Greetings All...
Bob, your note motivates me to throw caution to the winds and dive into The
Great Techno-Babble Debate. I think you're equating nonequals.
IMHO, thousands of good amps in daily use testify to Tom's credentials as a
competent designer. I don't agree with everything he says and it makes me
uncomfortable when exchanges get nasty (reminds me of 75M). But the vast
majority of Tom's amplifier-related technical comments correlate very closely
with my own experiences over (too many) decades of designing and debugging
power amps spanning the 807, 6146, 813, 4-250A, 4-400A, 4-1000A, 8122, 8874,
8877, 3CX1000A, 3CV1500A, 3CPX5000, 5CX1500, etc.
On the other hand, IMHO RIch often gives the strong impression that he thinks
no amplifier designer other than himself has any clue how to suppress/avoid VHF
parasitics. He seems to imply, if not stating it outright, that every
commercial amp has parasitics and that they're killing tubes enmass. He might
lead you to think it's the greatest issue in RF engineering. That just ain't
so.
Sure, some sloppily-designed amps have been produced by engineering- and/or
ethics-challenged companies in the past. But IMO the issue is pretty much a red
herring today. Despite having talked with thousands of customers since 1970, I
know of no credible evidence of VHF parasitics in any Alpha ever built and only
one isolated instance of a VLF parasitic - in 1971 in one of the first Alphas
delivered. I flew from Florida to the W6's home to fix it.
The 3CPX5000 is well known to be susceptible to internal cavity-type VHF/UHF
parasitics, which Tom and others have discussed here recently. Yet, with more
than 3000 of these tubes in medical/MRI service for up to 7 years now, in our
experience catastrophic failures (as in "VHF parasitics! The sky is falling!")
and premature end-of-life are virtually zero.
The average useful life of thousands of 8877's and 3CPX5000's in our
MRI-service amps is about 18,500 hours. What does that say about implications
that tube-killer parasitic oscillations are endemic so that every amp needs a
"fix"? The figure reflects normal old age end-of-life due to gradual loss of
emission and the like, as well as catastrophic failures (open heater, shorts,
loss of vacuum, etc.) It excludes ONLY the late-1980's epidemic of 8877 g-k
shorts caused by the disastrous tube design change. That screwup was corrected,
probably in '89, and our experience with 8877/3CPX1500A7 reliability has since
been excellent.
More frequent AC on/off cycling typical of ham radio use may reduce life
modestly, but lots of 15-23 year old 76's, 374's & 78's are still out there
cookin' with their original 8874s. 3CX800A7 failures from all causes as
reported to us are extremely low - we estimate the rate at less than 1% per
year of all tubes delivered in the last 10 years. There's no way to know how
many people may have replaced tubes without contacting us, but we're generally
our customers' lowest-cost source of new tubes and in any case most guys aren't
shy about calling when something fails. I can speak only from ETO/Alpha
experience, but would expect that other responsible mfrs using the same or
similar tubes have similarly good experience with them. Is anyone else willing
and able to offer first-hand feedback regarding experience with large
quantities of tubes in your products???
I hope that sticking my neck out like this will help y'all understand why I -
as well as Tom and most of the other professional amp designers who frequent
this site - scoff at innuendos and insinuations that a great epidemic of
parasitic-prone HF amplifiers has been foisted on you by an industry populated
entirely by incompetents. Again, it just ain't so! BTW, I haven't talked with
Tom and my intent isn't so much to defend him, which he does very well without
my help, as to present the facts as they look from another perspective.
Finally, Rich, I didn't say that 8877s at ARRL failed due to grid-cathode
shorting. If you read it carefully, I said that a great many - perhaps nearly
all - 8877's built during a lengthy period around 1988-89 did fail prematurely
from grid-cathode shorts caused by the ill-conceived heat dam change. It nearly
killed us. My additional point was that virtually all 8877 failures WHICH WERE
DUE TO GRID-CATHODE SHORTS in that time period were a direct result of that
design fault. Presumably not EVERY 8877 failure during that time resulted from
a grid-cathode fault, but I'd bet that over 99% did.
Holiday greetings and More Power to All!
73, Dick W0ID (ex-W4ETO, WA4NGO, K6CTV, W0FID)
----------
From: bob alexander[SMTP:realex@flash.net]
Sent: Thursday, November 27, 1997 6:37 AM
To: amps@contesting.com
Subject: [AMPS] techno-babble
Howdy,
To paraphrase one of Tom's postings; between the two of them,the
postings of Rich and Tom are 198% nonsense. Perhaps, if the King of
Parasitics and the Master of Techno-babble got together we would
end up with parasitic technobabble. This new form of parasitic can
easily be suppressed by pressing the delete key on your computer
keyboard.
Come to think of it, maybe thats why there are two DEL keys on most
keyboards.
73, Bob
W5AH
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