On Mon, 25 Nov 2024 at 18:44, <flynth@gmail.com> wrote:
>
>
> On Mon, 25 Nov 2024, 18:15 Victor Rosenthal 4X6GP, <k2vco.vic@gmail.com>
> wrote:
>
>> I think you said the minimum capacitance of your tuning capacitor was 15
>> pf.
>>
> It is 12pF now (it was 20pF total in the previous configuration - close to
the case wall, as it has a big cap on the end. Just moving that thing 7cm
away took 5~6pF away, then there are these large metal holders, just
reverting them so they face away lowered it by further 2pF.so it is 12pF
residual now (nominal) .
>
> But too big residual capacitance is not really a problem. I can either put
> 10pF capacitor in series with the plate capacitor (either on the ground
> side, or the other side and this will pull it down to 5pF to 10pF). It's
> basic capacitance in series thing. If that causes some hidden problem I'll
> make an adjustable pcb cap 5 to 10pF. It's very easy to make such small
> caps with two pcbs.
>
>> There are vacuum capacitors with a minimum capacitance of 5 pf, which
>> would be a better choice.
>>
> Indeed it would, but that's what I could get for reasonable money. These
caps you talk about go for about 250 EUR. Just for comparison. I bought the
entire 6M amplifier (from the R-140 military transceiver) for about 200EUR.
And here it is a single cap. Insanity...
>
> In my experience, stray capacity is more like 10 pf in a practical
>> amplifier. The plate choke also influences the circuit.
>>
> 10pF is what I estimate as having before. I'm hoping to get way lower.
> You didn't say what tube you were using.
>>
> Didn't I? It is a GU-43B.
73,
F
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