Re: Some work with Toobs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 10:57 04/06/03 -0600, you wrote:
>Original poster: "sundog by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><sundog-at-timeship-dot-net>
>
>Hi All,
>
>   I've been working with my vacuum tubes as time allows.  I got my 
> horizontal bipolar VTTC running nicely, then promptly dismantled half of 
> it to work with some experiments.  (ain't progress wonderful?)
>
>   One of my experiments is to run the tubes in grounded grid 
> configuration, and use a mosfet for cathode switching.  Similar to the 
> staccato mode, but better in some ways.
>
>   For most tubes, this requires either a full wave rectifier on the 
> filament (and usually drops the filament voltage below it's needed 
> level), or a center tapped filament transformer.  I'm working on finding 
> a way to hook up a bridging transformer to provide a centertap without 
> re-winding.  6.3V center taped transformers are hard to come by, and 
> using a variac to run a 12v CT tranny at 50V input isn't practical for 
> most people.

You only need the special filament supply arrangement for tubes with 
directly heated cathodes. Unfortunately that's exactly what most of the 
bigass tubes used in VTTCs such as the 304TL, 833A, etc, are. The reason 
for the special supply (AFAIK) is to try and make the RF current divide 
equally between the two ends of the filament. A big RF bypass capacitor 
across the filament terminals of the tube would probably work almost as 
well and allow you to use a regular transformer.

Steve C.