Marshall Super bass red plating

[todor]

Hello everybody!

I'm reparing a Super Bass amp and am stuck with a weird problem at the moment. I've replaced all electrolytes in the amp and also had to put in one new tube socket as the original showed signs of arcing in the past (the pcb has lotsa burn marks also).

Now, the amp is ok when idling (power stage bias at around 72mA@450V per side), voltages look good everything seems fine, but when driving signal in, the power tubes start to get hotter and eventually red plate. Also, the negative biasV starts to go down, from about -40V to -60V.

When scoping, the signal looks good all the way to power tube grids, but looks very exploded on the anodes. Sounds and smells terrible. The third set of tubes (used but in known good condition) on trial, so that should be eliminated already... Gone through some solder joints already, but going to double check some.

Any ideas appreciated, thank you in advance!

-Tommi

[Mars Amp Repair]

hello,
I'm not sure which marshall is the Super Bass, but the 'Murder One' model 1992LEM pops up when I put Super Bass in the Korg site.
How did you measure the idle current on the power tubes? do you know if it is set properly to start out with? If it is too high to begin with, that can cause a run-away condition, too.
If the idle adjusts, then I would ask 1st if you replaced the filter caps in the bias supply. Also you might have a leaky coupling cap between the PI stage & the power tubes C14 & c16 on this schemo, albeit generally rare for that to happen in a Marshall.

If this is one of the Marshall's that uses a cap in series with the power transformer to supply AC to the bias rectifier, that cap can be bad.

glen

[Enzo]

You said your bias goes down -40 to -60. First off, as a convention I think most of us would consider that going up. I think it is counter-communicative to call -100v "less" than -50.

In any case, take that measurement at the bias supply, at the grid, your meter will also be measuring the signal there. You could watch with a scope to see if the DC average is wandering. But bias moving from -40 to -60 would make your tubes run cooler, not hotter.

Or did you intend to say the bias started at -60 and dropped to -40?

Go over to RG's Geofex site and look up his transformer tester. Disconnect all the leads of your output transformer and check it for shorted turns with his tester. Also make sure none of the windings short to the frame.

ANy chance that ugly wave form is oscillation?

[todor]

You said your bias goes down -40 to -60. First off, as a convention I think most of us would consider that going up. I think it is counter-communicative to call -100v "less" than -50.

Sorry about that, I was thinking about things going more negative is things going lower...

In any case, take that measurement at the bias supply, at the grid, your meter will also be measuring the signal there. You could watch with a scope to see if the DC average is wandering. But bias moving from -40 to -60 would make your tubes run cooler, not hotter.

That was the thing I was wondering about, bias is going more negative and still I'm getting anodes glowing red as hell. And I was measuring from the bias supply, not from grids.

Originally Posted by Mars Amp Repair

hello,
I'm not sure which marshall is the Super Bass, but the 'Murder One' model 1992LEM pops up when I put Super Bass in the Korg site.
How did you measure the idle current on the power tubes? do you know if it is set properly to start out with? If it is too high to begin with, that can cause a run-away condition, too.
If the idle adjusts, then I would ask 1st if you replaced the filter caps in the bias supply. Also you might have a leaky coupling cap between the PI stage & the power tubes C14 & c16 on this schemo, albeit generally rare for that to happen in a Marshall.

If this is one of the Marshall's that uses a cap in series with the power transformer to supply AC to the bias rectifier, that cap can be bad.

glen

1992 model -76 is the amp in question. All the caps were replaced, including bias supply.

But I got rid of the problem, soldered the whole power stage again just in case and took out a res I had to solder in parallel with the bias supply shunt resistor to get more range. Put in a whole set of tubes also, now the bias settles and everything seems quite fine. Have to keep it running a few hours with new tubes in and see what happens.

Thanks anyway guys!!

-Tommi