I agree that the components, esp tubes, have improved since the early
designs. Capacitors are similarly lower L, and higher self-resonance.
For instance, to bypass the filament leads coming out of a Burle 4616
tetrode running at 200 MHz, we used to have a ring of 0.001 ceramic
disk capacitors around each stem of the tube. It was like a pair of
garters made from a two springs, that held the caps from the hot side
to ground side of an insulator ring. 4616 has internal mica bypass
capacitors in the vacuum envelope for both filaments and screen grid.
In this manner, this tetrode can run over 20 dB of power gain, at
200-450 Mhz range. Try that with any other gridded vacuum tube. But
the internal caps are not enough to prevent a little rf leakage from
the stems. In 1993 we rebuilt the amplifiers, and used a small
assembly with 2 x 390 pF ATC 100E ceramic microstrip capacitors,
with very wide ribbon leads. This lowered the RF level to nearly
unmeasurable levels.
In a 200 kW amplifier, push-pull, using a pair of Thomson TH555A
tetrodes, at 2800 KHz, I have a ring of 12 x 2700 pF ATC capacitors
(or Murata Erie, they work too) between the two filaments of the
Thomson Socket. This is in parallel with the built in Pyralux
(copper-clad Kapton film) capacitors that work at VHF in the socket
itself. I tried mica blocks, doorknob ceramics, disk ceramics,
nothing worked as well as the low L high Q ceramic capacitors in
presenting a nearly short circuit across those terminals at my
operating frequency. I have pages of printouts of plots taken with
the Hp 4193A vector Z meter, from 1 to 100 MHz.
That amplifier has no parasitic suppressors, although my network
analyzer pointed out a large anode resonance at 40 MHz, due to the
first output shunt capacitor, a Comet 450 pF model across the input
to a pi network. I built suppressors, finalizing the design on using
a copper hairpin U strap, loaded with a Carborundum resistor, and
also used a ferrite tile from Philips. It worked beautifully on the
cold measurments, having knocked down the effects of the resonance on
S21 by 20 dB or more, but with less than a few hundreths of a dB loss
at 2800. But, when we first powered up, we thought of running without
the suppressor, then adding it if necessary. It was not needed, the
amplifier never 'invoked' oscillation at HF besides amplifying what
it was driven at. Perhaps someday I'll hook it up just for a power
test..... But the amplifier has been running for 2 years now on these
tubes.....
The tube socket does have a radial ring of ferrite cores, around the
screen connection, to suppress a waveguide mode which is in the UHF
region. This is needed as we operate Class A for extreme linearity.
Power gain is over 30 dB in this one stage. Drive comes from an ENI
300 watt amplifier, loafing.
I have provision for neutralization, being a push pull amplifier
thats easy to do. But it, too, was not installed. Some might say this
is lowdown and dirty, but if it doesn't need it, why add more
components? Lucky this was a single frequency amplifier, that helps a
lot.
W8JI said:
>Exactly. Let me tell you what I've been doing the past few months.
>I put a 1200Z7 in an RF deck of the AL1200, and with multiple
>fingerstock contacts the tube was totally stable with no
>suppression at all. That allowed me to add inductance between the
>tube and the tank, to reduce Q on ten meters and bring efficiency
>way up. The only reason that works is because the Z7 has virtually
>zero grid impedance to the chassis. A 1200A7 will oscillate like
>crazy with no suppression, or with suppression and that series coil.
>
>I did the same to a 3CX800A7. I added a grid collet instead of a few
>fingers on the grid, and I could remove all suppression on a
>3CX800. That let me use the same technique to put an AL-800
>amp on six meters (bandswitched from 160-6 meters) and get 1200
>watts out on six meters!
>
>I have a 3CPX800A7 with 3000 volts on the anode, no suppression,
>a VHF Q of over 1000 in the tank system, and it is completely
>stable. That PA runs 1100 watts out on 2 meters with 30 watts of
>drive, and is rock stable. The reason? The grids are very well
>grounded both inside and outside the tube. The grid "problem" is far
>above the operating frequency so stability isn't an issue.
17 years ago, I helped design an FM broadcast transmitter for 1500
watts, using the Eimac 8877/3CX1500A7 in the PA. By modifying a
Johnson socket to get very low inductance around the grid connection,
we made a very stable cathode-driven triode amplifier for 88-108 MHz,
without any suppressor circuitry. Hundreds of those rigs were sold.
By my SB220 does have anode parasitic suppressors.
K5PRO
John
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