On 7/23/06 at 5:48 PM R L Measures wrote:
>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.
If some may remember, we picked apart an 11 meter amp on here which was listed
on Ebay that used six or eight 3-500Z's. Also, we picked apart one using eight
4CX250B's. There was one model using four 4CX250B's that was built in some
quantity for several years. I have seen a some using as high as 10 sweep tubes
in parallel. All of this at 11-10 meters. Anyone remember the amateur amp
called "Galaxy" which used 10 sweep tubes if I recall?
Rich, you better watch as someone will start pointing their finger at you like
they did me for even mentioning an 11 meter amp HI HI! Remember, those are
cheap junk. The thing is, I've seen some of those built back in the 60's that
has outlasted many amateur amps like are being built now by a few manufacturers.
>>
>> 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
>
>
Best,
Will
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