On 23 Jan 03, at 12:16, Dave Haupt wrote:
(and thanks Dave!)
< snip, snip >
> So, after doing the conversions, the various flavors
> of 3CX1200 end up with the following values:
>
> A7: Input, 20pF; output 12pF, feedthrough 0.2pF
>
> D7: Input, 17.6pF; output, 9.7pF; feedthrough 0.57pF
>
> Z7: Input, 17.6pF; output, 9.7pf; feedthrough 0.8pF
>
> On first blush, the A7 has the least feedback
> capacitance.
That's what I get, too.
> However, that's almost irrelevant.
> What's more important is at what frequency does the
> grid structure cease to effectively shield the output
> from the input. That is a function largely of the
> grid geometry inside the tube, and the low-frequency
> capacitance measurement tells us almost nothing useful
> about that structure.
>
> There is a fair amount of anecdotal evidence
> suggesting that the Z7 is superior to the A7 in terms
> of keeping it from oscillating at VHF. However, that
> results is obviously not due to reduced feedback
> capacitance.
OK. So Rich is not correct that the z7 has less feedthrough
capacitance, but he's SPOT ON about the z7 being more stable as an HF
amplifier.
> Whether a given tube oscillates in a given circuit
> depends on a large number of factors. The mu of the
> tube, the dimensions of the elements inside the tube
> (which we are never given), the method of grounding
> the grid and how the cathode is terminated out-of-band
> are probably more important than the anode-to-cathode
> capacitance.
You mean layout and building methods can be important?! 8>)
[please note the smiley thing]
> Input and output capacitances are
> largely subsumed into the matching networks, so
> they're relevant only to the extent that we can design
> matching networks to accommodate them.
Yep.
73,
George T. Daughters, K6GT
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