Hi John,
-
Regarding the "no parasitic suppressor" portion of
your story below...
-
I would think the potential for RF amplifier parasitics
are from quite the mixed bag of sources. Each chassis
layout and potentially each varied tube combination
as an example...
-
As some of the reported VHF unstable SB-220's
were tamed with "updated suppressors", some
same circuit, different tube - amp Combinations
were for the most part, always stable with the
same basic physical layout.
-
I've found what I belive to be unofficial similar results
with SB-220 and other amplifiers. My BTI LK's
included. One LK wants to sing like a dog while
the other has always been pretty much rock
solid stable.
-
The point is you might have just gotten lucky not
including VHF suppressors. Although you had
good results, could you be sure there would be
no surprises in various tube circuit combinations..?
-
I would expect the only real proof of a stable circuit
would be a track record over time. I would also
consider the addition of suppressors a practical
addition and some measure (no pun intended)
of prevention. Pictures of damaged tubes make
me nervous.
-
cheers
skipp
-
-
From: "John T. M. Lyles" <jtml@lanl.gov>
I got the RFPP HFS1000G converted to 5 MHz a week back, at work. It
was at ISM freq 13.56 MHz. Now it is driving a dedicated experiment,
easily providing a kW output. Efficiency is about 69% not counting
drive feedthru. The 8877/3CX1500A7 takes plenty of abuse without even
flinching. It self biases via cathode resistor only, at about 200 mA
idling. Dumping about 600 watts in the anode in this case. With drive
it is better.
Changed the input to a simple pi, low Q, with no tuning needed.
Output L was replaced with 3/16 refrig tubing coil, 12 turns, about 7
uH. Added 150 pF padding with 15 kV ceramic doorknob caps, one NPO
and one N750, for the C-tune. C-load padded with a stack of heavy
duty surplus Erie RF chip caps at 670 pF. Used Hp impedance meter
looking at the anode, with a 50 ohm load on the output. Set the
controls for 2500 ohm loadline at the tube, and then it came on first
time, hardly needed any tuning. By the way, the 8877 doesn't have
parasitic suppressor. Its very stable - no surprises. Using an ENI
240L 40 watt solid state broadband amp as the driver for the 8877.
73
John
K5PRO
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