Tony King wrote:
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.
For the tubes with grid rings, there seem to be two basic solutions:
either clamp the ring directly onto a simple hole in the chassis (which
the shape of most Russian triodes makes simple); or use finger-stock.
On the other hand, tubes that bring the grid out to pins inevitably have
a very much longer path (and therefore inductance) from the grid to the
chassis. Like you, I would very much like to know what happens to the
feed-through from cathode to anode when the socket is mounted below the
chassis on standoffs... with long, generous loops of wire from the tags
on the socket to the tags bolted to the chassis.
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.
I hope somebody has a wide range of amps, the test equipment and the
time to do it.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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