Re: tank circuit of VTTC

["Bert Hickman" <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Tesla list wrote:
> 
> Original poster: davidt-at-pond-dot-net
> 
> Hello,
> 
<SNIP>
Bert:
> I'd suggest beefing up the tank circuit by going
> to
> larger diameter copper tubing or flat copper strap, and possibly
> silver
> plating it as well. You are likely going to melt the PVC insulation
> on
> your primary under extended running conditions.
> 
> ..............................................
> 
David:
> Well, I would have to take issue with this. I have used #12 PVC
> insulated wire on my primaries without any problem. Again, with
> up to 3 833C's filted DC 2+ KW in. "I" believe #10 would be fine,
> if you go and use a larger diameter pri conductor, then you may
> run into problems with size.
> 
> ...............................................

<SNIP>
Bert: 
> However, I have no doubt that a properly constructed MMC could
> safely
> handle this task. 
<SNIP>

> .........................................
David:
> Sure can. Been, there, done that. My current MMC has been in
> use for over a year and a half, with many hours of run time (I
> have used my Audio Mod Vac coil for mood music in my shop while
> working on other projects ;-)). MMC is a single strand of Panasonic
> 0.015 uF caps, it does get warm, and I sugest using two strings
> for longer life.
> 
> .........................................
> 
<SNIP>
Bert:
> However, there's not very much empirical experience with the
> long-term
> performance of MMC's in a CW RF environment...
> 
> .....................................
David:
> Like I said: I'v been running them for over a year and a half.
> It is very true that while running with just filtered DC everything
> gets hot after several minuts, I would not recommend running
> a setup like mine (#12 pri, single strand MMC, 2-3 388C's -at-5KV)
> for more than 2-3 min runs, on filted DC alone. I must point
> out that the meathod of Audio modulation I use (grid), I run
> the tubes at almost cutoff, with 40-50K Resitance in the grid
> leak, and the effective "pulsing" of the tube must cut the duty
> cycle down. That must be how I can get 10 to 20 minute runs (
> not full power with longer runs, of course). I did "burn" up
> a secondary once, it seems to get the hotest af anything (besides
> tubes ;-)). My Schematic can be seen at:
> http://216.36.6.9/webdoc2.htg/Tube_coil/Audio_mod_sch_sm.jpg
> 
> Regards,
> 
> David Trimmell
> www.ChaoticUniverse-dot-com
<SNIP>

David,

Thanks for the additional information! At 10 kV plate voltage, the
design that Herwig was proposing would have subjected his tank inductor
and capacitor to about twice the RMS current of your system (if running
flat out at 5 kV). And, all other things being the same, this would
result in 4X the amount of joule heating in his tank circuit. 

I am very surprised that you are not seeing substantially greater
heating in your tank circuit. The high value of grid leak resistance
used in your system (40k versus 1-3k typical) may be reducing the
overall RF amplitude seen in your tank circuit. If you were indeed
running at about 15 A RMS RF (running flat out at 5 kV plate voltage in
your system), I would expect your single string tank cap(s) would get
VERY hot. The Panasonic MMC caps showed almost a 20 degree C.
temperature rise when carrying only 3A RMS current in Terry's earlier
experiments. If your system WAS actually hitting 15A RMS at almost 400
kHz, that's 5X the level that Terry was running, and it would imply at
least 25X the joule heating that Terry saw. Your single string of caps
would NOT survive this type of abuse for any length of time. 

This is really a poser...! 

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-com
Web Site: http://www.teslamania-dot-com