On Thursday 17 February 2005 19:27, Larry Carman wrote:
> Steve,
> I just copy the text from the ARRL book about Cmin. Here goes:
> Cmin is the minimum practical capacitance you can expect to achieve with
> your circuit, based on your knowledge of the tube output capacitance, stray
> circuit capacitance, the minimum capacitance of the variable tuning
> capacitors and a reasonable amount of capacitance for tuning. Start with a
> minimum capacitance of about 35 pF for vacuum variable capacitors and about
> 45 to 50 pF for air variable capacitors.
>
> Steve, the Cmin will be the minimum amount of capacitance your tuning
> capacitor allows, plus the tube and stray circuit capacitance. So, a Cmin
> of 35 pf could be 10 pf of Cout from the tube and 15 pf stray capacitance
> and 10 pF minimum tuning on the tuning capacitor.
>
> Please, if anyone sees any discrepancies with this, please let me know.
I wouldn't call this a discrepancy, but I measured some ceramic/external anode
valves mounted in normal situations but with nothing at all connected to
them. As a rule of thumb, the capacitance from anode to ground was about
double the manufacturer's Cout/Ca figure from the data sheet. Wiring strays
and tuning Cmin are extra.
One day I'll find time to do the same with glass ones.
Steve
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