On Jul 30, 2006, at 8:18 AM, Tom W8JI wrote:
>> Tom, I am not aware of what amp you might be referring to
>> here, can you lwt
>> me know...PLEASE! Thanks 73 Lou
>
> When the Chinese were making 3-500Z tubes with regular glass
> and holes were sucking in the sides, Rich concluded it was a
> VHF parasitic that couldn't be seen or measured. This VHF
> oscillation supposedly caused dielectric heating of glass at
> VHF.
Correct. Rus Healy at QST was the first who told me of this
phenomenon. However, it took Eimac a while to come up with a glass
recipe that would not melt at VHF, so I was not surprised that the
Chinese initially encountered the same difficulty.
>
> Paul Hrivnak of Viewstar, Palstar, Vectronics fame was
> building a single 3-500 amp using that particular Chinese
> tube. He changed to nichrome suppressors and tubes kept
> right on failing.
Since the VHF gain of a HF amplifier's tube is proportional to the Q
of it's VHF suppressor, merely changing a suppressor's conductor
material from copper (Cu) or silver (Ag) to Ni-Cr will make no
difference in VHF gain if the suppressor's VHF Q is not lowered.
Example: The Q of the suppressor in a 922 or a 2K-4 is c. 4.5.
Since one can easily build a suppressor using Cu that has a Q of 0.5
at 100MHz, why not make the mod? The sticky wicket is that a Cu
suppressor whose Q is 0.5 would have so much L that it would need a
resistor that can dissipate c. 45W at 29MHz. The benefit of Ni-Cr is
that due to its having 50x more R than Cu, it lessens the dissipative
burden on the suppressor resistor, not that is has magic power.
>
> 73 Tom
>
>
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>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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