...
>IMHO, thousands of good amps in daily use testify to Tom's credentials as
>a competent designer.
Mr. Rauch's 8877 amplifier design, the MFJ/Ameritron AL-1500 uses no VHF
suppressor whatsoever. I know of no other HF 8877 amplifier on this
planet that does not use one. During the parasitics debate, Mr. Rauch
stonewalled the issue of whether the AL-1500 even used a VHF suppressor.
Finally, an owner e-mailed with the information that it did not use such
a suppressor.
Mr. Rauch claims that his single-ended grounded-grid amplifier designs
are "neutralized". Are yours?
...
>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.
Any ARRL member can submit a manuscript to *QST*. Prior to 1988, there
was no information in any ARRL prublication on how to design a VHF
suppressor. If you have knowledge on the subject, Mr. Erhorn, why not
write an article about it and submit the manuscript to *QST*?. How
about you, Mr. Rauch? Your friend Reid Brandon, who works at Eimac, said
he was not aware of any article on VHF parasitics. Why didn't he write
an article on VHF parasitics?
>He seems to imply, if not stating it outright, that every commercial amp has
parasitics
>and that they're killing tubes enmass.
References?
>He might lead you to think it's the greatest issue in RF engineering. That
just ain't so.
True. However, it is an issue. One of the gold-sputter damaged tubes
shown in "Parasitics Revisited" was removed from an Alpha amplifier. Say
it ain't so Mr. Erhorn.
>
I have never heard of a VHF parasitic in a 30S-1.
>... Despite having talked with thousands of
>customers since 1970, I know of no credible evidence of VHF parasitics in
>any Alpha ever built ...
Have you ever taken apart a kaput tube that uses a gold-plated grid? .
. Have you ever performed the Funderberg test for gold-sputtering in a
kaput tube?
When *QST* was copy-editing "Parasitics Revisited" for publication in the
9/90 and 10/90 issues, I received a hand-written inquiry from Mr. Erhorn.
According to our records, he placed order #1265, which was entered on
Wednesday, 4 July, 1990. We sent him some parasitic suppressor retrofit
kits and a copy of the manuscript for "Parasitics Revisited".
Subsequently, I received another letter from Mr. Erhorn. In this letter,
he said that he did not agree that 8877s, 8874s and the like were being
fatally-damaged by intermittent VHF parasitic-oscillations. He said
that since Eimac replaced the kaput tubes he sent in, no questions asked,
the failure must have been due to a manufacuring defect. . Mr. Erhorn
said that if these tube failures were ETO's fault, Eimac would not have
replaced the tubes -- hence, Alphas do not have parasitic oscillation
problems. . . . . I telephoned Eimac. An Eimac-engineer said that even
though Eimac engineers know about gold-sputtering damage, Eimac's
warranty dept. does not. When a returned, under-warranty tube shows any
kind of internal leakage, it is simply tossed into a scrap bucket and
replaced - no questions asked. I commented that it was nuts to replace
tubes that were damaged by a semi-stable amplifier design. He agreed and
explained that Eimac's warranty dept. does NOT talk to Eimac's
engineering dept. I got the impression that Mr. Erhorn has never taken
apart a kaput 8877, 3CX800A7 et cetera to see what's what. OTOH, Mr.
Rauch has -- although he seems to be a bit reluctant to talk about the
distribution of gold-blisters on the damaged grid.
>
>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 totally-unsubstantiated rumour that I heard about why GE cancelled
the contract went a bit different -- as in Not Invented Here syndrome.
>
...
>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.
You do not know unless you take the failed tube apart or perform the
Funderberg test.
>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.
I test kaput 8877, et cetera that amplifier owners send me. On average,
about 2 out 3 oxide-cathode high-Mu triodes with gold-plated grids fail
from gold-sputtering. Heat dam g-k shorts are relatively rare. These
tubes were only produced for a bit over a year. IMO, you are blaming
everything on Eimac, which is essentially the same thing you said in your
letter.
Hmm. The owner of a large company hand-writes letters. . . .
>
Rich...
R. L. Measures, 805-386-3734, AG6K
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