Silver Jubilee Hum

[plexi50]

I have a Marshall Silver Jubilee 50-100 watt. It has a slight low level hum when switched to the low power side off the Hi -Lo switch

The High power side is nice and quiet

I have replaced the cap can PS capacitors

I am thinking power tube

[LooseChange]

Attributes of a Pentode/Triode switch... In Triode mode.  Try switching to a different set of power tubes or adjusting the bias may help.

[plexi50]

Thanks LC. I changed the power tubes and the hum is gone. I was probing the grid for voltage and i noticed when i touched the probe to the grid resistors the hum would go away