----- Original Message -----
From: "Eddy Swynar" <deswynar@xplornet.ca>
To: <donroden@hiwaay.net>
Cc: <amps@contesting.com>
Sent: Wednesday, December 21, 2011 11:01 AM
Subject: Re: [Amps] Fwd: Linear Amplifier Tuning---PROPERLY!
>
> On 2011-12-21, at 7:55 AM, donroden@hiwaay.net wrote:
>
>> Isn't it true that if you re-peak the tuning control, then the loading
>> is no longer "over-coupled" ?
>>
>
> Hi Don,
>
> I think you've hit the proverbial nail right on the head with that
> observation---which, BTW, was pretty much the very same opinion that the
> majority of originators of the last batch of e-mails that I received here
> on the subject had, as well...
>
> Probably the best easy-to-grasp response (for me, anyway!) that I read
> here as to why it is NOT desirable to re-tweak the plate TUNING capacitor
> as a final step in the tuning process of a linear amplifier, came from
> Steve (KK7UV), who was told---when asking a similar such question himself
> some time ago---that "...the answer was something like 'if you do that,
> you change the load line and you play cat and mouse between the tuning and
> loading caps'".
>
> Makes sense to me, I s'pose...
>
> Too bad the movers & shakers in the Amateur radio world of
> literature---from Bill Orr back in the day, to the present crop of
> technical gurus on-line---couldn't be as descriptive! Hams, by their very
> nature, are inveterate knob-twiddlers & adjusters----so why WOULDN'T we
> re-adjust things "...one final time" during a tuning-up process...?! A
> simple & final "HANDS OFF!" warning after the over-coupling procedure in
> the descriptive literature would suffice! Hi Hi.
>
> ~73~ de Eddy VE3CUI - VE3XZ
Its a shame so many feel they have to reply in a private email which does
nobody any good. Thats as bad as the chap who has a web page to spout on but
there is no peer review permitted. Just a take it as gospel attitude.
Since I have doubts about those comments I just ran a 20M SA IMD test on the
LK-500ZC ( pair of 3-500Z's) between max power out and heavier loading to
drop power about 50W. The IMD improved by a repeatable 2-3dB; it was a
pretty broad result with no change between 40-60W that I was able to tell.
Slightly moving the Tune cap did nothing to the IMD and the power dropped if
I went further.
Once you tune for an absolute maximum and then just slightly overcouple for
the IMD sweet spot the Tune cap does nothing since the peak point with a
typical low Q is broad enough.
Just as I said yesterday. If I were to do it to my flat plate 2M 3CX1000A7
amp with some ridiculosly high Q the results would be different. The same
with the 432 cavity amp.
This was your basic 2 tone test, nothing fancy or accurate to show what the
actual SSB IMD really is.
Where some amps may have a problem is when they are not completely
neutralized. IOW, the control or screen grid peak doesnt occur at the plate
current minimum. Then all bets are off I would think.
Carl
KM1H
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