Fender Supersonic: Bias going CRAZY, Weird Sound...

[otokomae]

OK, now I'm really perplexed:
I have a Fender Supersonic that, until a week ago, was a great amp! Then after changing tubes and resetting the bias, it stopped making sound altogether. So I took it home, did the "Chopstick Test", and found that when I touched the 2 straight wires that connected the speaker jacks, the sound would come on, then off again when I took pressure off. The solder joints looked weak, so I resoldered them with new wire, checked and double-checked to make sure I'd done it right and connections were solid, and played it to make sure it was working. Everything seemed fine as of yesterday...

Then today, I started playing it, it sounded fine, and suddenly it cut out! I turned it off, hooked up the Weber Bias Rite, and checked the numbers. At first everything was normal (470 plate-volts, 17ma), then suddenly the numbers went CRAZY!!! The plate-voltage dropped slightly to 450-455, and the bias SHOT UP to 100+ !!! Now, when I turn the amp on all I hear is a loud "Waaahhhhh....." sound, and there is a slight burning smell.

I tried changing tubes, btw, and it made no difference. Can anyone offer any advice as to what might be causing this, or where to start looking? I've just gotten home, so I'll open it up soon to see if anything has obviously burned out, but what would cause something like this to happen in the first place? And where does the noise come from?

I'd really appreciate any help, or any ideas anyone might have! Thanks in advance guys!

[Structo]

I had a hard time finding a schematic for this amp.

It has a lot of opamps and transistors in it so it may be hard to work on verses an older all tube amp.

Does this amp really have 10 tubes?

I was reading a thread over at TDPRI and a guy was hinting that this amp had some bias issues on the first run of amps. His was a May 2007 and he said his didn't have the bias issues, which led me to believe they have had some design problems.

In looking at the schematic, it has 1 watt 470 ohm screen resistors.
1 watt on the screens can be a bit low on the power side if the tubes do any funny stuff.
So I would pull the chassis and without touching anything in there,look to see if you can find the screen resistors to see if they look burnt.
They are R142 and R143.

According to the schematic I believe the bias should be around 32 mv.

Here's the schematic.

http://www.fender.com/support/amp_schem ... dated).pdf

[otokomae]

OK, the resistor @ R142 has a brown spot in the middle. I can replace it simply enough (don't worry, the amp is already drained.) But what would have caused it to go in the first place? Is R142 burning out really enough to cause the bias to go all crazy like that?

Thanks so much for your help, btw!

[Wayne]

... Is R142 burning out really enough to cause the bias to go all crazy like that? ...

Probably the other way around - the bias circuit screwed up allowing the tubes to draw excessive current, taking out R142. If this is what happened the tube associated with the burnt resistor is probably either dead, or at least much closer to the end of it's life.

I would replace both 142 and 143 with 3 or even better 5W units, remove the output tubes, then look in the vicinity of R154 - R157, C85 - C87, D43 & D44 for the bias problem.

Someone with more specific experience with this particular amp may be able to narrow it down more than that.

As should always be stated - if you aren't damn sure what your doing in there, please take it to someone who is. A 500V shock is at best unpleasant, and at worst fatal.

W