I agree, having a net reactive load does not make any sense. The Plate Tune
capacitor should be peaked.
Remember when you were tuning up a transmitter with more than 75 watt plate
input capacity when you were a novice.
You tuned for the proper plate current for 75 watts but always dipped the
plate current last.
How much load mismatch must you have to get this "sweet spot" for better
linearity?
Maybe it would be better not to drive the amplifier so hard in the first place.
Is it better to mismatch the load or to tune for Z match and not drive it so
hard?
My solution is to have an amplifier capable of 2500 Watts and drive it to
1500 Watts PEP.
73
Bill wa4lav
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
TexasRF@aol.com [TexasRF@aol.com]
Sent: Wednesday, December 21, 2011 8:12 PM
To: deswynar@xplornet.ca; donroden@hiwaay.net
Cc: amps@contesting.com
Subject: Re: [Amps] Fwd: Linear Amplifier Tuning---PROPERLY!
Hi All, it makes no sense to not peak the tune C as the last step in a
matching procedure.
The tuning network is fully adjustable, allowing a match from your nominal
50 ohm load to what ever plate load impedance you want to use. If the tube
needs 2000 ohms for best efficiency and power output, it is a simple matter
to provide that. But, the transformed impedance is resistive only when the
complete network is resonant. The only way that can happen is when the
plate tune C is adjusted last, for maximum power transfer. The complete
network includes all sources of reactance, including tube output C and stray C
as
well as the effect of the rf choke and tank inductor and tune C.
When you tweak the tune C for maximum output, you are making all of the
reactance contributors parallel resonant, leaving only a resistive load for
the tube.
Sure the plate load impedance changes when you tweak the tune C, that is
what is supposed to happen. Over coupling slightly is just another way to say
that the plate load impedance is reduced slightly. If you don't end up
with the desired over coupled value, then the plate load C is not set to the
required capacity and needs further adjustment.
Operating a tube into a reactive load just doesn't make any sense in my
mind. It may not be reactive enough to hurt anything but how can it help in
any way?
Standing by for other view points and arguments.
73,
Gerald K5GW
In a message dated 12/21/2011 2:50:05 P.M. Central Standard Time,
deswynar@xplornet.ca writes:
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
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