Bad Cat Panther with creeping bias

[ahamay79]

Hi guys.

I've got a Bad Cat Panther on the bench. All new tubes, all good, resistors in power section all check out, no schematic, cheap bias pot replaced with 50k / 20 turn cermet pot.

Bias creeps up from 30ma per tube to 40ma per tube in less than 3 minutes. I turn it down and it continues to creep up.

Any ideas? Diode maybe?

thanks,

Dave

[black_labb]

could there be a bad resistor that is effected alot by heat? (ie heat increasing the resistance or silimar).

i assume someone hasnt put a way too big cap in there?

[ahamay79]

All the components are factory stock.... except for the new bias pot.

[Enzo]

Well, first, watch the grids. Does the bias VOLTAGE remain stable there?

The power tubes may well be good, but did you try a different set of good tubes?

WHat about after 15 minutes, does it stabilize? I never bias an amp until it has warmed up at least that long.

[ahamay79]

Tried 2 different sets of new tubes.... same problem.

I chased the bias for about 15 minutes. I eventually ran out of bias pot range.

I'll check the grids.

Thanks,

Dave

[cudamax2343]

Hey, how are-ya. I really don't know much about amps but someone passed this site on to me when I was having a problem. It's all about run-a-way Bias and heating up certain componants with a blow dryer and about PC board material. I know its on Marshall but maybe it will help you. Good luck.
http://www.lynx.bc.ca/~jc/TSL122.html
Let me know what fixed your problem.

[ahamay79]

Thanks for the link!!! I work on a few of these Marshalls on occasion, too. Any extra info is ammunition for the brain.

Dave

[cudamax2343]

You bet-cha. Got it from Marty54 at the Marshallforum.com Real smart dude.
Don't forget to let me know how it turns out.

[ahamay79]

Hey guys,

I finally got to crank up the Panther again to day. Here's what I see:

I'm using bias probes in the sockets.

After 2-3 minutes warm up:

Bias: 30ma
Plate: 425 vdc
Pin #4: 417 vdc
Pin #5: -36.4 vdc
-82 vdc across the bias cap
----------------------------------------
... a couple of miutes later:
Bias: 34ma
Plate: 417 vdc
Pin #4: 408 vdc
Pin #5: -34.9 vdc
-82 vdc across the bias cap
------------------------------------
a couple of minutes after that:

Bias: 36ma
Plate: no reading taken
Pin #4: no reading taken
Pin #5: -33.9 vdc
-82 across the bias cap
----------------------------------

I adjusted it back down to 30ma and I watched it creep back up to 36ma in a few minutes.

And again I adjusted it back down to 30ma and I watched it creep back up to 36ma in a few minutes.

And when I adjusted the bias again back down to 30 ma it started to creep DOWN. Okay...

I noticed it was doing that when the A/C was running. I covered the chassis with cardboard and it started to creep up again.

I sprayed compressed air on the diode and the bias started to drop a little, then started to creep back up.

So.... heat sensitive parts? bad parts? Ghost in the machine?

Headache. yes.

Thanks,

Dave

[Enzo]

Notice how your bias voltage keeps dropping? When the bias voltage drops, the tube current rises.

DO both tubes act the same way?

How about the bias supply itself. Is that dropping? or is it only dropping out at the tubes?

[ahamay79]

Enzo,

The voltage at the bias cap never changes.

All four tubes act the same.

Also, something in there is heat sensitive. When my A/C turns on, the air blows a little on my bench and I see the trends start to reverse (ie, bias creeps down, plate creeps up, etc).

I waved a fan over the chassis and the bias crept down. turned the fan off and the bias crept up.

I put a 100 watt lamp over the chassis and the bias creeps up faster. I turn off the lamp and the bias slows down.

... Yep, I can adjust the bias down just by blowing air on the 470 ohm/5watt resistors.

[Enzo]

Do you have freeze spray? You can localize what you chill with that. Furthermore, using a small piece of carboard, you can shield some parts from its blast right next to other parts. See if you can find exactly what is the sensitive element here.

[ahamay79]

I'm going to get some right now.... Thanks !!

[ahamay79]

I found the problem (I hope).

470 ohm / 5 watt resistor tests fine, but when I hit it with the freeze spray the bias drops. The other resistors do nothing when hit with the spray.

I have some resistors coming in in the next few days and will give update.

Thanks !!!!

Dave

[ahamay79]

Hey guys,

I've been busy with other repairs and haven't got back about the Bad Cat bias issue. Well, I changed out the suspect resistor only to have the bias creeping up again, as usual. I'm about to just start replacing parts until the darn thing acts right.