On Nov 4, 2004, at 11:47 AM, Ian White, G3SEK wrote:
R. Measures wrote:
On Nov 4, 2004, at 12:23 AM, Ian White, G3SEK wrote:
K7RDX wrote:
My hf GS-35b AMP has a supressor only in the cathode and has been
super stable for the last four years.....
GS-35B amps on 432MHz, 144Mhz and 50MHz are unconditionally stable
with NO parasitic suppressors at all. That's with the anode and
cathode deliberately tuned to the same frequencies, and any
combination of input and output tuning and loading.
-- But of course -- the only amplifiers that have VHF parasitics
are those that have two resonances in the anode circuit. In a VHF
or UHF amplifier, there is but one resonance in the anode circuit
because the anode-C itself is all or most of the Tune-C. HF and MF
amplifiers are unique in that a separate Tune-C connects to the anode
by a conductor with a DC-blocker cap. in the middle. The L in this
conductor plus the L in the DC blocker -- in conjunction with the
anode-C in series (via the chassis) with the Tune-C forms a VHF
resonant circuit that is Not on the schematic diagram. - note -
The lowest anode circuit resonance that I've measured was 42MHz in a
40kW pep, 12MHz broadcast amplifier and the highest was 160MHz in a
1, 8873 HF amplifier.
With the grid ring solidly clamped to the chassis, the GS-35B will
not oscillate at VHF, regardless of *any* input or output
resonances.
Exactly the same can be said for the 8877 - another tube that can be
used in tuned amplifiers from HF though VHF. If its grid ring is
grounded directly to the chassis, there will be no on-frequency
oscillation in a VHF amp, and no VHF parasitic oscillation in an HF
amp.
During the testing phase, the Eimac engineering team that developed
the 8877 discovered that it was capable of sustaining an "oscillation
condition" that caused thin layers of the gold plating on the grid to
boil off and condense into meltballs. An 8877 has 0.1 pF of
anode-cathode C. A GS-35b has 0.12pF of anode-cathode C.
- note - The presence of loose gold can be non-destructively
confirmed with a high-pot tester by measuring leakage-I, reversing
polarity and re-measuring leakage-I. If the leakage current is
higher with pos. on the anode than with neg. on the anode, the
leakage is quite likely from loose gold. If the leakage is = for
both pos. and for neg. the leakage is from a tube flatulent or from a
bad metal-ceramic seal solder job. .
Neither of those responses addresses my point: if VHF amplifiers using
these tubes are stable under worst-case conditions,
Ian -- I know of VHF amplifiers that oscillated, but there were no
fireworks.
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.
I have autopsied too many tubes that were removed from HF amplifiers to
agree
I had a 4cx250R 70cm amplifier oscillate, and a friend had a 8938 70cm
amplifier oscillate. However, in each case there was no damage because
the oscillation's energy simply passed into the load. - note - In each
amplifier, the problem was solved by using fewer pF in the HV bypass-C.
- HF amplifiers suffer damage from parasites because there is no way
for the VHF energy produced thereby to reach the load.
(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,
Agreed.
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.
Yea, verily.
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.
makes sense
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.
Excellent idea.
- Since the grid-resonance of a 3-500Z is c. 88MHz, has anyone tried
lengthening the anode-lead to move the anode-resonance below 88MHz?
--
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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