Rich responded with:
>Not just any amp, only HF ones because VHF amplifiers do not have
>the problem since there is only one VHF resonance in the anode
>circuit
>and that's where the amp makes power.
I agree that VHF amplifiers tend to not oscillate at HF, but I have
seen cavity amps that did oscillate at VHF or UHF while tuned to a
single high Q resonance at VHF. Unfortunately I been there and done
that, i should add. It wasn't fun, and it cost the company some time
and money to solve it.
The Eimac 4CX3500A and 4CX7500A were both new tetrodes from the 1980s
that required a bit of work to get them to play well at VHF in a
particular PA that I am very closely associated with. A bit of
nichrome and other tricks was required. The RF choke was made from
nichrome, and the LC second harmonic suppressor was also made with
nichrome to solve the problem.
I am presently designing a 3 MW 200 MHz cavity PA using the Thales
TH628, double ended tetrode (Diacrode is their name for that). It
will have integral waveguide parasitic suppressors in the walls, due
to the large geometry of this tube supporting TE21 and TE31 modes
inside. They can both be modelled by computer codes and measured in
actual amplifiers running AB1 with this tube. The CPI/Eimac 4CM2500KG
(X2174), and their other large diameter bottles with eye hooks also
are easy to coax into damaging oscillation in these high order
transverse modes, but the single-ended construction (socketed at the
bottom only) of these tubes makes them easier to tame with appropiate
Eccosorb or ferrite tiles. Like I said earlier, it is an interesting
and sometimes astounding field, building PAs and dealing with
parasites.
Anyway, i tend to agree that putting conventional L//R suppressors in
series with the anode connection of a cavity or line-type PA circuit
is not a common practice.
>>The Broadcast Electronics FM1.5A/B fm transmitter
>>which i helped design back in 1983
>
>What freq. range?
88-108 MHz
>
>In my experiences, intermittent VHF parasites are immune to mistuning.
Interesting.
>Name another Ham, HF-MF 8877 amplifier besides the AL-1500 that uses
>no VHF suppressor.
I see several hams have commented here that their own homebrews
didn't. I don't know the market for amateur 8877 amplifiers so I
cannot name one at present. I do know of those commerical rigs and
there may be others that copied them. I believe that there were some
Italian FM transmitters built in the 1980s and 1990s that also used
the 8877, when the explosive growth of lower power FM broadcast
stations came about in that country. It would be interesting to see
their circuits too, but again, they are most likely VHF line circuits.
>Mmmmm. You are interested in pottery and you needl an outlet for a
>240V-50A electric kiln?
How'd you guess. Also, I am designing a new electric chair for my shed.
73
john
K5PRO
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