>Hi Folks,
>This amp has a 10 ohm wirewound 10 watt resistor in series with the plate
>RFC, with no cap to ground between them,
** No cap is necessary, but 100 - 300 pF would help to attenuate
harmonics in the VHF region.
>and I burned one up badly this
>morning before it went pop, and took out the power supply main fuses.
** The chain of events: Loose gold meltballs which evaporated from the
gold-plated molybdneum grid cause an anode to grounded-grid arc inside
the 3cx800A7s. If the glitch R is not up to the job -- i.e., it comes
unglued during the arc. It should either be replaced with two
glass-coated 10 - 15 ohm resistors in series or replaced with a
surge-protection resistor that is rated for >than the # of
Joules/watt-seconds of energy stored in the HV filter C. [Globar SP-type]
>I replace it tonite, went back on 15, took tuner out of line, this time I
>saw the flash, got the pop, and then the fuses blew. It cracked the
>ceramic standoff holding the resistor, and the resistor was warm..
> Any ideas ? Company says it in the antenna- bull !
** Spread some of this around fruit trees to get sweeter fruit.
-- The laugher is that when Ten-Tec read the 1988 article on low VHF-Q
parasitic suppressors, they tried the idea but quickly abandoned it when
the VHF suppressors got real-toasty hot at 29MHz. However, if VHF
suppressors don't get hot at 29MHz, they are obviously not going to do
much to suppress/waste energy in the VHF region. The result was that
Ten-Tec stuck with their original high-Q suppressors which are lovely to
behold but sometimes fail to sufficiently lower the parallel-equivalent-R
at the anode's resonant frequency. The fix may be as simple as tapping
the anodes in the vertical plane (base-down) with the flat side of a
ball-peen hammer in order to move the loose meltballs out of harm's way.
To prevent a re-occurence, install lossier VHF parasitic suppressors.
The trade-off is c. 2% less P-out on 10-m. [note -- after tapping, if
the tubes are ever inverted to base-up, re-tapping is in order]
- There's more on gold-migration as well as non-destuctive testing for
the problem on my Web site.
good luck, John
> Got a problem in the
>other direction. Any ideas? Only a 15 meter problem.Had this amp from the
>factory 15 years now.
> 73, John WE2F
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