Marshall JMP Master Model Bias

[Steelwitch]

Hey folks,

I have a Marshall JMP Master Model MKII on the bench.. and I happened to leave my bias probe at home. So! This amp needs to be properly biased before it goes out. I've read the transformer shunt method is not the most accurate technique for measuring plate current for marshalls, but works very well in fender amps. How would ya'll say is the best way for a vintage marshall? Is this a myth or true? Plate voltage on each EL34 is 380v which seems a bit low. Thanks!

[Steelwitch]

here is a schematic https://drtube.com/schematics/marshall/2204u.gif

[pdf64]

Why not fit 1 ohm cathode resistors? It’s but a minutes work. Adjust bias to achieve ~35mV across them.
I suggest to measure the HT voltage rather than plate; hanging a probe/lead/meter off the latter can provoke oscillation, which will screw up any measurement.

[Steelwitch]

As in putting a 1 ohm resistor between pins 1 and 8?

[Steelwitch]

I think I may have shorted the output transformer on this amp.. After taking a voltage reading on pin 3 of the output tube, I am getting 0 output anywhere. Did I just create a direct short to ground. Is it possible I damaged the OT by doing this?

[Steelwitch]

Nevermind, it was a blown HT fuse. But wondering why this would blow the fuse...

[g1]

Did you by any chance have the meter set to a current range, like mA or A ? This is one of the dangers of trying to use the shunt method.

[Steelwitch]

Did you by any chance have the meter set to a current range, like mA or A ? This is one of the dangers of trying to use the shunt method.

Yup. That’s exactly what happened

[g1]

Yeah, lucky not to do more damage to the amp, your meter, or yourself. One of the three often occurs.
Try this way, with power off, measure the 2 halves of OT primary resistance. Write it down.
Now with unit running, measure DC voltage across each OT primary half.
Voltage divided by resistance (you wrote down) gives the idle current for each side.

Or, to install the cathode resistors that pdf64 mentioned, remove the jumper that connects pin1&8 to ground. (leave pin 1 connected to pin8). Put the resistor so it connects the pins to ground instead of the jumper.

[Steelwitch]

Thank you! This is exactly the info I needed