Bad bias pot - Hot Rod Deluxe

[lowell]

Hey all,
This amp left my shop in great working order only to come right back to me w/ glowing power tubes (5881WXT). I immediately thought that it must be that pot because all was well and good when it left.
So I opened 'er up and got 190ma!!!!! ouch... I clipped the meter on so could mess w/ bias pot a bit and the MOMENT I touched that bias pot w/ the screwdriver the current shot down to 30ma. This is possibly the solder joints on the pot, but if not should I replace w/ the same type pot? Mod w/ better pot in different location? Or set bias w/ fixed voltage divider resistors? Just wondering what others find the best solution here. I'm sure many have dealt w/ this issue before.

[Enzo]

What would you do if it were a volume control that jumped if you touched it? I'd squirt some cleaner on it and turn it back and forth a couple times. See what that does.

Never seen cracked solder on one of those, but there is a first time for everything, can't hurt to check or just reflow it anyway.

[tim]

I had one of these pots fail last year on an amp I repaired....they glyptoled the wiper at the factory (paint to hold wiper from moving) and when I tried to move the wiper part (it took a bit of force I might add), it must've cracked the substrate, causing an intermittent contact. I just replaced the pot with resistors and sent it on its way...I ordered a couple of pots from Fender in the mean time so when it comes back (HRDlx always come back eventually), I'll replace the resistors with a pot.

[Diablo]

That looks like a doomed bias design. If the wiper fails to make contact, the bias voltage is gone and the tube runs away. I don't know why they wouldn't have used one of the standard old designs where the wiper is used to bypass the excess bias voltage to ground.

[ahamay79]

If it's the same that's in the Deville, which I'm sure it is, you can get an exact replacement from Digi-Key for about 50 cents. Part # 201UR253B-ND

Dave

[Enzo]

Also, check closely the solder on the powr tube sockets. That likes to crack on those amps, and cracked solder to a grid pin would look just like a bad bias supply.