65' twin reverb reissue hum with reverb

[überfuzz]

When I ground the output from inside the tank the hum goes away. What does this mean??

I have the footswitch unplugged from the amp.

I'm not sure of what you grounded... You have the hum only when the tank is plugged, right? And you're sure none of the tubes are noisy? Not meant as an insult. ;-)

[pontiacpete]

The footswitch is disconnected.

Changed tubes many times over.
The only thing that works is an AT7 for the return in place of the AX7.
Might be a good work around, plenty of reverb and way less on the humm

[TimmyP1955]

Make sure that the tank's output RCA ground is actually continuous to the tank chassis. I've had a couple on which this connection was faulty. And make sure that the input isn't inadvertently continuous to the tank chassis.

[pontiacpete]

when moving the tank around there is no change.
Measuring with an ohm meter from the black wire inside the tank ,the shield wires are continuous with the tank and the amp chassis.