Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Jason -
I think this is your question!...
The tank circuit of a transmitter is essentially a "matching network" to
match the high impedance of the tube plate circuit to the low impedance
of an antenna feedline. The plate circuit impedance is approximately
equal to the plate voltage divided by the plate current, or
approximately 2000 ohms. A typical antenna feedline is 50 ohms, so you
need a transformation ratio of about 40. Adjusting the TUNE and LOAD
controls "tunes" the network for a match between these two impedances.
The network's "job" is to couple the RF energy from the plate circuit to
the feedline. When the network is tuned, (plate current "dipped",) the
maximum amount of RF generated is being transferred to the antenna.
When the match is NOT correct, (plate current NOT dipped,) the network
isn't transferring RF efficiently, the tubes are attempting to drive a
mismatched load, and have to dissipate the "wasted" power as heat. Any
time the plate current is NOT dipped, indicating the match is not
correct, any additional plate current must be dissipated by the tube
plates as heat.
Hopefully this isn't too convoluted to follow...
73, Garey - K4OAH
Chicago
[EMAIL PROTECTED] wrote:
On Wed, 30 Nov 2005, Garey Barrell wrote:
I ran three 4 Lines on autostart RTTY. They ran 24/7, and often made
transmissions in excess of 30 minutes, AT FULL POWER. In this service,
the finals lasted a little over two years, typically, with "end of life"
defined as 100W output on 20M.
This piqued my curiosity again - Let's say you're running your T-4XB at
full power for 30 minutes at a time at 300mA and the plate is perfectly
dipped. All is good. But if the plate isn't dipped at 300mA and would be
at 240mA if dipped properly there would be 60mA of current wasted as heat
running at 300mA. But it's still 300mA, whether resonant and dipped at
300mA or splattering at 300mA - the thing i'm wondering is if the tubes
are suffering more at 300mA undipped or if the life expectancy the same as
300mA dipped perfectly, being paced at a rate of current (heat) travelling
through the metal.
And I suppose the other question is "what is a plate dip" anyway - can I
think of it as if the tank circuit is sucking the power out of the tube or
is it best to think of the tank circuit as a valve that passes the power
when at resonance, and the current increases as the RF power output is
"backed up" and can't get out?
Sorry for the rudimentary questions but I can't find this online anywhere
and don't have an old ARRL handbook that explains it.
thanks
--
73 Jason N1SU
http://n1su.com/
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