Laney supergroup 100. Grounding

[diydidi]

Hi guys.
I recently started repairing a Laney Supergroup 100W amp from the 60’s.
I pretty much completed the restoration, but I’ve been wondering about the grounding scheme the designers implemented. There is a slight hum with all controls full.
Turning down the the two channel volume controls, kills it.
The amp uses a form of star grounding at the presence control. The presence control body is the only connection made to chassis. A buss bar runs from input socket grounds to all volume control and tone control grounded terminals. The input and speaker output sockets are plastic and isolated from chassis.
Are there any modifications to improve the grounding, that I can do?
Any problems with the way they did it?
Thanks in advance!!

[Helmholtz]

The presence control body is the only connection made to chassis.

If so, make sure that the presence pot shell measures zero Ohm to chassis.

But are you really sure?
How are power tube cathodes and filter caps grounded?

[diydidi]

They are also grounded at the presence pot.

[Jon Snell]

The Laney Supergroup 100 mk1 was first built in 1969.
Both volume controls attenuate the first ECC83 V1 and the control grids of each triode are not shorted to ground by way of the input sockets allowing hum pickup. Placing a shorting jack into either input jack, both channels, should remove the hum. If it doesn't, replace the ECC83 for a better quality one with less heater cathode leak. I include a schematic for the pre amp, it is a recent drawing so the RCA valve numbers are used.

Back in those days, if your amp wasn't humming, it was probably not switched on!