Burnt 100r filament resistors

[Deric]

I've got a silver face Pro Reverb (AA1069) that smoked the 2 100r resistors for the faux center tap. One of them shorted and the mains fuse blew. The other didn't look burnt but was cracked and tested open. Checked all the tubes for shorts to the filaments and they all look good. All looks good on the heater string. No arcing in the sockets.

I assume (and we all know where that'll get me) that the resistor shorted and then burned. But why did it short? It's carbon comp. Is it likely they just went bad or should I be looking for other problems too.

Thanks

[Phil_S]

Here's what I'd probably do. If those are soldered to the chassis, clip the leads as long as possible. Most people don't have the required heat to solder to the chassis. Then, use the left over leads to form hooks for soldering in replacements. Take all the tubes out and power it up. Test the filament supply at every socket. OK? Put in the rectifier and the power tubes. OK? Put in the preamp tubes. Good luck.

[rfgordon]

They most likely shorted because the power tubes had a failure.

I had an amp with a KT88 that shorted plate-to-filament, and it smoked the 100 ohm resistors. After the short, the power tube measured 1k4 ohms from plate to filament pins, when all the pins, except the two filament pins, should measure as open circuits.

If you try the amp again with those power tubes, be ready to fry the resistors/fuse again.

[Deric]

Thanks for the help guys.

I did check the power tubes for shorts. All the pins (except the filaments) do measure open.

Just curious, what would happen if you only had 1 of the resistors hooked up? One of them was just cracked and measured open. Would this create a problem?

[Phil_S]

If both filament wires go to ground, but only one has the 100r, you'll probably get an annoying hum.

[Deric]

Well... sure enough. Even though all the pins measure open, it's the power tubes. Swapped power tubes and all is fine...

Thanks for the help!