 years, with the help of Ampage guys.
I don't have special tools like a scope or a variac. Only a Fluke meter, a couple solder irons and lot of parts and jumper/connection wires.

[slidincharlie (Carlo P)]

Should I definitely buy a new power tranny without any trial to tell if the current tranny is toasted?
Is there any chance that the tranny is good and the trouble is somewhere else?
I confirm anyway that the tranny was very hot after playing (and it stayed hot for maybe half an hour), and it never went hot before.

[mykey]

Should I definitely buy a new power tranny without any trial to tell if the current tranny is toasted?
Is there any chance that the tranny is good and the trouble is somewhere else?
I confirm anyway that the tranny was very hot after playing (and it stayed hot for maybe half an hour), and it never went hot before.

are you sure this isn't a mesa boogie?

[Konny]

Perhaps the tranny isnīt damaged but if you arenīt familiar with such problems you should go and buy a new one.

Even if the tranny isnīt damaged itīs isolation is probably weakened and about to short if it gets stressed by any means.

[Tom Phillips]

Should I definitely buy a new power tranny without any trial to tell if the current tranny is toasted?
Is there any chance that the tranny is good and the trouble is somewhere else?
I confirm anyway that the tranny was very hot after playing (and it stayed hot for maybe half an hour), and it never went hot before.

If it were my tranny, I'd unhook all the seconday windings and power it up. If it din't get overly warm (unfortunatly, that's a judgement call but a good one will barely rise over ambient temp) after a couple of hours, then I would jugdge the PT OK to use. Then, of course, you need to still find the root problem.
Tom

[slidincharlie (Carlo P)]

Tom,
this is exactly what I wanted to hear!
I'll need a couple days to have the job finished (all the primary wires at the VAC selector's end are burned), then I'll post the results.

[Alex/Tubewonder]

This is a dubious advice. If you're going to follow it use a fuse not stronger than 50mA. It will still not give the transformer a clean bill of health.

[Enzo]

50ma? You mean 500ma? A 50ma fuse would be quite expensive, and surge current in the iron would likely pop it.

If flames came out of the transformer, I would replace it. If the transformer made bubbling sounds inside I would replace it. Yes, even if it tests OK.