Marshall DSL100 bias creep

[milosch]

Amp came in with the HT fuse blown and V5 and V6 showing signs of heat. I replaced the tubes, C46 and an open R76 (from the other pair).

I've since replaced C36, C37, C6, C7, C29, C28, R67, R69, R66, R7 in that order. I found that one of the two bias pots was reading 18K instead of 22K, so I replaced them both. I've also touched up every connection on the board from the phase splitter through all power tubes.

The bias on V5/6 is still creeping up, while V7/8 bias is solid. Yes, there are a couple of parts I have not yet replaced, but this thing is killing me with all of the dismantling. Yes, I've swapped the pairs on the new tubes. Any ideas?

[Rick Erickson]

Check the grid swamper resistors - they should be 5.6K. Marshall put 220K's in some of the early DSL's. Also check the sockets for contamination and clean them thoroughly if necessary.

RE

[milosch]

You're right, they are 220K instead of the 5.6K on the schematic. I wasn't confident in changing them since the other pair works just fine. But, I will definitely try it.

[milosch]

I replaced R7, R66, R70, R10 with 5.6K. Tube sockets look ok. Still rising, ever so slowly.

[John G]

Hi,
Trace the path of the grids back to the coupling caps of the phase splitter. its a torturous long path passing close to HT volts on the way and eventually ends up 1mm away from the splitter anode resistor.
Cleaning the pcb can help but usually I have to cut the tracks at the grids to isolate them, and at the splitter end. I lift the bias side of the coupling cap and run a hard wire to the grid thru a 5K6 stopper ( all up in the air off the pcb just like the old days ) and also lift the bias 220K and attach to the cap side of the stopper.
Crude but desparate measures for desperate times.
Cheers,
John

[milosch]

I wonder if this would necessarily affect one pair over the other as it is actually laid out.

[Mars Amp Repair]

I was wondering...I am supposing you are monitoring the 'bias' at the test points on the back when you're watching the bias creep up. Have you monitored the actual grid of the pair of tubes to see if it really is the bias voltage that is creating the bias creep?

What if the cathode resistor were changing value or something like that. The bias reading would creep up, but the bias voltage would remain unchanged...just a thought. glen

[milosch]

I decided to check the voltage across the G2 resistors to see if I could see the creep. Imagine my surprise when there was almost no voltage across R76, the first one I replaced. I'm so blind, I put a 1 ohm instead of 1K. I am surprised it didn't flame out.

I guess the other pair are trying to compensate.

Please pardon my boneheadedness.

[milosch]

Sadly not a miracle fix. I suppose I may need to try the isolation recommended by John.

[milosch]

I was wondering...I am supposing you are monitoring the 'bias' at the test points on the back when you're watching the bias creep up. Have you monitored the actual grid of the pair of tubes to see if it really is the bias voltage that is creating the bias creep?

What if the cathode resistor were changing value or something like that. The bias reading would creep up, but the bias voltage would remain unchanged...just a thought. glen

I just checked that and the voltage across R6 is rising, whereas R9 is solid.

[milosch]

Oh well... Waiting to hear back from the customer regarding his patience level. I may try replacing tube sockets, then worst case the trace bypass operation.

[Steve Conner]

Have you tried swapping round the tubes themselves? Ie, to see if the problem is with the tubes currently in the V5 and V6 sockets, or with the electronics attached to the other sides of said sockets. It's not unknown for (mostly old) tubes to go bad and start drawing grid current as they heat up, which causes the bias to creep. (You'd be able to measure a small volt-drop across the grid swamper resistor too, in this case.)

It's also possible for the problem to follow the socket even when it's in the tube. The creep is triggered by heat, so if all the tubes are old and tired, but one socket is in a hotter position than the others, the tube in that socket will start to run away first.

[milosch]

Yes. The first thing I did was replace the output tubes because of the customer's complaint about heat and the HT fuse blowing. He had been replacing it when that happened. Also, the orange labeling on V5 and V6 (GrooveTubes) was burned. Swapping pairs on the new tubes does not move the problem. It is still with V5 and V6. If the customer can wait, I'll probably be replacing all 4 sockets.

Have you tried swapping round the tubes themselves? Ie, to see if the problem is with the tubes currently in the V5 and V6 sockets, or with the electronics attached to the other sides of said sockets. It's not unknown for (mostly old) tubes to go bad and start drawing grid current as they heat up, which causes the bias to creep. (You'd be able to measure a small volt-drop across the grid swamper resistor too, in this case.)

It's also possible for the problem to follow the socket even when it's in the tube. The creep is triggered by heat, so if all the tubes are old and tired, but one socket is in a hotter position than the others, the tube in that socket will start to run away first.

[kg]

imo this is not a socket issue. what are the grid leak resistors in this amp? can someone throw up a link to a schemo?

[milosch]

Everything here:
http://www.drtube.com/marshall.htm

Power amp:
http://www.drtube.com/schematics/mar...jcm2-60-02.pdf