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
>
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