Transformer Cross Coupling/ Shielding

[arledgsc]

My JTM50 emits low-level hum from the speaker when the Mains are switched on irregardless of the Standby switch. It is just loud enough to be annoying. As a matter of fact I can hear this hum when all tubes are pulled including the rectifier. Wiring has been inspected, routing verified, and any error ruled out. I confirmed cross coupling between the PT and OT by placing a mu metal ring on top of the PT end bell. Adjusting the ring in vacinity of the PT end bell quietens down the hum to normal levels.

The PT is a MM P4550JT-G2 and the OT Marstran 784-128. It is funny that I use the same MM PT in my JTM45 with a Marstran 784-103 and that amp is spooky quiet. Both amps are wired similarly with Larry ground. The chassis material is different though with aluminum on the JTM45 and the JTM50 is steel. And the OT is orthogonally mounted in relation to the PT on both amps.

Anyway does anyone have any ideas for permanently quieting down the cross coupled hum? I guess I could fashion a mu metal halo to sit on top of the PT. Moving the OT over further away from the PT is an option however difficult that would be. I have also read that placing short aluminum spacers under the PT to magnetically isolate the PT from the steel chassis may help. Any other ideas? Thanks, Scott

[arledgsc]

An update... I finally got rid of the hum. Moved the OT away from the PT another 1/2". The Ceraitone chassis placed the OT closer the PT than standard. Something to consider if planning to purchase a Ceriatone chassis. Fairly nice chassis otherwise.

This JTM50 is killer and glad to not worry about this any longer

Scott