Resend due to a bounce earlier...
On 2023-02-06 16:34, Chan Gade wrote:
With machine unplugged, tip a selection pin, plug machine in. It will start scanning as normal. Does the hum begin immediately OR does it only begin as the tubes warm up?
I ask because these amps have multiple voltage sources for the entire machine. Sometimes poor grounds can cause AC (60Hz) to leak into other non-related circuits.
Thanks for the suggestion. The hum only occurs once the amp is warmed up. As far as I can tell the hum is not from the B+ supply. The hum is only there when it is unmuted. I pretty much have isolated the source to the 6AU6. I will point out that it is not a loud hum. I really do think that this is due to a ground loop on the PCB itself.
On 2023-02-06 15:49, Britt Abbott via Jukebox-list wrote:
Check the braided cable from cartridge to amp to insure braid is grounded and live internal wire is not grounded. You might want to try using another new or rebuilt cartridge to see if it fixes your hum.
I've been doing some testing on the bench, so the hum source is not input into the app. Now that you bring this up, several years ago I converted the amp from Cobra to a magnetic cartridge + preamp. The input to the amp is basically line level now which makes it easier to test on the bench.
Carl Haines