[Amps] Not the Diacrode

[John Lyles]

The filament is DC powered. De big tubes often use DC as the filament 
may mechanically resonate with powerline frequency and short to grid or 
itself. I heard and old story of Radio France having a problem of 
shorting out their medium or short wave transmitter tube on a particular 
note of a particular song due to vibration. Thomson company eventually 
figured this one out, and in the iterim may have told them to avoid that 
song - lucky it wasn't "La Marseillaise". With these tubes there is a 
very slow ramp up (minutes) when warming up the filament, to allow the 
filament basket to stretch inside of the grid basket without touching it 
and shorting out the control bias. The ramping is a lot easier to do 
with DC supply than AC (which might use variacs, saturable reactors, or 
other ancient technologies). I use an SCR regulated mains supply with 
good filtering.

73
John K5PRO

> But you won't need any high voltage DC. The other guy will hear your filament.
> :-)
> 
> 73, Bill W6WRT