[ARC5] ARC-5 TX problem --DC on fils gives hum on signal?

[Tom Bridgers]

Have been working to clear up a hum problem on my 3-4 mHz BC-696.  It has been converted from 24 volts to 12 volts on the filaments.  The tube lineup is orginal:  1626 plus 2 ea 1625's.  In my situation, I supply filament power to all three tubes; however, I am only supplying power (150 VDC) to the oscillator.  No power is applied to the PA plate and screen.  

 

When I key the oscillator, I can hear the hum in my receiver.  There is no antenna connected to the ARC-5.  I am just supplying 150 volts B+ to the oscillator and 12 volts to all filaments but I just key the oscillator.  

 

I've tried various combinations of power supplies for the oscillator and 12 volts DC and AC voltage to the filaments, and have determined thereby that the hum is being produced by the 12 volts DC and not either the 12 volts AC or any of the +150 power supplies I tried.  

 

Said differently, I hear no hum on the signal when using 12 volts AC on the filaments.  However when I use 12 volts DC, from a well filtered power supply, the signal definitely has audible hum.

 

Why in the world would 12 VDC on the filaments cause hum and 12 volts AC not?

 

Any help or suggestions would be most welcome.

Thanks,

-Tom  KE4RHH