Help with Fender Ramparte

[Falken]

Greetings Everyone!

I have a Fender Ramparte that I use as a "living room" amp. It's lower wattage and looks more like furniture than a typical amp, so it works when I'm sitting around in the living room and wanna jam quick. If you're not familiar with the odd-ball Ramparte, it's a Single Ended 6L6 powered amp, about 9ish watts, with a pair of 12ax7s in the preamp, no tone stack or master volume control.

The other day I was playing it and the amp suddenly went quiet and some of the magic blue smoke escaped. After inspection, the two 100ohm resistors off the secondary that supplies the heaters were toasted. Visually, everything "looks" fine, save for the 2 resistors in the heater circuit. I've checked the voltages on both the secondaries off the PT, and they seem fine. Heater is 6.3v. Both secondaries, and primary, have a bit of resistance, and aren't shorted. No blown fuses anywhere. No caps are popped. I replaced the to 100ohm resistors with half watt metal film (exact replacement). I powered it up without the tube in it, and everything appears fine. The B+ voltage is within spec, the voltage across each resistor is a little over 3v, the 6.3v powers the lamp just fine. I put the preamp tubes in, everything is still fine, same story. As soon as I add in the 6L6, those resistors start heating up and are ready to smoke again.

I have a new 6L6 on it's way, but is there anything else I can/should be testing before I risk damaging a new tube? The old 6L6 tube has continuity across the heater pins, and the other pins (plate, grid, screen, etc) are not shorted to each other or to the heater pins. Granted I'm not testing this tube under load or once heated. Anyone seen a tube blow out the heater circuit resistors before?

Thanks in advance for any help or suggestions!

[Enzo]

As soon as you said the 100 ohm resistors were burning, in my mind I said - bad power tube. AS I read further, you verified that. get a new power tube.

[Randall]

What Enzo said. The filament in that 6L6 is shorting out.

[Leo_Gnardo]

Besides the output tube being a major suspect, also have a close look at its socket. There may be arcing between pins 2 and 3, and that may have left a carbonized track which would cause the same problem again if a new tube was installed. If you see an arc track it would be best to replace the socket as well as the output tube.

[Falken]

Besides the output tube being a major suspect, also have a close look at its socket. There may be arcing between pins 2 and 3, and that may have left a carbonized track which would cause the same problem again if a new tube was installed. If you see an arc track it would be best to replace the socket as well as the output tube.

Appears to be clean as a whistle.

Thanks for the replies everyone.

Is it a common failure for a tube to short out in this manner? Previously my tubes, preamp or otherwise, have failed gracefully.

Thanks.

[g1]

It's not all that common for power tubes, but it does happen.
It can also happen if the tube's guide pin at the center of the pins gets broken off and the tube gets installed in the wrong orientation.

[Enzo]

And really, it doesn't much matter if it is a common failure. If it is one in a million, if there are a million tubes, SOMEONE gets that bad one. Remember, if you are a one in a million guy, there are ten of you in New York. As a pro bench tech, I never worried much what happened to others, I mainly focused on the amp in front of me. Statistics do not apply to individuals.