note -- quote level format manually corrected by yours truly.
On Aug 7, 2005, at 10:07 PM, k7fm wrote:
> Rich said:
>
>> 1. In order to reduce VHF gain, parasitic suppressors need to have a
>> low-Q. In your opinion, do Ag and Cu contribute to a low-Q?
>
>> 2. Even a medium-Q parasitic suppressor is better than no suppressor
>> whatsoever.
>>
>> 3. Can you name a commercial HF 8877 amplifier besides the AL-1500
>> that does not use a VHF suppressor?
>>
>> 4. So Dick Ehrhorn made a mistake in the Alpha 77 by using a VHF
>> suppressor?
>
> Response:
>
> 1. If an output circuit does not have stray reactances that act as a
> high impedance for vhf signals, and the tube does not have internal
> stray reactances, there will be no vhf instability and no parasitics.
Correct. However, a tube typically exhibits 5 - 15 pF of anode-C, and,
and in all HF amplifiers, there is a Cu conductor connecting the anode
to the Tune-C through the DC blocker C. The L of this arrangement is
typically 50 - 100 nH. The Tunc-C typically exhibits a few tens of pF
to a few hundred pF, depending on the freq. in use. The resonant
frequency of this circuit is typically 80 to 160MHz in Ham amps. For
example, in a Henry 2K-4, the anode circuitry resonance is c. 95MHz and
in an AL-80 or SB-1000, it's c. 150MHz. At the resonant freq., this
part of the anode circuitry exhibits high-R. Since the gain of the
tube is proportional to RL, the resultant VHF gain will be high at this
resonance. If the tube had no internal feedback-C, regeneration could
not occur at the resonant frequency, but - as we know - all tubes do.
Also, whenever the anode-I increases or decreases, this resonant
circuit rings and produces a damped VHF wave that is fed back through
the internal anode-cathode (feedback) C. To me, it kinda sounds like
the stage is set for an oscillation condition.
> In that case, it does not matter if the parasitic suppressor is made
> of silver, gold or pork bellies - except that pork bellies would
> attenuate the rf rather substantially.
I rather like the smell of cooking bacon and sausage. A bacon
suppressor would be impractical, but a sausage suppressor just might be
doable if the Ni-Cr coil were the right dimensions. Cooking breakfast
in the shack would definitely be an advantage during a very important
radio contest.
>
> 2. The point was that the Al-1500 appears to be proof that any VHF
> suppressor is not necessary.
Did the first dozen Space Shuttle missions prove that no modifications
or improvements were necessary?
> If it was necessary, then every AL-1500 would fail every time it was
> used, since it does not have what you say it requires.
The trouble with intermittent VHF parasites is that, like the stock
market and dropdead gorgeous women, they are not predictable. Another
complication is that the VHF gain of tubes seems to vary -- even within
the same production lot.
>
> 3. My statement was "Many high power amplifiers used in commercial
> services do not use parasitic suppressors." I did not mention the
> 8877, nor did I mention amateur amplifiers. The point is that the
> 8877 is similar to many larger tubes in construction.
You need to provide examples of those that don't.
>
> 4. I did not say Dick Ehrhorn made a mistake in his Alpha 77 - you
> tried to imply that my statement lead to that conclusion. Dick
> Ehrhorn was a respected amplifier builder that knew a lot more than I
> do about amplifier construction.
Why did Dick decide to put a parasite suppressor in the Alpha 77 if one
was not needed? My guess is that he realized that a UHF triode with an
amplification factor of 200 is nuttin' to sneeze at.
> He also put a high power amplifier and power supply in a relatively
> compact desk unit and perhaps the circuit layout had sufficient lead
> length to require parasitic suppressors. Perhaps he put one in
> because he did not wish to spend a lot of development time trying to
> do without.
Is there a layout technique that can reduce the anode lead length to
zero mm, change the 8877's 10pF anode-C to 0pF, and eliminate the 0.1pF
of anode-cathode (feedback) C ?
>
> Let me twist the words like you do. Are you saying that the designer
> of the AL-1500 is stupid because he did not use a suppressor, or in
> other words, that you are smarter than he is?
The designer of the AL-1500 assumed that Eimac's grid-grounding collet
(p/n 135305) has zero L. I did not. He also assumed that the feedback
C to the cathode was insignificant in the upper region of where the
8877 exhibits gain. I reasoned that it was probably a couple of k-ohms
or so. If this was stupid, I plead guilty.
>
> Seems a bit like all of the scientists that published papers proving
> that man could not fly while the Wright Brothers were doing just that.
During the Grate Parasitics debate is was claimed that an 8877
absolutely can not oscillate without external feedback. Shortly
thereafter I received a telephone call from a guy whose company designs
amplifiers using a computer app, He ran the numbers and discovered
that an 8877 can oscillate with its own feedback C above about 105MHz.
Has anyone ever measured the anode-resonance freq. in an AL-1500?
cheers, Colin.
>
> Colin K7FM
>
>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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