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[Amps] Re: a7/z7 ??

To: <amps@contesting.com>
Subject: [Amps] Re: a7/z7 ??
From: gdaught6 at leland.stanford.edu (George T. Daughters)
Date: Thu Jan 23 17:13:03 2003
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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