On May 29, 2006, at 2:51 AM, Chris Pedder wrote:
> At 09:29 29/05/2006, R.L. Measures wrote:
>
>> The 3-1000Z has a grid that does a marginal job of shielding the
>> input from the output -- which means it has more feedback-C than need
>> be. The 3cx1200A7 is the external anode version of the 3-1000Z and
>> it apparently uses the same grid and filament. Both tubes have a
>> reputation for squirreliness -- especially with higher anode PS
>> voltages. Eventually, Eimac discontinued making the 3-1000Z and the
>> 3cx1200A7 evolved into the 3cx1200Z7, which has less feedback-C --
>> and better stability. Unfortunately, the ...A7 and the ...Z7 are not
>> interchangable.
>
> I cannot help but wonder why you keep pedalling this one. I have
> been, yet again, to the Eimac/CPI data sheets which read as follows:
>
>
> grounded grid capacitance in pf
>
> in out feedback
> 3CX1200A7 20 10.3 0.6
> 3CX1200Z7 17.6 9.7 0.8
>
> This does not accord with your comments. Are the Eimac figures
> incorrect?
Yes. On other sheets the Z7 has 0.08 pF, not 0.8pF. Does it make
sense that the improved version would have more feedback?
>
> Chris
>
>
>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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