>
>On Wed, 22 Apr 98 22:56:49 -0500 Jon Ogden <jono@webspun.com> writes:
>
>SNIP
>
>
>>Please, if you have a view explain why and give me some scientific
>>reason behind your science. I am engaging you, Rich and the whole mail
>>reflector on this topic because I am trying to learn what is actually
>>happening. So if I have technically gotten something wrong, please
>>point it out to me. Don't just tell me I am wrong. How will I learn what
>>the correct answer is?
>>
>>73,
>>
>>Jon
>>KE9NA
>
>Jon, I guess I didn't make my point clear...sorry but when I am real
>short for time I over simplify.
>
>First of all I have no particular disagreement with the use of a somewhat
>lossy material in the suppressor.
At least, not since N7WS published his vhf suppressor measurements.
>I have commented since the beginning of
>this reflector that National Radio went that route in a military tetrode
>amp back in the 60's. I use that same strap today in my work.....bought a
>roll of it at the final National IRS auction several years ago. The main
>difference is that Rich tries to get by with #16
#18 Ni-Cr-Fe alloy
>wire whereas National
>used 1/2" wide strap around a big 50 Ohm non-inductive resistor.
There is no such thing as a non-inductive resistor. A big globar type
resistor has over 100nH, which is roughly the same L as a typical
suppressor inductor has.
>I do object to using that material in the complete plate to blocking cap
>path and prefer the traditional low inductance wide silver plated
>strap/tubing strap.
The two, parallel resistance-wire arrangement we use has about the same
inductance as a silver-plated strap of the same width, which is why the
anode-resonace changes little after the resistance wires are installed.
Sure, in medium size amplifiers it gets pretty warm at 28MHz due to its
inherently high VHF-ESR. . For tetrodes with handles size amplifiers,
we supply nichrome-80, 0.25" ribbon.
>If the suppressor is doing its proper job right at
>the tube then there is no reason to continue nichrome to the blocking
>cap.
Every VHF ESR-ohm in the anode resonant circuit contributes to Decreasing
the VHF parallel-equivalent resistance (Rp) load presented to the anode,
which decreases the VHF amplification of the tube. .
>Layout is critical and in a sloppy version it may be necessary to use
>more nichrome or whatever. But in a good layout I would think that the
>lowest possible inductance in order to move resonance above the tubes
>ability to oscillate would be the "good engineering" approach.
>
Indeed. The Tune C should be close to the anode. To minimize L, vacuum
capacitors should be mounted vertically , as Jennings recommends, and
driven through a Multronics right angle drive under the chassis.
>Secondly, I have harped for a year about VHF resonances in the tank
>circuit with nary a bit of comment from anyone. Now I see that Rich has
>finally added that possibility to his ever varying repertoire.
As I recall: I discussed this with Mr. Rauch during the grate
parasitics debate, and we semi-agreed that a typical HF tank inductor
acted pretty much like an RF choke from 90-150MHz. The major VHF
resonances are in the Tune capacitor.
>National Radio solved that problem very neatly in the NCL-2000 and NCX-1000 by
>adding a SERIES cap...not a shunt as has been discussed about here the
>past few days. The National engineers added a 10pf capacitor across the
>20 and 40M shorting bandswitch contacts which eliminated a particularly
>nasty proclivity to arcing.; perhaps you would wish to analyze that
>circuit.
It would be better than using no 10pF capacitor, however, there would
seemingly be less stray inductance if the capacitor made a less
circuitous path to gnd.
..........
>Thirdly, my dis-belief centers around the big bang and that is where I
>suggested ( or that was my intent anyway) an article. You are the
>engineer, show me how a VHF parasitic has enough energy to bend a 3-500Z
>filament.
The parasitic apparently produces a surge in DC grid current because an
underloaded high Mu triode oscillator exhibits large grid current. In a
TL-922, the grid current surge is large enough to burn out the 1A RFC
from the grid to gnd. Since all grid current comes from the thoriated
tungsten filament, and all electric currents produce mechanical force,
why couldn't enough current produce sufficient force to bend the hot
(1820 deg. K) filament helices? How does one know that a parasite took
place during the big bang/shorted 3-500Z filament/grid? For starters, I
would measure the resistance of the VHF suppressor resistors and inspect
their appearance.
>I say it is a gas/plasma discharge
However, in a G-G amplifier a plasma discharge would be from anode to
grounded grid. Such an arc would not reach the filament. In the dozen
or so kaput 3-500Zs, with bent filament-helices, I have autopsied, I have
not found one with an arc mark on the grid cage. {see page 15 of 9/90
*QST*}
>and partially caused by barnacling
It is my opinion that if there were "barnacles" that produced internal
arcs, one would undoubtedly find arc-marks when one finds bent filament
helices. . So far, the barnacles theorm is not supported by evidence
-- i.e., "barnacles" may be a 'sea-story'/ 'junk-science'.
>
>.........
>And finally....No, I have not built a ton of amps but there are certainly
>plenty of established companies that have. I would prefer to give their
>engineers the credit they deserve and cite the megatons of stable amps
>that they have shipped to military and other customers. A few persons act
>as if they are the sole sources of these revolutionary "new ideas" but in
>reality there is absolutely nothing new at all.
Agreed. F.E. Handy wrote about it in 1926. Another chap apparently
wrote about in 1935, I think in the IRE journal.
>.........
>My own suggestions for an amp would include:
>
>1. HV surge suppressor resistor of a value and wattage that can limit the
>instantaneous PS discharge current to a safe non-destructive value.
agreed
>2. Use of a suitably placed series capacitor in the tank circuit to
>provide a low impedence VHF path to the load.
What kind of a VHF load is a 20m yagi? It seemingly makes more sense to
put 10pF from the output end of the Rs/Ls vhf suppressor to chassis gnd.
>3. Use of a suitably sized parasitic suppressor resistor and phase out
>carbon composition. A 5W metal oxide should be the minimum for 3-500Z
>size tubes; a pair of them for the 4-1000A.
However, one should know how much intrinsic inductance Rs has. Not just
any old MOF resistor has low L. The resistors we use in our vhf
suppressor retrofit kits have c. 11nH/0.011uH.
>4. Use of a resistive suppressor L material only if necessary to tame a
>known problem tube such as the 3CX1200A7 or to compensate for layout
>problems.
Why wait for trouble to come knocking?
- later, Carl.
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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