Fender Supersonic: Bias going CRAZY (120ma), 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! This forum has always been so helpful; thanks in advance for any ideas you guys might have!
(^_^)

[txstrat]

I don't know the layout of the pcb of the supersonic but can you make sure you didn't "hurt" the bias circuits solder joints or some conducting path while resoldering the speaker cables? It might've thrown the bias out of range.

Hope this helps

Matt

[psychotech]

Fender supersonic Bias.

I use a GT Bias Tool and a DVM. There is a table that shows the mv and plate voltage ratings of most amp circuits and more importantly the tubes and their ratings. Using standard tubes it's easy to see the correct voltage settings. I think the problem you are having is a bad resistor solder joint in the bias circuit or a burned resistor indicated by a burning smell. The smell can also be just heat making the chemicals in caps and on the board and components surface to give off chemical vapors that can sometimes smell like burning components. what you can do is check the bias circuit by desoldering the ground connection to the resistor network which is pretty easy if you can solder and read a schematic. once you have the circuit open you can check it end to end and anywhere in between the resistors and the blue variable resistor that controls the bias shift for adjustment. It is possible that the bias adjuster had been overheated causing it to change value or a bad solder joint causing resistance to fluctuate in the bias circuit causing flucations in the voltage readings and strange audio results. I use my bias too on both tubes starting with the one closest to the main xformer. It will be extremely cold from the factory, about 22.5mv or so instead of the proper 32mv Usually the tubes differ slightly so you can find a balance between the two by checking back and forth a couple of times to get them balanced as close a possible. Set the tube by the xformer at about 33.5mv and read the other one. It will read about 2mv at abour 30 or 31.x or so and if you move the bias around you can get them closer or further apart depending on how well your tubes are matched. once you have them both between 30mv and 33.5 you should have a good push-pull valve balance and this will be reflected in proper frequency response, tonal body and warmth and volume. This brings the tube from about 23watts each to about 30 watts or so which is the proper rating for the amp at 60 Watts. I performed this bias adjustment on my SS and I got so much more body, bottom and warmth with crystal highs and chime on clean and burn I am more than satisfied. I have owned many tube amps and currently use 2 Marshall MKII 1977 Master Models with orginal cabinets and/or 2 Peavey 5150s slightly modded. I traded a Triple recto for two 5150 and am completely satisfied. My SS is on top of an Avatar Oxblood contemporary 2x12 cab with Gold Basket weave and Heritage 30w Celestions 55hz, the expensive ones. I use a BBE SM and Intellifex LTD. The loop is the warmest I have ever heard. The fender is more tonaly dynamic than any other amp I have ever heard. It is now my main rig for live performing. I play the Hardest Metal to Blues to the smoothest Reggae and I hate to say it but it sounds better than my fav marshall through the same cabinet. Definately a bad ass amp and not a toy. A keeper for sure. Hope this helps.

Regards,
PSy

[Enzo]

The SUpersonic is hand wired, so the connections are less reliable. PC boards are far more consistent. I do far more resolders on Fender hand wired amps than on the mass produced ones.

Having said that: First inspect the solder on the power tube sockets very closely, and wiggle the wires to see if the soldered part moves. or just resolder them all.

Bias is important, but if it is off adjustment, it doesn;t result in a loss of sound.

Fender amps come with a 5 year warranty. Are you the original owner and is the amp within that period?

[Mars Amp Repair]

Well,
I don't know if this will be any help with your issue, but I have worked on a few of these amps & both had very intermittent crackling & popping sometimes loud enough to drive the speaker to full excursion.

The problem is very intermittent sometimes disappearing all together for days. It turns out to be on the front control pcb. On the trace side of the pcb the leads of the components have been folded over and end up over other traces. Normally this isn't an issues as the traces gave the green paint to insulate them, but in these amps the leads have been cut off & are actually sharp enough to pierce the traces they are overhanging.

Intermittently they short to the overlapped trace. This will drive you NUTZ! I have decided on every one of these amps I get I'm now using an exacto knife to pry all those overlapping component leads up as to preclude any future issues. They don't all overlap, so it really doesn't take that long.

I've seen 450V impressed on signal lines, so you just don't know what weirdness that could create in your case. eventhough the audio line is cap isolated, a short could still somehow cause an issue in the bias ckt....ya nevah know!

glen