Good to hear from you again, Mike.
I've had pretty good luck with this particular sweep tube, not so much the others. Compared to your basic 6bq6 or similar, a 6kd6 is a monster (3x the specs) - they were only made at the very end of the big screen tube TV/CRT era. You just have to work within it's limitations (like anything). My first big use of them was a tube tesla coil thing that used 6 in parallel, no special issues. No question a modern tetrode could be better (but higher knee on the plate current curve, almost every tube has a higher knee than this one), and would probably take more drive effort. The 6kd6 (depending on the screen volts) needs under 100v grid drive for full power and full cutoff at HV on the plate, and I'm planning to drive it with some semiconductor VGA cathode driver chips (but drive the control grid) I found that do 100v at some ma. That gives me a broadband driver, no tuned circuits at all there. The problem no matter what device I use is really the magnetics. Until I have things nailed down to inside an octave (and I may need non-sines here) that's the big issue. Since I'll probably only have about 3kv pk-pk plate swing and need some stepup (I think, this is uncharted turf) that's a real problem. Broadband magnetics are one of the blacker arts. Funny how audio guys easily do 10 octaves, where the RF boys get all proud with a fraction of one.
Of course, one "advantage" of the glass bulb on this is you can see when you're getting the plates too hot. We picked up a 1.5kv switcher supply originally meant for cap charging for lasers, which looks nice for this. I have the heavy old iron as well, but if I can make this efficient, why not? I plan to use 4 tubes here (after a one tube prototype) as that switcher has a 28v 8a aux output to run things like filaments. The 6kd6 is quite a power hog there.
Could it be that most of the sweep tube amps you saw, being homebrew by cheapskates, simply weren't built that well? I've beaten these puppies like a rented mule and they don't seem to have any real bad issues. Easy to neutralize in the bargain, and few parasitics. You can mostly pretend those issues don't exist if you have the old 47 ohm R and coil in the plate and make the circuit "tight" re RF and grounding issues.
Note the dual pinouts on the grids to help with parasitics. I'm guessing the designer had a little glint in his eyes when he did that -- no need for that for the intended use!
- 6KD6.pdf
- 6kd6 datasheet
- (383.37 KiB) Downloaded 527 times
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.