Champ Build No Longer Working

[yodaeatsfishstx]

So I completed my first build, a 5f1 tweed champ. Everything worked fine for about a week.

I went to go use it one day and it would not work, it had power, the fuse was fine, the tubes all lit up, nothing obviously wrong...

The 10k 2w B+ resistor on the bottom left of the board was burnt out.

http://4.bp.blogspot.com/_BYIkYo9cnn...5f1_layout.gif

It looked like it was slowly baked as opposed to just burning out abruptly.
I ordered the replacement for it, but what would cause this? How could I prevent this from happening again?

Thanks!

[dchang0]

Drawing too much current through that resistor somehow... Too bad its so early in the circuit--it could be anything behind it that's sucking current. That's practically everything to the right of it, LOL. It could also be everything to the right of it.

Might be that it's only that resistor that went bad. Replace it and then check voltages without the tubes installed. I had a cathode bypass cap blow up and thought I had miswired something but didn't find anything wrong. Put another one in and it worked fine, without changes to the rest of the circuit. It was even oriented properly. Later, I measured the voltages, and it seems that I'm just barely over the rated voltage limit.

BTW, I upgraded both the 10K and the 22K resistors to 3W. Probably shoulda gone to 10K wirewound 5W to change the tonal character a bit.

[ThermionicScott]

While I'm totally in support of careful engineering, 5W seems like overkill to me. The preamp tube shouldn't be drawing any more than 3-5mA total, and with the 6V6 screen voltage so far below the plate voltage, it shouldn't draw more than a couple mA unless it's cranked up all the way. I think we should be checking for miswired components or accidental ground connections before slapping really big resistors in there.

I smoked the 10K resistor in my 5F2A once, but that was when I accidentally plugged in an incorrect power tube and shorted the screen to ground!

- Scott

[dchang0]

After double-checking the wiring without any power, he needs to get something in there resilient enough so that he can begin to check voltages. Considering that the amp worked for a while, it's not all that likely that there's a miswiring (or at least not a major miswiring like an outright permanent short).

I get what you mean, though, in that slapping big resistors in there is not a permanent band-aid for a serious underlying problem.

[mooreamps]

I wonder if either :

1. One of the High Voltage Caps failed.
2. The polarity of one of the High Voltage Caps is reversed.

-g

[MWJB]

YodaEFS: It's difficult to ascertain what's wrong, precisely, because you have just linked a layout (that's wrong anyway) & not actual pics of your build. If you built the amp exactly to that layout it would not work right...even if it was correct, it's still not the layout of your amp, because yours has another issue. Please post pics of the circuit board, tube sockets & pot wiring.