Hi Phil,
The Ten Tec Centaur measured -39dB (3rd) and -45 dB (5th) in the ARRL
review, and uses no "special length" cable or tuned input! The
AL-811H measures about -41and -48 when operated at the same power
level of the Collins (it uses neutralization and tuned input).
I've tested several dozen amps for IMD over the years, and find the
feedline length makes no difference at all in IMD results on lower
frequencies. Feedline length does affect stability and IMD at
the high frequency end in PA's without or with poor neutralization.
> How can furnishing a 20.5 piece of coax with each radio require an excuse?
> Was there an over-stock of RG-8 at Collins, or a conspiracy with Belden?
> (I don't recall any other manufacturer of amps giving me any coax of any
> length.) I am sure the price of the coax was justified in improvement of
> specs.
Of course there was a reason. It helps terminate the input in one
of the few four 811A tube PA's manufactured without neutralization.
Improving stability improves IMD.
If a PA is properly designed and constructed, input cable length
or exciter type has no effect at all on PA performance. Failing
to meet this criteria is NOT always a fault with engineering or
construction. Sometimes cost, time, and component availability limits
the results.
The 811A was an offspring of a tube originally designed for audio
service. Initially the tubes were low-mu 810 triodes designed for
class B modulator applications. RCA added a hi-mu grid and made a
zero bias triode, and eventually the tube got pressed into RF service
despite the long thin grid lead and poor internal shielding.
When a less than optimum designed tube is the heart of ANY PA, the
design will almost always be full of contradictions and compromises.
A pretty round emblem and good reputation can't correct all the tube
problems.
If you look at the Heathkit Warrior and Gonset 811A PA, you'll see
they are neutralized. So is the AL-811H. In ALL three of these PA's,
you can unterminate the input and rotate the tuning through all
positions with no instability. In all three of these PA's, input
cable length has little or no affect on IMD.
If you try this test with the Dentron and Collins, most of them will
oscillate on ten and 15 meters. They will all oscillate if you select
the correct input cable length.
I'm not saying the 30L1 was poorly constructed, but it certainly
would have been a better PA if they neutralized the PA. In that case,
any length cable could have been used.
Let's just agree to disagree. From an engineering standpoint, I view
the problem differently than a consumer.
73, Tom W8JI
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