On Jul 23, 2006, at 12:17 PM, Tom W8JI wrote:
>
>> I have an article using 6 of the 811A's in parallel for 160
>> meters from 73
>> Magazine, early 1980's. I would imagine the output
>> capacitance of more than
>> 4 would be too much to tune the plate circuit on say, 15
>> and 10 meters.
>
> The real issue is feedback capacitance, not output
> capacitance. Amplifiers using two tubes are barely stable on
> upper HF because feedthrough capacitance is far too high.
> Four tubes actually require neutralization to be stable at
> upper HF, so six would the well beyond the limit.
Example: -- Goal: build a 12kW amplifier using a 4000v anode
supply: To do so using 3-500Zs: c. a dozen would be needed. Since
the feedback C is 0.15pF per 3-500Z, the total feedback C for a dozen
is 1.8pF. OTOH, using a 3CX10,000A7, only one tube would be needed
to produce 12,000w out. Since the feedback C for one 3CX10,000A7 is
0.2pF, the design using a large single tube would be more stable than
the design using 12 smaller tubes. Thus, Mr. Rauch is right regarding
feedback.
However, as to the statement that six tubes is well beyond the
limit, there have been many successful amplifiers built for 11m that
used 5 or more tubes. Also, Tektronix oscilloscopes used a 10 tube
"distributed" amplifier to drive deflection plates.
>
> Output capacitance of the tubes is not an issue at all.
> Adding tubes increases capacitance in the same proportion as
> it reduces optimum load impedance, and tank operating Q
> isn't a critical parameter at all as long as it exceeds a
> certain amount.
>
> 73 Tom
>
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R L MEASURES, AG6K. 805-386-3734
r@somis.org
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