I suppose Rich is referring to amateur radio
power amplifiers that he is 'aware' of when this
was stated:
>
>Every manufacturer that I'm aware of on this planet, except?
>Ameritron, uses a parasite suppressor on their 8877. I'm doubting?
>that everybody else is out of step and only Ameritron got it right.??
>Just how the AL-1500's designer came to the conclusion that 5000-ohms?
>of feedback XC at the tube's max freq rating could not cause
>oscillation is a mystery.
>
>R L MEASURES, AG6K. 805-386-3734
I will provide some other examples which don't,
to improve this awareness. Several successful
commercial power amplifiers that I know well do
not have suppressors between the anode and tank.
The Broadcast Electronics FM1.5A/B fm transmitter
which i helped design back in 1983 with Bill
Harness and Geoff Mendenhall in Quincy, IL, ran
8877 in class B in a transmission line circuit at
1500 watts TPO, and didn't require a suppressor.
The RF plasma Products HFS-1000G 13.56/27.12/40
MHz industrial RF generator had none either. I
converted one of these over to 5 MHz operation in
October of 2002 ( see archives here for my
comments then). It worked beautifully and had no
instabilities. With this 5 MHz amplifier, the
idling current (Zsac) was set for class AB1
setting. I ran it into horrible mismatches at 1
kW output, adding an AGC loop to maintain full
output even into mismatches, as I was testing
ferrite cores for a new synchrotron accelerator
design, and needed consistent voltage across it
as it swept frequency and tune.
I built a 100 kW class A pulsed amplifier here at
work in 1998, using a push pull pair of Thales
TH555A tetrodes, each rated for 200 kW in the HF
range. Using ferrite cores mounted around the
socket (basically bolted to the screen ring) this
amplifier was also stable without drive and
mistuned. I originally designed a parasitic
suppressor using ferrite tiles made by Fair Rite
company, that sat bolted within the fold of a
hairpin inductor in the anode to tuning cap
strap. This worked well on the cold sweeps using
a network analyzer, in getting rid of a pesky
high Q peak at 17 MHz. However, under power, the
amplifier proved that it was not needed, so the
parts were not installed. The amplifer runs at
2800 KHz.
I am not saying that every amplifier can be built
without suppressors, but the blanket statement
that every 8877/3CX1500A design has them, is
unrealistic and also not correct. Layout and
component selection of the final tuning capacitor
and tank circuit, as well as input network
design, affects amplifier sensitivity to
parasites, as well as the G-BW product of the
tube, as Rich also refers to - for the 8877
triode.
Keep up the good work, I rather enjoy discussions
over arcane topics like parasites (which are the
bane for every tube power amplifier designer)
instead of how to wire AC circuits in a ham
shack. Although, I am about to embark on a new
garage project - planning to have 220VAC single
phase outlet in the back for you know what....
73
John
K5PRO
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