OK, it's perfectly fine to have such a diversity of thought on parasitic
suppressors and on parasitic generation itself. By the way, I have built
VHF solid state power amplifiers where they oscillate at LF. These are the
bane of bipolar power amplfiers, as the LF beta is high, and inadequate
bypassing and decoupling networks on the collector and base bias can cause
it to want to take off a KHz to low MHz.
It's a paradoxical that with tube amplifiers it's just the opposite! Get
them to work at HF, and they want to oscillate at VHF/UHF. The principles
are, of course, different due to the large structures involved with tube
circuitry.
In the present amplifier I am developing using 150-250 Kw tetrodes, the
bias is pulsed up from cutoff only 5% of the time, so the average
dissipation is 30 KW or so. It is CLASS A during the on-time. This thing is
for 2-6 MHz tunable. When I told one tube supplier that I was planning
Class A, they gulped and said I would be lucky if it doesn't excite UHF to
L band oscillation. The class A gain of big tetrodes is high, over 20 dB
possible from the curves. The parasitic oscillation is mainly from the
large radial geometry of these tubes in the plate/screen to grid region. A
TE11 or TE21 circular mode can be excited. It is commonly seen when trying
to make characteristic curves in the test of big tubes. So one version of
my tube socket will have with a ring of lossy ferrite around the screen
grid (out of HV harms way of course). I will report how it comes out, in a
few months, once we fire it up. The beauty of using low DF testing, is that
we can reduce our bias pulse to just a few hundred microseconds, and the
repetition rate to 10 Hz or so. Any breakup will happen in the short
intervals where the average power is low. This is where building CW/SSB
linear amplifiers for ham/industrial applications have a challenge - to
keep the parasitics from doing major harm since the HV stays on, along with
the RF. Lucky that the glass and smaller ceramic-metal tubes (without
handles!) have small interelectrode spacing, and these radial field modes
are above a GHz.
John
K5PRO
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