Randall RGT100ES Burned Resistor

[kizzlecake]

I have a resistor on the power board that was getting hot enough to desolder a wire attached to it. I must have resoldered it well because the resistor burned up. Any ideas on what could cause this or is it just an almost 30yr component that had enough. Check this post for more pics and schematics. You'll see the white 5w 470ohm resistor next to the power caps. http://music-electronics-forum.com/t38976/

[The Dude]

Resistors burn up because something after the resistor is drawing too much current. They rarely fail for any other reason, and I only say that because I hate to say never. The only 5W resistor I see on the schematic is the 5W/500 between B+ nodes A and B. Is that the one?

[kizzlecake]

That's the guy. Maybe the caps before it are weak and need replaced?

[The Dude]

Measure from node 'B' to ground and see if you detect a short or something that might be an excessive load (low resistance). Does the amp have 6L6's?

[kizzlecake]

It has El34's at the moment but the problem existed with the 6l6's as well. I just never really played the amp more than a few minutes at a time. Resistance from Node B to ground is higher than the meter can read. I just put the probe on the board at the point where I took the resistor out and put the other lead on the case and couple other ground wires.

[The Dude]

If you are running EL34's, you may simply have too much screen current (node 'B' is the screen supply). Screen current should be negligible. If you are going to run EL34's, I would change the screen resistors to 1K. That is why I asked if you were running 6L6's. I would replace the burnt resistor and run the amp with the output tubes removed. See if the resistor still gets hot with no tubes.

[kizzlecake]

Screen resistor being the one that failed correct?

[The Dude]

The screen resistors are the 4) 470 ohm 1W resistors on the screen grids of the output tubes.

[kizzlecake]

Ahh I see, I'll switch them out when I replace that other resistor and see what happens. Thanks for the help LeBowski