At 02:47 PM 11/4/2004, Ian White, G3SEK, wrote:
<snip>
Neither of those responses addresses my point: if VHF amplifiers using
these tubes are stable under worst-case conditions, where the input and
output tanks are unloaded and tuned to the same frequency, then it is
unreasonable to expect HF amplifiers using the same tubes to be liable to
VHF parasitic oscillations.
(Still provided that the grid ring is solidly grounded to the chassis, and
all other feedback paths have been eliminated by shielding and RF decoupling.)
My HF GS-35B provides a kind-of-backhanded proof of this. It has the
usual parallel L&R in the anode circuit, but by courtesy of Steve
G8GSQ's network analyser, we discovered that doesn't provide any
significant damping at the VHF parasitic resonance frequency! But there
is no parasitic oscillation... because the grid ring is solidly
Ian -- What is the resonant frequency of the grid in this fixture? Does
the grid-grounding fixture have zero inductance?
Nothing has literally zero inductance, but the GS35B is a planar triode
with a shallow domed grid which is 360deg bonded to the grid ring. The
grid ring is dropped onto a hole in the chassis, and then clamped down
with 360deg contact. Grid inductance in a large power tube doesn't get
much lower than that.
I'm curious about what kind of mounting fixture you are using. There are
two basic fixtures seen in the many HF amps on the web:
1) Directly clamped to the chassis
2) Clamped to a plate which is fastened below the chassis with threaded
standoffs.
Both of them are popular if the pictures are any indication. The plate
below chassis using threaded standoffs appears that it would have a much
larger stray inductance than the other.
The resonant frequency where the grid becomes totally ineffective is
probably some way above 1GHz... which is precisely why the tube is so
reluctant to oscillate at lower frequencies.
Much the same applies to the 8877, but this is not such a high-frequency
tube. It has coaxial construction with a tubular grid, and this will act
as a tuned line, loaded by the anode capacitance. Quarter-wave resonance
of the grid structure would be at a several hundred MHz (at a guess), but
this is still high enough to ensure that - again, always, with solid
grounding of the grid ring and no external feedback paths - VHF amplifiers
using the 8877 are unconditionally stable.
If you want to know where the grid resonance is, you'd have to put the
tube (cold) into a fixture with totally shielded cathode and anode
compartments. Ground the grid in your chosen manner, and then make a
frequency swept measurement of the RF leakage between the anode and the
cathode. The grid resonance should show up very clearly as a peak in
transmission.
Actually, that would be a very useful set of measurements for someone to
make. Start with a 572B in a regular socket, and work upward to the
VHF/UHF tubes with a solidly grounded grid ring.
Looking forward to seeing the results of that.
Watching/reading with much interest... enjoying it a lot!
73, Tony W4ZT
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|