>>/\ 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.
/\ thanks, John. Others on this group would have stonewalled.
> 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.
/\ Were any of these amplifiers repaired by the factory?
>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.
/\ Gold-sputtering failure cannot be determined without either autopsy
or the Funderberg gold meltball test.
>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.
/\ Damage from VHF parasitics is rare in VHF amplifiers.
>BTW There have been NO parasitic chokes in ANY
>of these VHF amps.
/\ VHF parasitic suppressors are undoubtedly bad news in VHF amplifiers.
>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--
/\ My guess is that you have a longer fuse and that you would be
unlikely to cut corners by:
1. using coupling-type caps, in place of more expensive padder-types,
2. use less than stable carbon-composition resistors as electrolytic
"equalizer" resistors. .
>but I have used (roughly) more than a 1,000 Eimac
>oxide-coated cathode type tubes without any issues
>due to VHF parasitics.
/\ Did you peruse the Eimac Letter on my Website? After the feculence
hit the cooling fan - due my amplifier article in the January, 1994
*QST*, an Eimac employee (Reid Brandon) told *QST* that the Eimac
employee who wrote me the letter was "not authorized" to tell me about
> Maybe I am just lucky or maybe
>the design was good. I'll let you decide.
/\ Designing an unconditionally-stable HF/MF amplifier is more difficult
than designing a stable amplifier for VHF. Although I have heard
reports thereof, I have never seen parasitic damage in a VHF or UHF
>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.)
/\ True, but the 8877 heat dam problem lasted c. 14-months. The laugher
is that some amplifier manufacturers are still using this to "explain"
tube failures whose date-code is not within the 14-month period that the
problem existed. Eimac exacerbates the problem because they cheerfully
replace all failures within the 12-month warranty period. In a letter
Dick Ehrhorn wrote to me, he said that he knew no tube had ever been
damaged by parasitics in an Alpha amplifier because Eimac always replaced
damaged tubes. Dick assumed that the failures were always Eimac's fault
or they would have refused replacement. Harry Truman called this sort
of thing passing the buck.
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
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