Bias problem- 1 socket shows extremely low bias!

[otokomae]

Ok, I'm working with a Fender Supersonic amp:

http://www.fender.com/support/amp_sc...0(Updated).pdf

I thought I'd put the schematic up front, as people often have a hard time finding it. Anyway...

I've run into a problem with the amp, where one socket (V8) has started showing an extremely low bias of -0.04 ma no matter what tube I put in it! I've used a couple of sets of tubes that match up fine in other amps, but keep getting this weird -0.04 ma reading in just 1 socket of the Fender Supersonic. Does anyone have any ideas about what might cause this? I'm not really sure where to start looking for this, even; negative bias circuit? Any help is greatly appreciated, and I'll be checking this forum at least once a day if anyone has any ideas! Thanks!
(^_^)

[otokomae]

http://www.fender.com/support/amp_sc...0(Updated).pdf

[Chuck H]

Start by pulling the tubes and checking B+ for the plate and screen and the -bias voltage for V8. Compare your findings to the other sockets. That may give you a clue where to look for the problem.

How are you derriving a Ma draw for an individual tube in that amp?

Chuck

[otokomae]

I'm using a Weber Bias Rite: http://taweber.powweb.com/biasrite/br_page.htm

I'll check those things again tonight! Thanks!

[Albert Kreuzer]

Could be an open screen resistor (R143) or open cathode resistor (R141). Without these the tube will not conduct properly.
I suppose you already checked the bias voltage (pin 5).

Cheers,
Albert

[Amp Kat]

Have to agree with Albert here on an open screen or something open because if you had a bias failure you would be reading high ma's and the tube would be going cherry on you or if you had a bad coupling cap would also result in high ma's or even an open current sense resistor.. Check the connectors and the voltage on the screen resistor before the tube and at pin 4 of the tube socket. Either a bad connection or possible open screen.

[drray]

Bias Probe

I had a simular problem a couple of years ago. I was biasing my Tremolux, and one tube acted dead like yours, I discovered it was not the amp but the bias socket from the tester that was bad. I would swap the tester sockets in the amp and see if the problem follows the tester socket.
Just my 2 cents, Ray

[bnwitt]

What millivolts dc do you get on test points 49 and 50? there are 1 watt resistors already on that board for bias testing.

[Enzo]

I'd be looking for an open screen resistor also. Is there B+ voltage on pin 4 of that socket.

And I didn;t notice, did we try swapping tubes to make sure it wasn;t a dead tube?

[otokomae]

Well, I changed the screen resistor at R143 to a 5 ohm, and the amp seems to be running fine now! Does any one have any ideas of what would cause a screen resistor to burn out on a Fender amp that's only 2 years old (it was clearly burned, almost completely brown outside)? Is it just from playing a high volumes, or is the 1 watt resistor that was originally in there really too low wattage for that position? If anyone has any ideas of examples of things that could cause this, I'd love to hear them so I check things to make sure I don't have the same problem again! Thanks for all the help from everyone who wrote on topic, it was really helpful!!!
(^_^)

[otokomae]

whoa, I meant 5 WATT, not 5 ohm! It's still 470 ohms~! lol

[Enzo]

The age doesn't matter. For any resistor to burn up, excess current must have flowed through it. There is only one current path through a screen resistor, and that is through the tube. Unless your socket arced to ground at the screen pin, I can think of nothing other than a failed tube that would burn up a screen resistor. Tubes can fail at any time.