[Central-Electronics] 100V audio Hummmmmm

[GRANT YOUNGMAN]

One of the issues with the 100V I’ve seen reported elsewhere and haven’t yet fully resolved in mine is a low level transmit audio hummmm.  I did run a new rerouted shielded audio cable from the rear mic jack to the 6U8 audio preamp stage as I’ve seen recommended (high quality cable, no cheap stuff).  That reduced it a bit, but has not eliminated it.

Any suggestions on what else can be done?  I think I’ve seen somewhere some info on rerunning the 6U8 filament feed, lifting the ground side off the chassis at the tube socket and tying it back somewhere in the power supply area, but can’t find the reference again.  I’m thinking maybe converting the 6U8 stage fils to DC.

Any wisdom out there on this?  I suppose at some point I’m going to have to let go and admit that it simply is what it is :)

Thanks .. Grant NQ5T