Marshall JMP 100 bias winding .

[Luthin You]

I have a marshall JMP 100 which has been blowing the mains fuse randomly every 2 weeks or so . I noticed some variation in the bias , I now have the bias winding (white) disconnected from board and its reading around 77Vac , I have replaced the bias caps and checked the resistors . would a bias fault blow the mains instead of the HT ? Thanks.

[Helmholtz]

There were a number of different 100W JMP models and circuits since 1968.

We need to identify the exact model and find the correct schematic.

Using an amp that blows the mains fuse (assuming correct fuse rating) is likely to make things worse.
Do you have a bulb limiter?

If the PT has an internal (intermittent) short it needs to be replaced. But there are other more likely reasons.

Before doing any further tests, pull the power tubes.

[Luthin You]

Helmholtz , thanks , I must apologise , it seems when i measured the bias DC and it was failing , i quickly put the meter on the AC supply but left it on DC reading and it showed about 30mv DC which i mistook for ACV. I now read 77V ac on the bias supply . I have a current limiter but the amp didn't blow fuses or light up the bulb , its a 150W bulb but still glows a bit and so reduces the supply to amp. The bias dc does fail now and then but its so rare that its hard to find. The amp is super Lead 100W II , so SL/A 5759F mid 1974 . i have edited my post now . Do you think a bias fault would blow the mains fuse instead of HT ?

[Helmholtz]

Do you think a bias fault would blow the mains fuse instead of HT ?

Not sure, but that kind of speculation doesn't help to isolate the problem.

[Enzo]

You noticed some variation in the bias? Or some variation in the idle current? Bias is the voltage on the grid. The milliamps through the tube you adjust the bias for is the idle current. I point that out because they are very different things, and any cure for them would matter on which thing you saw.

I rarely think bias itself blows fuses. Even runaway red plating doesn't usually blow the fuse. Of course anything is possible.

Most common reason for a fuse to blow in a tube amp is a failed or failing power tube. By far.

If your bias is actually coming and going, I'd be looking for a connection or solder joint first, myself. But a power tube shorting internally can also drag down the bias voltage.

[Luthin You]

Enzo , I noticed the bias voltage dropping , at the diode , resistors and caps . I haven't seen red plating , The amp was blowing fuses previously and I replaced power tubes and reservoir and filter caps , but it came back still blowing HT fuse . I am an old tech but limited experience on Marshalls . Hard to locate when it can be happy for a week or two , then pop.