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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