On the parasitic suppression front, I have some very good data plotted now
that show conclusively the effects of several suppressors on a 40 Mhz plate
strap/cap parallel resonance, that is very strong. Other resonances are not
so important to me right now, as they are things like the pi network
inductor, and the output capacitance resonating with something or another.
The 40 Mhz mode is so big that it will show up in the plot of the entire
input to output response, with the amplifier turned off. The fundamental,
of course, shows here also. I measured the output to input isolation
yesterday, using the output connector to drive the amp, and the HP high Z
probe on the plate, then on the grid. From plate to grid I see about 45 dB
of cold isolation at operating freq in tune, and about 40 dB at the
parasite freq...
The gain is expected to be high, calculated to be over 30 dB class A from
the curves. However, due to things like finite cathode to ground impedance,
I can assume that it will be lower, so it may require more drive. That's
OK, we have 1 KW available, and only planned to use about 250 watts for
starters.
The latest parasitic suppressor is still a big hairpin of copper, with two
shunt 885AS Cesiwid globars across it, and it effects a 30 dB loss to the
mode without touching the normal op freq. (well, about 0.01 dB affect, and
this is a 5% duty amplifier). Today, I received samples of a new Fair Rite
ferrite tile, #42, which is used in Anechroic chambers. It is incredibily
effective when placed next to the plate strap, knocks the >10 Mhz
resonances down another 10 dB. It is possible that this stuff may be useful
for ham amplifiers, but should be used carefully so it doesn't absorb the
desired RF power. It is heavy, fragile, and slightly magnetic.
Once we turn on, will start with the short low Z strap, then if the mode
kicks up, will install the hairpin. I don't want to put bandaid in without
finding the wound first. Will keep posting when next step occurs. Amp will
be installed at the load tomorrow. Have to terminate cables, 15 pin
control, 2 screens, 2 bias, 2 plates, 2 filaments, etc.
John
K5PRO
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