When there are spurious oscillations in the PA tubes, is there a particular frequency this usually occurs? In other words, does changing bands change the frequency of the oscillation? I'm just wondering what frequency I might try listening to in order to hear them.
Thanks,
Barry - N4BUQ
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It is NOT normal. It is a sign that the amplifier needs to be
neutralized. Plate dip and power peak should be simultaneous. The
coincidence of these two is a very sensitive measure of neutralization.
For the T-4 and some other models Drake specifies setting the
neutralization using the coincidence but if the neutralization is not
almost correct this may be a difficult adjustment to make. A better way
to check neutralization is to measure the feed through of drive power to
the plate by disconnecting the screens of tetrodes or the plates of
triodes. Use something sensitive like a receiver or scope to see what is
coming through, then set the neutralization adjustment to null it. Once
that is adjusted you can check the neutralization via the Drake method,
that is see if the plate dip and power peak are simultaneous.
There is a great deal of information on neutralizing on the web.
You should be familiar with the theory of what is being done. The idea
is roughly to eliminate any positive feedback through the tubes by means
of a sort of bridge circuit. It is done to prevent the tubes from
oscillating. If that happens the oscillation will be uncontrolled and
destroy the tubes. In general neutralization is more critical at higher
frequencies. You can get approximate neutralization at, say 20 meters,
and then check it at 10 meters making any small adjustments necessary.
BTW, this is not a matter of the type of circuit used to match the
output to the antenna, neutralization is necessary for any type of
output pi-network or otherwise.
On 4/3/2025 6:53 AM, n4buq wrote:
I don't think I've ever owned a power meter and now I have one in my MN-2000.
Tuning up my TR-4, I notice that maximizing the output power doesn't
necessarily coincide with the "dip" in plate current but, instead, seems to
occur on the "edge" of that dip. Is this normal?
From many, many years ago, with only a plate meter, I always adjusted with the
dip at its minimum value but now I'm wondering if that wasn't yielding maximum
power output. Furthermore, I presume that getting the dip first and then
tweaking for maximum output is the most efficient way to run. Is that correct?
Is what I'm seeing normal? I presume so but after all these years, I'm
beginning to wonder.
If it matters, I'm currently only tuning into a Drake dummy load.
Thanks,
Barry - N4BUQ
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
