On Mon, 4 Jan 1999 23:08:58 -0800 "Jim" <jdc@ieway.com> writes:
>"Have you ever noticed" (Now I sound like Andy Rooney) how different
>the plate load impedance/component value charts (C1, C2 L1 L2) for a
>given plate load change markedly between different publications ?
>
>I have noticed in the 23 edition of the "Radio Handbook" the chart for
>the Pi-L network has values for the Q of 10. In the 1998 "ARRL
>Handbook" the chart for the same plate load impedance has about the
>same values (within 5 % ) and that chart is for a Q of 12. I have
>also noticed that different publications give the formula for
>calculating the palate load impedance different as well. Some books
>give a "K" value of 1.8 (23rd radio handbook), and the some give the
>"K" value of 1.57 ???
>
>In the information sheet that I have from Eimac on the 3CX800A7, this
>sheet says that the plate load impedance value for the tube operating
>at 2200 VDC and a plate current of .5 amp is 2700 Ohms. This makes the
>"K" value 1.56.
>
>All of the values for "K" are for the tube operating in class
>AB2......
>
>On another note, I also have observed that the overall efficiency and
>low quantities of heat, and the reliability of the tank components of
>H.F. amplifiers that have a relative low value of plate load impedance
>(1500-2000 Ohms) seem to be more efficient and run much cooler than
>the high plate load ( 3500-4000 Ohms) amplifiers.
>
>
>I would like to hear what Rich, Dick Erhorn (sp. ?) and Carl have to
>say about the conflicting information.............
>
>Just curious and its time for a "new" discussion here on the amps
>reflector........
>
>
>73 and thanks
>Jim W7RY
>
Far from a new discussion, in fact it was discussed at length about a
month or two ago. Check the archives for some backround.
As far as efficiency is concerned I suspect that you are only partially
correct. Low voltage high current tubes are pretty much grouped around
the modern ceramic metal types with full ratings in the UHF region. At HF
they tend to be very efficient so the bit of extra tank circuit losses go
unnoticed.
OTOH the tubes commonly used in high impedence tanks are often the
ancient glass jugs such as the 4-1000A that really strain on 10M. I'm
talking about power levels in the 1 to 3KW range, not off the wall stuff.
Transforming 5K Ohms to 50 Ohms in one step is not easy either. If you
look at some of the old 4-1000A 2-30MHz military amps you will notice
there was often a change in network design at roughly 18MHz resulting in
an intermediate transfer step. Not a Pi-L but a 5 element HP filter.
The 3-500Z is a nice middle of the road tube that works efficiently at
least thru 50MHz.
An article by Irvin Hoff, W6FFC, in June 1978 HRM is the best I have read
in ham literature. He uses a K value of 1.31 for AB1 designs that are
approaching Class A; a K of 1.57 for B. I have used his charts and other
data for years with no problems; dial in the values with the GDO and
power up.
He covers Pi and Pi-L 10-160M, a 2-30mHz broadband design, why Q is not
magic at 12, etc. His math is understandable without a degree also!
73 Carl KM1H
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