Marshall DSL100 bias creep

[Award]

Hey Folks, please check out the website:http://www.lynx.net/~jc/TSL122.html, an expert modify TSL bias circuit and other, it should solve the damn problem...

[Mars Amp Repair]

Well,
I read the fix for the Marshall TSL bias creep issue. I have to say that I've worked on dozens of these DSL's & TSL's & have never had a bias creep issue.

One issue I do have with fixes like this is that this is just ONE with bias issues of HUNDREDS that don't. I'd have to see a real pattern before I'd go to these lengths on every one I see.

However, that said of all the things suggested on the website you posted I will definitely heat the bias resistors & see if they cause drift.

As for a single bias adjustment, I would not bother with that one unless I owned the amp myself. If you're looking to make a living at this, there's no time to make an amp 'perfect' & quite often no need to, either.

I am also very surprised that the author has not run into problems with those tiny capacitors that feed IC-5b (C36 & 37) that get leaky as a result of shorted power tubes. Those little guys cause more bias issues than I can say. I do replace them with at least 100V caps (space is an issue) whenever I run into power tube problems. The tubes just have to short once to over voltage these little guys & cause leakage.

Anyway, my 2 cents. glen

[Pedro Vecino]

The fight continues...
A brief introduction: the problem about which we speak is that in a pair of EL34 (one side) the flow rises over the time whereas in another side itīs stable.

My experience:
> With all tubes installed adjusting every side to 36mA per tube, in ten minutes one of the sides rises to 41.5 mA... And continues rising. The other side remanis stable.
> I have observed that without PI tube installed the amp recovers his stability and returns slowly to 36mA per tube in both sides.

John G idea seems to be very related to this effect that I comment:

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.

But... it does not seem to explain completely the real reason. The tracks continue being in his place. The only difference is that there is no signal circulating along them.
What type of contamination can generate this effect? Can someone explain why without PI tube this problem does not appear?

All ideas are welcome

Thanks to all

[Mars Amp Repair]

Hello,
sorry I didn't have time to read all the prior posts, but I wonder if it's possible that the amp is oscillating at a very high RF frequency & that's why when you pull the PI tube, the problem resolves itself (oscillation stops).

these amps are also notorious for the anti-oscillation disc cap (22pf @ 1KV) that's connected from screen grid to plate on the power tubes that burns up. Perhaps the entire issue could be resolved by replacing that cap. Just a though ;-] ...g

[Pedro Vecino]

Thanks, Mars Amp. Yes, this cap has been replaced with one of 1000 volts with no results (In other TSL/DSL simply I have withdrawn and I have not found problems).
Also I replaced caps 36 and 37 of preventive form with 100n/250V (you have reason: they are in dangerous zone).
The idea of the oscillation is the first thing that I thought but I dīont find explanation to the relation effect/time (5mA up in ten minutes and rising...)
Some explanation must have this phenomenon...
Thanks again. All comments are welcome

[Enzo]

Pedro, we ask the same question we asked the other guy - what is the grid voltage doing at pin 5 on all each power tube. Is the bias voltage slowly dropping? Monitoring the current at the test points doesn;t tell us why it happens, only that it does.