Marshall DSL100 Humming

[bajaman]

still have not resolved this biasing problem - and now 12 months later I have another one - read up and I see that I am not the only one having problems with DSL 100 amps - the solution appears to be to ditch the board and replace it either with a proper fibreglass board from Marshall service or hard wired tube sockets mounted direct to chassis.
Any one know how to fix one of these (stop bias climbing until tubes burn out) yet !!!!!!
bajaman

[Enzo]

Haven't found one yet I couldn;t fix, and have not noted any systematic failing in this area.

This affects all four tubes at the same time or only one side at a time. Either the same side always or one side now and the other another time. or it affects only one tube. ANd that tube is either always a particular one, or it moves from socket to socket.

The bias voltage drifts or not. If it does, then either the supply itself or the circuit path from the supply to the grids is compromised, or there is a leaky coupling cap. Or of course a tube can be bad. More than one tube can be bad. Brand new tubes can be bad.

There are several small caps under the trimmer board in the drawing. ANy of those is suspect, as are the resistors in that area. The bias supply draws from the power transformer through capacitors. If they let you down, the bias supply ebbs away.

And did you get rid of C46? That is the little disc ceramic cap between pins 3 and 4 of the end power tube nearest the power transformer.

[bajaman]

Just had an interesting phone call from John G (a member here) - he, like me lives and works in New Zealand (Northern Antarctica).
Anyway from our telephone conversation, it appears that the problem is mainly confined to the earliest board layout (the one with the 220k grid stoppers fitted).
John's solution which he has posted here is to cut the tracks at the grid of each power tube and fit stand up 5ks stoppers direct to the valve pins. He also lifts the 220k bias feed resistors and the PI coupling caps on the -ve bias grid sides, and runs wire (like telegraph pole lines) between, connecting the grid, biasing resistors and the coupling capacitors. After these mods he says he has never had a DSL come back with the runaway creeping bias syndrome problems. I won't tell you what he said about the TSL amps though.
Anyway - thanks John, I am going to try your suggestions and I will let the forum know the results - after all, I have two of the fckin' things here in the workshop.
bajaman

[j45]

dsl

Hello,

Had a really hard time with one of these. Bias creeping up.
You would do yourself a great service and pay very close attention to what Enzo replies. He is able to point you in the right direction , for sure.

My problem was a bad tube. It did fool me and I chased ghosts.

Hope you resolve.

J45

[AmpFix]

So was this ever resolved?

[bajaman]

Yes
I purchased a brand new replacement board through the local Marshall service agents - they admitted they had seen similar problem DSL 100 amplifiers. My board was an issue 5 - the replacement was issue 10!!!
AND when I set the bias it balanced perfectly with all the old tubes - 5 hours later the bias current had actually DROPPED slightly
My advice - if it is less than issue 6 and the tubes test okay - replace the bloody board and save yourself a lot of anguish.
Cheers
Steve

[bajaman]

The faults ( apart from the creeping bias) completely disaappeared with the installation of the new board AND after 5 hours on soak the power transformer was stone cold to the touch unlike previously after 30 minutes ou could fry an egg on it!!!!
I must emphasise - ALL THE OLD TUBES WERE fitted with absolutely no problems - customer has had it back for a month or so now and LOVES it
bajaman

[rf7]

Hooray, (sarcasm intended) it's back in the shop again and still humming like a beast. A power supply resistor went open, that was an easy fix, but this hum is extreme. It's got a 60Hz period, but there is a weird zero crossing in the middle of it too. It seems to originate in the PI section, but I seem to see it everywhere in the preamp section with the scope too. I tightened all of the ground connections, but that did not help. I had to replace the heater wires from the PT, they had looked like they were going high resistance. It's not tubes, I already swapped them out. If I disconnect everything before the PI stage it actually gets a bit worse.

I think I'm going to stick a hum balance pot on the heaters and see if that helps.

[AmpFix]

If the hum is 60Hz, a heater balance pot will not help, not counting the fact that the preamp valves are powered off of DC.

If you really want to eliminate the heaters as the source of noise (and they do cause some noise), disconnect the heater tap from the PT and connect a 6vdc power supply to the tube board. When powered this way, my DSL is dead quiet. Its a little older (1997) and I think that the heater supply caps need replacing.

Does the amp hum when you plug in directly into the effects return jack?

[rf7]

If the hum is 60Hz, a heater balance pot will not help, not counting the fact that the preamp valves are powered off of DC.

Only the first 2 tubes are run off of DC, V3 and V4 (the PI) run off the AC bias along with the EL34's.

I think that the heater supply caps need replacing.

Tried that yesterday.

Does the amp hum when you plug in directly into the effects return jack?

Yeah, it does not affect the hum.

[rf7]

Ran tubes V3 and V4 off of 6Vdc heater voltage, and I still get hum. It's a little bit smaller on the scope but not that big of a difference. Oh well.

I actually see the same hum on the chassis with a scope, with the scope grounded to the chassis??