Hum balance pot in Marshal 1987x reissue amp emits FIRE

[asfi]

Frankly, I'm finding this a bit baffling. The hum balance circuit in a 1987x reissue amp suddenly self-combusted in a spectacular manner. Now, if this beast had been struck by lightning I'd expect other components to be charred as well, but nope......

At any rate, I guess it makes for some fun "disaster porn" photos!

[pdf64]

It may be that a tube has had its filament short to another element, thereby sending a lot of current to ground via that resistor.
My thinking is that those ground ref resistors need to be a high power rating in order to maintain a solid reference in the face of such faults, to protect the other tubes.
Otherwise they may get referenced to VB+ and the heater-cathode insulation damaged.
Consider 3 or 5W replacements.
Pete

[big_teee]

Damn!
That's Scary Looking.
T

[Enzo]

Agreed, burnt up heater balance resistors almost always are caused by a short to heater in the power tubes or their sockets. If pin 3 arcs to pin2, you get that. Inside the tube, something shorting to heater does that.


Or someone has a EL34 with a broken off center pin, and the tube got stuffed in the hole with the wrong pins.

[asfi]

Hahaha, of course. One of the EL34's has about 600 ohms between heater and plate.....nope, that's definitely not supposed to be there.

Replacing the fixed resistors with great big beefy ones is all well and good but wouldn't that just make the trimpot ever more vulnerable to death by magic smoke emission?

[pdf64]

Yes, sorry, it's obvious now you point it out but I assumed it was a bias pot or something, never seen a heater balance trimmer before.
Maybe use chunky higher value resistors to ground, eg 2x470 ohm forming a secondary ground ref, to take over the next time one of the trimmer circuit resistors blow?
Pete

[Enzo]

When a power tube shorts to heater, SOMETHING is going to burn up. I don't look at every circuit failure as a design flaw. We can put high power parts in place of the small ones here, but that just pushes the failure to some other point. In all amps, how often do you see this happen? I see it now and then, but not really often.

You put 5w resistors in and even a 5w rheostat in place of the pot, and next time this happens, those won't burn up, but then you'll have a copper trace burn off the board like a fuse blowing.

[pdf64]

Yes, something has got to give, at least Marshall have a tradition of fusing the B+, so hopefully everything else taking the fault current avoids being too compromised by the time it pops.
My concern is that an amp's tubes may all have their heater-cathode insulation damaged by this failure mode and that represents quite a significant replacement cost, equivalent to a transformer.
Pete

[Enzo]

That is a valid concern, but this failure mode is CAUSED by bad tubes, so they are going to be replaced anyway.