Marshall DSL401 defective?

[drewl]

Got one on the bench that had the internal 6.3A heater fuse blown, replaced it and biased and all seems well.
Spoke to the customer who's had it serviced like 3 times this year, had the output tubes replaced, fan installed and it keeps having the same problem.
Is there some inherent defect to these amps why they keep blowing the heater fuse?
He had a Marshall tech tell him the rectifier bridge was too large and draws too much current....anybody hear of this problem?
Maybe try a different brand of EL84's?
Just curious if anyone else ran into similiar problems.
Thanks.

[Enzo]

Rectifier bridge too large and draws too much current??? That's a good one. The rectifier doesn't draw any current, unless it is shorted. Usng heavier rectifiers certainly won't stress anything. Of course heavier rectifiers allow the rectifier to survive longer than maybe some other part when faced with a shunt. But really...

I wouldn't be thinking there is some inherent problem with THESE amps so much as I would be thinking THIS amp has a problem.

Put your ammeter in place of that fuse. Under normal conditions, what draw is there. should be pretty steady I'd think after a few seconds warmup. Is the current elevated? That is a large percentage of the fuse rating?

FOur EL84s at about .75A and thre 12AX7s. That sounds like about 3.9A through the fuse, more or less. If the current reads about that, then we look certain places. if the current is a lot closer to 6A, then we look elsewhere. Either there is a steady problem, or there is an intermittent spike.

EL84s are not the most robust of tubes. When tubes fail things short to other things inside. Any HV short to heater will take that fuse most likely. SO occasional tube short on that spike list. The small tubes run on DC, so while I don't expect a heater short in one it is possible, but an intermittent short in the bridge or filter caps is on the list. SO is a bead of solder loose in the wrong place.

On the steady failure list, a very leaky filter cap could drag extra current. FOr that matter a failing bridge could as well.

That little heater bridge could be bad, but not because it was too large. Possibly too small, but never too large. You could stuff a 35A bridge in there and it would work fine.

[drewl]

Yeah, I just posted what the guy was told.
I've found other posts on this board and others about that bridge failing or the leads getting corroded and causing problems.
What you suggested was the route I was going to take.
If I find something odd I'll report back.

[drewl]

Status report for anyone who may run into this problem.

Amp draws about 8a momentarily through the heater fuse at power up dropping down to about 4.3 amps after warm up.
At full output the current drops down to about 4a.