Super Reverb problem

[stompjunkie]

I've got a fender silveface super reverb thats been confusing me. It came to me popping fuses. I noticed that it had a burnt quick blow in there, so I used a slow-blow and everything was just fine...till a few moments later when I noticed that one of the power tubes was goin all lava glow.... So I figured that the tube was probably going, and swapped for a new set of GTs. A few seconds after the standby is thrown the fuse pops. Argh.....

So with the old tubes, it will stay on, but the one looks like it's gonna blow. and when I tried the new ones again, pop goes the fuse.

As an interesting side note, I measured the voltage at the tube. Old tubes 458 (glowey) and 463. With the GTs it was 389 (in glowey's spot) and the fuse popped before I could get the last reading.

Any help will be greatly appreciated. My many thanks.

[dehughes]

This is probably not your issue, but I just worked on a Silver Face Pro Reverb that kept randomly blowing fuses. It turns out that one of the older, cloth covered PT primary wires had a very thin lining that had worn nearly through, and was in such close proximity (basically touching...) to the polarity switch that, depending on how the amp was handled, it would short to the switch and pop the fuse instantly. This probably won't explain your glowing tube, but....

[stompjunkie]

I appreciate the tip. Issues like that can be very frustrating, being so dificult to find...... I went through a few times checking connections and lead dress. It's a bit of a sloppy piece, but nothing looks like it's touching, and the connections look ok. Thats my usual first step. It's right up there with the poke-it-with-a-stick test.

[Bob-I]

Sounds like a potential bias problem. Check out the bias voltage at the tube socket.

[arwokc]

Stompjunkie,
You can find a layout with voltages here: http://www.ampwares.com/ffg/schem/super ... layout.gif

As you can see, your negative bias voltage should be around -51v. You can find that figure up by the bias balance pot. It would probably be easiest to check where the lead runs to the 100k resistor towards the left end of the circuit board. Also check your resistors across the power tubes to make sure they are within range of what the layout shows they should read. Don't forget the bias range resistor up on the bias board that is wired between the bias lead of the transformer and the bias balance pot. Remember, only a handful of the very early silver face super reverbs had a true bias system. After that, they had a bias humm balance type of adjustment. You might check this link out to convert it to the black face spec if you feel you can pull it off. http://www.el34world.com/charts/bias_conversions.htm
Personally, I would be suspicious of the side of the bias that leads from the tube that is glowing. I would start at that tube socket and work backwards. I hope this helps.

Andy