>> Don was pretty insistent about installing the AG6K Low VHF-Q supressor kit
>>based on the numerous articles written and also based on other amateur's
>>reported success with the kit. He also told me tonight that he knows a guy
>in >Wyoming with an older AL-80 who apparently had similar problems 'till he
>installed >a different supressor. We did not install any of the other items
>in the kit yet. I left >him with a semi-worn Eimac bottle to use 'till his
>replacement arrives.
>>
>
>As I see it, the one problem with relying on tube failure as
>evidence of the efficacy of parasitic suppressors, is that
>there may be no way of knowing if the new suppressors are
>the cure or just coincident with the installation of a new good
>set of tubes. In other words, if someone has a string of tube
>filters and then plugs in a set of nichrome suppressors and
>the failures stop, it doesn't mean that the suppressors are the
>key ingredient in the cure. It's also possible that the failures
>stop because the new tube(s) installed with the new suppessors
>are from a different manufacturing lot or just don't have the
>same inherent defect as the previous tube(s).
3-500Zs that are capable of oscillating at VHF with the built-in 0.15pF
feedback C would seem likely to have above average gain. In amplifiers
that use a grid RFC per Bill Orr, blaming the tube for blowing the grid
RFC doen't hold water because shorting the grid to the filament during
operation with a clip-lead will not blow the RFC. The only notable
effect is that 0V grid/fil bias causes idle current to flow on RX And TX,
and the short prevents driver RF from reaching the cathode on TX. And
so, I must plead guilty to starting the Parasite War with a stupid
clip-lead.
>To control the
>experiment, you would have to run the amp for a while with the
>new tube and nichrome suppressor to satisfy yourself that the
>tube was through infant mortality, and then switch back to the
>old suppressors to see if a failure ensued. Even then, you don't
>have 100% proof that the suppressor was the cure. In any
>case, the aformentioned scenario could explain why there is
>a body of anecdotal evidence that the nichrome suppressors
>have provided a cure for high failure rate amplifiers when in
>fact the suppressor had nothing to do with the failure.
Good logic. To do a bullet proof test, one would need to remove a
shorted tube, install lower-Q VHF suppressors, centrifuge the tube to
straighten the bent filament helices, re-install the tube and operate the
amplifier for a number of years. If there's no second short, perhaps the
difference was lower-Q suppression. However, one could seemingly presume
that since low-Q suppressors have been shown (by a Ham in Arizona who
probably wishes to remain anonymous) to reduce anode VHF-Rp by c. 50%,
that VHF gain is also reduced by about half, one could forego the
centrifuge, buy a new tube, and use the time saved to talk with one's
friends and enemies on the radio.
>Of course
>in the few case I have seen cited where the improved stability is
>directly observed (smoother tuning, no signs of oscillation), this
>pitfall doesn't come into play (I seem to recall that VK6APK
>recently cited a case where the nichrome suppressors helped
>stabilize a squirrely homebrew amplifier, but again this amp
>exhibited poor tuning and tendency towards non-catostrophic
>parasitic oscillation - e.g. no tube failure was involved).
I've seen a SB-220 with double-dip tuning that occasionally ate
suppressor resistors for lunch, wherein no tube failure occurred.
Lower-Q solved the double-dip problem and the amp is still being used
several times a week 30-years later.
>
cheers, Mike
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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