Re: Hybrid Solid State Vacuum Tube Design [LONG!]

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Shad Henderson <sundog@xxxxxxxxxxxx>

Hi All, Steve,

I haven't tried this on a *large* coil yet.  I've sucessfully driven a
6SN7 tube (dual triode, both sections in parallel, about 10W plate diss
total) with this setup, but it was unstable due to using 555's to supply
the timing, and running the FETs at around 800khz (ouch!).  The coil was
a bipolar, and ran before that as a conventional VTTC.  Output of both
the conventional setup was about 1/8" on each side of the secondary, and
the same for the solid state modulated test run.  The sparks *sounded*
different, more raspy, but being so small and quiet, I couldn't be 100%
sure of anything other than "It Works!"
The little VTTC's specs :
Tube : 6SN7 dual triode, both sections in parallel
Plate/filament supply, 6.3v @ .6A,  310V pulsed DC on the plate
Tank cap - 250pF air variable in parallel with a 300pF ceramic doorknob
Tank coil - 25-30 turns of 26ga wire
Grid coil - 14 turns of 32ga
Grid C - 250pF ceramic bypass cap
Grid R - 8kohm 1W carbon comp resistor
Secondary - 1.25" thinwall PVC wound (IIRC) 9" with 32ga wire
toploads - .125" brass balls (found in lamp section of Home Depot)

The modulator was run off of a +/- 30V supply and I used IRF510's for
switching.  2 555's provided 75uS pulses at 120-ish pps.  When using the
tiny VTTC with the modulator the plate supply was changed to smoothed DC
with 2uF of filter capacitance (370v 2uF motor run cap, .056uF 1600v
poly cap in parallel for fast current needs)

Both sections of the tube in parallel pulled about 50mA pulsed with +12v
on the grids, and shut down completely with -30V on the grids.  I used a
cathode resistor (single digit ohms, can't remember exactly) to let me
measure cathode current via voltage drop.

I've run an 811A into a dummy load with the modulator, pretty much to
smoke test the solid state part of it (plate diss about 45W during the
test), and it performed well.  I've just got to find the time to build
the primary and finish wiring up the base of the coil.  Already have the
secondary (3" PVC wound with 14" of 32ga wire), but need to build the
rest of the coil.

Once I get sparks with an 811A, I'll move up to a larger tube.  I chose
the 811A for testing because it's easy to feed, easy to drive, and low
enough powered that I won't be in danger of damaging the expensive mica
transmitting capacitors I commonly use in VTTC's.   Once the 811A coil
runs to my satisfaction, I'll either break out a 4-125A, the 833A, or
the RS1026 (graphite plate).

Hopefully, if I can scrounge up some free time (try finding that in a
dumpster!), I'll be able to complete the coil this weekend.

Shad


On Mon, 2005-04-18 at 15:43, Tesla list wrote:
> Original poster: Steve Ward <steve.ward@xxxxxxxxx>
>
> Shad,
>
> i snipped the whole post, but just had one question.  Did you test it
> with a coil yet?  Im very anxious to see your results!  Also, a
> feedback design should really be a simple thing (at least it is for
> me).  Im not sure if primary or secondary feedback would work better.
> The primary's duty cycle is sorta messed up, so it might need to be
> secondary based where its sinusoidal.
>
> Keep us updated on your progress.
>
> Steve Ward
--


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