>/\ John - what kind of HF amplifier do you use?
I'll bite. I had an Alpha 86, and have three Alpha 87As, and
an Alpha 77SX. I have never had ANY tube failures in any
of them. The 87As are used quite heavily in contests and
the 77SX is only used in RTTY contests. No tube failures--
period. Nada--in any of them.
I have also designed and built HUNDREDS of wind profiler
transmitters at 49 MHz (class C, 2% duty cycle, 50 KW to
100 KW peak power output). Early ones ran an 8874
as a driver (4CX250 driver in some of the very early ones)
and a 3CPX1500A7 as a final. The later ones ran a 3CX800A7
as the driver and a 3CX5000A7 or a 3CPX5000A7
(depending on vintage) as the final.
I have never seen any tube failures from VHF parasitics.
Many of these transmitters have run 24/7 for DECADES
(first ones were delivered to a NOAA site in 1978 in
Alaska and are still running in the Pacific at NOAA sites
for the study of El Nino). Tube failures are just lifetime
issues. Emission drops after many years. NOAA planned
to replace tubes on the original transmitters every year
(24/7 operation in outside enclosures north of Fairbanks)
but the tubes weren't deteriorating that fast. So they
decided to only change when the performance started
dropping and they got multi years of service in the
harsh environment with continuous operation at 49 MHz.
BTW There have been NO parasitic chokes in ANY
of these VHF amps. And because of the high peak
plate currents there are no resistors in the HV supply.
When the tubes are new there may be a tube thump
or even two--but there has never been any damage to
a tube due to a tube thump--even without current limiting
resistors. (Not recommended but necessary since
these are pulse amps and the voltage drop and power
dissipation in the series resistor is high with high peak
Dick Ehrhorn clearly has more experience than I do--
but I have used (roughly) more than a 1,000 Eimac
oxide-coated cathode type tubes without any issues
due to VHF parasitics. Maybe I am just lucky or maybe
the design was good. I'll let you decide.
Of course this does not cover any Eimac tubes that have
had manufacturing problems--such as the heat dam issue
for which the 8877s are notorious. (Occasionally Eimac
has had brain-flatulence issues with which to contend.)
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