[Amps] AL-80B questions

[rlm]

>> 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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