Hi Chris,
I’ve stayed within a very similar frequency range.
Teslista555 is the best source I’ve found regarding a variety of frequencies in VTTCs. https://www.vn-experimenty.eu/teslov-transformator/vttc.html?filter_tag[0]=11 <https://www.vn-experimenty.eu/teslov-transformator/vttc.html?filter_tag%5B0%5D=11>
The GU-5B pages in particular:
https://www.vn-experimenty.eu/teslov-transformator/vttc/vttc-11.html <https://www.vn-experimenty.eu/teslov-transformator/vttc/vttc-11.html>
https://www.vn-experimenty.eu/teslov-transformator/vttc/vttc-12.html <https://www.vn-experimenty.eu/teslov-transformator/vttc/vttc-12.html>
JMartis also has a higher frequency VTTC video with a GU-39B: https://www.youtube.com/watch?v=CH_0H_-UZIc <https://www.youtube.com/watch?v=CH_0H_-UZIc>
Magic Tee has a smaller low MHz VTTC: https://www.youtube.com/watch?v=BRj2RGca36w <https://www.youtube.com/watch?v=BRj2RGca36w> https://www.youtube.com/watch?v=UEmmOXIQwZw <https://www.youtube.com/watch?v=UEmmOXIQwZw>
Best,
Calvin
> On Jan 15, 2023, at 15:16, Chris Reeland <chrisreeland@xxxxxxxxx> wrote:
>
> Hi Fabio,
>
> Those ceramic transmitting capacitors you linked are exactly the type I was
> thinking of and I find it interesting that you have had good luck on some
> MMC polypropylene caps so far.
>
> Just wondering, since you are in Italy (I think I have this correct), what
> kind of mica cap availability do you have over there in that part of the
> world and made from what countries out of curiosity? For me/we in the USA,
> we have a large selection of good older USA made mica transmitting
> capacitors especially from the 1940's to late 1960's pretty easily obtained
> that are constructed with the "finest India ruby mica" dielectric. They had
> especially good quality control in my opinion of this era of manufacturing
> mica capacitors. And good sources of raw ruby mica, other types of mica is
> not as good, and from what I know, these days, good raw ruby mica is much
> harder to come by for manufacturers, so newer ones can be made of other
> types of mica. There is only "so much out there" of this mica type in the
> mines. For me, I have had only one mica cap destroyed because it was way
> too small for the current handling requirement in how the VTTC was set up,
> met voltage, but... learned the hard way...
> For me, when sized correctly for both voltage and current handling
> capability I have had great performance and I really push my tube coils
> hard at times and for really long durations too. And also, another note
> here, is these older ones have a metal brass nameplate which shows
> typically three different frequency numbers and the associated amps at that
> frequency. The amp handling goes down the lower the frequency. And when
> hitting a point in much higher frequency, it will again go down in amps in
> the rating.
> Just like those good Soviet ceramics, they get much larger and heavier the
> good ones.
>
> Another curiosity is, what are the frequencies of your VTTC'S.
> For me, the lowest running frequency I have experimented with so far, about
> 200khz and I have been all over in-between up to 700khz. I have yet, to
> experiment going into the MHz range... some day...
>
> Chris Reeland
> Ladd Illinois USA
>
> Sent from my Lenovo Tablet
>
> On Sun, Jan 15, 2023, 9:54 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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