JVM Forum

Santiall

There is a post phase inverter master with the dual pot (on220K) after the phase inverter caps. The resistors that feeds the bias has been removed and the pot is there instead but it´s not grounded. And no caps after the dual pot. This master should not affect the tone in any way when on full volume. Is this master dangerous?

The Marshall VM and old 2150 Marshall combo has a PPIM with 2 caps before and 2 caps after the dual master pot.

Hi, yes, it is dangerous. If the pots get dirty and the wipers don't make proper contact, the power valves will be without bias which most likely will either blow up the HT fuse or damage the power valves. In the worst scenario the output transformer could be risked too.

So I should have the master changed to the one that is in the VM with tha dual pot after the phase inverter valve and with 2 caps both before and after the dual pot? Wich values are best for it to not affect the tone with the master on full and using the Marshall Power Brake?

yes, something like the VM is the good practice. I don't know the values used there but as ballpark you can use a 1M or 500K pot and 47n caps.

Thanks you are the best!

Is it possible to put a high value resistor in parallell across the pot and if the pot gets bad it will get bias through the resistor?

you could do the same as a VM without the cap between the wiper and the valve grid and keeping the additional resistor but you may have noise while turning the pot if there is some kind of grid current. Ot course the master pot should be connected to bias and not to ground.