Fireworks

ICBM

It can if the amp isn't properly designed.

The initial fault was that the valve shorted between pins 3 (plate) and 2 (filament) - this is quite common but in an amp with an HT fuse that should be as far as it goes - the fuse blows at that point.

In this one the burning valve base coated the valve socket with soot which then caused a major arc to occur on the socket as well, generating enough heat to melt it, the plate terminal, the wiring and even burn the chrome plating on the chassis around it a bit.

It also burnt out the two 'virtual centre tap' resistors on the board above with further heat and soot damage.

At no point did the mains fuse blow - or the filament fuses, which wouldn't unless the melted wiring loom had shorted, which it didn't actually.

I don't know for sure yet if the transformers are OK - probably, but not guaranteed.

All because there's no HT fuse… for some reason it doesn't seem to be standard practice in American amps.

martinw

This is what happens when the fuse (mains) does blow, but you keep fitting bigger ones to "fix" the problem:

jonnyburgo

Pph!. Nerds...

menamestom

martinw said:

This is what happens when the fuse (mains) does blow, but you keep fitting bigger ones to "fix" the problem:

Is that PK's Blues Deluxe? I remember when he told me about it, his mate had just come round and said 'do you know your amp is on fire Paul?' If that's the one you did a good job bringing it back to health!

martinw

menamestom said:

martinw said:

This is what happens when the fuse (mains) does blow, but you keep fitting bigger ones to "fix" the problem:

Is that PK's Blues Deluxe? I remember when he told me about it, his mate had just come round and said 'do you know your amp is on fire Paul?' If that's the one you did a good job bringing it back to health!

Yeah, that's the one. A fair bit of Dremeling required, but I brought it back to life!

'"Do you know your amp is on fire Paul?"

"Dunno, what key is it in?"

ICBM

martinw said:

Yeah, that's the one. A fair bit of Dremeling required, but I brought it back to life!

Impressed! Although I assume that once you'd cleaned all the soot off it wasn't quite as bad as it looks there…

I've done a few Marshall DSL/TSL boards like that - cut and scrape out all the carbon and damaged traces, wash the board thoroughly, and jumper over the holes.

There does come a point where replacing the board is the only option though.

ChrisMusic

Where is the "ouch" button when you need one !

Dave_Mc

^ +1 wow

martinw

ICBM said:

martinw said:

Yeah, that's the one. A fair bit of Dremeling required, but I brought it back to life!

Impressed! Although I assume that once you'd cleaned all the soot off it wasn't quite as bad as it looks there…

I've done a few Marshall DSL/TSL boards like that - cut and scrape out all the carbon and damaged traces, wash the board thoroughly, and jumper over the holes.

There does come a point where replacing the board is the only option though.

Not quite as bad as it looks, but up there! Fortunately there wasn't that much at that end of the board, so with a bit of cleaning up, cutting out the damaged board, bypassing tracks and relocating components, it was just about salvageable. The fire occurred at the 2  x 100ohm 'dummy centre tap' resistors, which had just about disappeared!

The amp also had burnt out screen grid resistors and board damage at the other end too. :-S

ICBM

martinw said:

Not quite as bad as it looks, but up there! Fortunately there wasn't that much at that end of the board, so with a bit of cleaning up, cutting out the damaged board, bypassing tracks and relocating components, it was just about salvageable. The fire occurred at the 2  x 100ohm 'dummy centre tap' resistors, which had just about disappeared!

The amp also had burnt out screen grid resistors and board damage at the other end too. :-S

I'm pretty sure I've seen one of these (pics online rather than in person) where someone had replaced the lower board with a hand-wired circuit using chassis-mounted valve sockets and a tag strip, so I assume it had probably gone the same way. Few enough components that it would be just about economical.