Question about a Gibson Skylark Amp

[flythair1]

I recently bought a Gibson Skylark amp. It's pots are dated 1967 so I assume it was built 67-68. This model has a 6X4, 12AX7 and a 6BQ5. I cleaned it up and left the two wire power cord intact for now. I intended on changing it to three wire but I did not have time right then.

I tried it out and everything worked as it should except when I turned the guitar volume control down anywhere below 10, the volume would drop off very fast. The volume control on the amp worked as it should. I took the amp over to a friends house and it acted the same with his guitar. He also got a light tingle from the amp until he changed the ground direction.

Yesterday I changed the cord to a three wire and now the volume on both of the same guitars works like they should.

Can anyone shed light on why this might have happened? Did some kind of stray voltage get in the circuit to make the guitar volume pots act like this? The power switch for the amp is a three position rotary. I disconnected the reversing part of the switch and the death cap.

Thanks.
Scott

[Dilapidus]

Did you take out the 'Death Cap' as well? Generally a .22 mfd cap from hot AC to ground. The tingle would make me suspect that cap was leaking and without a real ground path to earth, you would know.