Hi Calvin and all,
I want to add with this MMC "talk" here is that actually inside the cases
of one these larger mica capacitors, it is actually various series and
parallel combinations tied together very efficiently as a MMC and this of
course gets the voltage and current ratings up and the wanted capacitance
level.
I agree, probably the cheapest route, would be these ceramic transmitting
Soviet caps currently.
Unfortunately, you have to be patient I will admit to get good larger mica
caps.
An example is most of these are overpriced on eBay by sellers and some of
these just sit at these prices and never sell. But bargains do come by...
when you see this do not hesitate... buy... before I do :-D:-D:-D
Actually, over the years I have now stock piled a lot of them and I am good
for any projects the rest of the time I have left on this big rock of ours.
So I do notice them, but I do pass now...but do notice they are not around
long...
Sorry to some of you out there in a quite a few years going back looking
into them on eBay at times possibly but then they we gone...it might have
been me :^)
And over the years Hamfests are a great place to get them just sometimes.
Saying, sometimes, because now I have noticed not seeing the larger ones
much anymore.
And I agree with you Calvin on the grid leak, I forgot to comment on this
on my previous post.
And for all, when I mentioned previously about sizing micas properly for
applied plate voltage, I do mean the same as Calvin says and always use a
higher voltage. Like him I can not give exact numbers and everyone makes
their choice here, just the same as caps and MMC caps voltages on SGTC's.
More is always better, but costs and budgets get in the way, so many
comprise here and well it may work or not for long.
And I feel if one is running way out of tune at higher powers, not knowing
it, for long periods is harmful to well several, several things.
Chris Reeland
Ladd Illinois USA
Sent from my Lenovo Tablet
On Sun, Jan 15, 2023, 5:44 PM Calvin Holmes <me@xxxxxxxxxxxxxxxx> wrote:
> I used an MMC of pulse (bad kVAR) doorknob capacitors due to availability
> (I got 42 40kv 2nF TDK doorknobs for $30). I had anywhere from 2-4 parallel
> strings of 2-6 capacitors, roundabouts, and the heating of the capacitors
> wasn’t too bad (they’d get a little warm after a while), even running at a
> few kW. Never had one fail. One big advantage to an MMC is that you can
> adjust the capacitance fairly easily. I would certainly not recommend going
> this route because it would cost a fortune. A small Soviet doorknob MMC is
> likely the cheapest/easiest route. Your tank cap needs to be rated for a
> voltage higher than your plate voltage. I’ve never had to worry about it,
> so I can’t provide exact numbers.
>
> Fabio, is it possible the smaller Mica capacitors you used failed because
> they weren’t rated for the voltage the saw in the tank?
>
> Are any of you aware of Tesla coils with MMCs of good SMD or THT ceramic
> capacitors (e.g. NP0/C0G)? If I’m remembering my conclusions from a few
> months back correctly, it would be a cost-effective tank cap, at least for
> QCWSS coils, if procured from the right sources.
>
>
> The grid leak capacitor does not needed to be rated to the tube's rated
> plate voltage or your HV supply voltage. Just use a couple/few kv and probe
> it if you’re concerned. Higher voltage ratings would probably be good for
> ceramic capacitors that aren’t class 1 so that they lose less capacitance.
>
> I ended up using a fixed cathode bias (seriesed MOSFET-buffered zener
> diodes) and no grid leak capacitor. I maintained some grid resistance to
> hopefully prevent grid melting from high positive voltage swings. I was
> doing kJ screen modulated QCW, though, so your results may differ.
>
> Regards,
> Calvin
>
>
> > On Jan 15, 2023, at 09:19, Joshua Thomas <joshuafthomas@xxxxxxxxx>
> wrote:
> >
> > Also, MMCs let you adjust up and down in size as needed, and if one of
> the
> > caps goes bad, it is a lot less expensive than the single doorknob cap...
> >
> > On Sun, Jan 15, 2023 at 10:53 AM Fabio Bragonzi.it <mcggyver@xxxxxxxxxx>
> > wrote:
> >
> >>
> >> Hi Jordan and all!
> >>
> >> I personally had some excellent results either with MMC capacitors
> >> (using MKP or KP type caps) or big ceramic capacitors (flat disk or
> >> hollow tube type) but i struggled to get proper results with doorknob
> >> capacitors (molded in plastic) or mica capacitors (molded inside a
> >> bakelite or ceramic case).
> >>
> >> For a better explanation, the doorknob i tried wasn't working as
> >> expected (quick overheating, resulting in a shift of resonant frequency)
> >> and all of the mica capacitors i tried worked just fine for a short
> >> period of time.... until they fail.
> >>
> >> I strongly suggest an old Soviet ceramic capacitor because are very
> >> strong and also quite cheap (something like these):
> >>
> >> https://www.ebay.it/itm/193245707970
> >>
> >> https://www.ebay.it/itm/284028401115
> >>
> >> https://www.ebay.it/itm/275420278203
> >>
> >> Please note that these capacitors almost always have the maximum
> >> reactive power rating printed on the case, but most sellers never give
> >> you this information
> >> (FYI, the cyrillic writings "kBAp" means "KVAr")
> >> The rated voltage of the grid leak capacitor shall be equal or greater
> >> than the max voltage ratings of the plate;
> >> almost any kind of capacitor (except electrolytic) is adequate for the
> >> purpose.
> >>
> >> Ciao!
> >> Fabio.
> >>
> >> Il 10/01/2023 18:11, jordan simmons ha scritto:
> >>> Hey All,
> >>> I have been looking for a while to find some suitable doorknob caps
> for a
> >>> GU 81M VTTC. Running ~350kHz probably around 1kW, 1.5kW if I really
> push
> >>> it. C-tank between 1.8-2.5nF.
> >>> Is it worth it to get doorknob caps? Or is it more wise to roll with an
> >> MMC
> >>> tank?
> >>> And what about the max voltage ratings on the grid leak capacitor? Is
> it
> >>> critical for it to be plate supply rated, or derated to the tube's
> >>> datasheet?
> >> _______________________________________________
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> >>
> >
> >
> > --
> > Joshua Thomas
> >
> > My new email address is: joshuafthomas@xxxxxxxxx
> > Please update your information if you have not already done so.
> > _______________________________________________
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>
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