Peter -
You have some good suggestions to try.
The first thing is to use a GOOD dummy load. It needs to be 50 ohms,
+/- 5% at a minimum.
A couple of things to consider. The 'standard' wattmeter, the Bird 43,
is only specified as +/- 5% of FULL SCALE. For the Palstar, which DOES
NOT even specify it's accuracy, the degree of uncertainty is no doubt
greater. The Breune Wattmeter, which is what most Amateur type
wattmeters are, is TYPICALLY +/- 5%, IF they are properly calibrated,
AND have not been 'adjusted' by someone other than the factory.
Your 75W drive for 200 mA of Grid current sounds a 'little bit' high,
and may indicate some mismatch at the amp input. Also, don't forget
that the transmitter wattmeter is ALSO +/- 5% AT BEST.
The PLATE mA meter is probably +/- 5% or so.
The PLATE DC voltmeter is also probably +/- 5%, AT BEST, if one or more
of the divider resistors hasn't gone up in value.
Quality of coax, and quality, installation and condition of connectors
ALL factor.
As you can see, power measurement is a tricky business. 5% here, 5%
there, pretty soon you're talking real error!!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
[email protected] wrote:
Thanks all for comments. Many good points, esp testing into dummy load.
I rebuilt the PS from the kit about 2-3 years ago; I bought a new pair of
matched 3-500Zs from a reliable source about 4-5 years ago when I put the amp
back into service after decades on a shelf. I don't see any difference in
output over time. I use an MFJ power-up delay switch and feed the amp with a
Mark 5 through an MFJ relay box. Takes 75 watts input nominal to get to 200 ma
on the grids. Output goes to a Palstar AT-1500DT transmatch, which also has the
RF power out and SWR meter. Everything goes through a Balun Designs 1:1 choke
unun, 5KW capacity, into the antenna switch to wires and verticals. I see minor
differences (about 10w max) in required drive o different bands.
My plan is to borrow a dummy load and test output on each band, recording key
down meter readings from amp and Palstar. This will also give me an indication
if the problem is in the antenna/feedline systems. I will report results to the
group.
Regards,
Peter W2CDO
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