I do not have the exact wording at this computer of prior statements, but I
do recall that Rich stated that the G2DAF failed two actuals tests. The
first was with an 813 tube and the second was with a 4CX250B tube. Since I
had no standard for the 813, I discounted it.
However, I was reminded that the G2DAF had failed two tests. Therefore, I
wanted to examine the 813 tests. My assumption (which I think was
reasonable) was that the 813 when grid driven with a constant screen voltage
was significantly inferior to the 813 when configured as a G2DAF amplifier
(grid driven with variable screen voltage derived from the driver).
However, it appears now that the test was in fact with a 813 configured as a
G2DAF amplifier compared to the 813 operating in grounded grid.
What sort of a comparison is this? Rich's statement that "The 813 is not
linear in grid driven service" would mean that a G2DAF test with an 813
would be faulty by the very rules stated by Rich. Thus that test is
meaningless. It is like comparing apples to road graders.
Suppose we take a 4CX1500 (which has superb 3rd order imd in grid driven
service) and operate it in a G2DAF circuit and compare that to a 4CX1000 in
grid driven service with regulated screen. Any bets on which amplifier
would win? I would not bet my pension on that one without a lot of
empirical tests and also a close reading of the rules.
And, what are the desired standards for 3rd order distortion anyway? This
would perhaps be a good discussion topic. -30 db 3rd order distortion
products do cause splatter. Is that the minimum level of distortion for a
ham to transmit with. For years I listened to W7FY (now deceased) who ran a
4CX1500 in grid driven service. His station was significantly cleaner than
other signals and a real pleasure to tune off of. Using his signal as a
standard, other signals were noticbly inferior. Unfortunately, a lot of
transceivers still use -30 db as the standard. Some of the newer
transceivers use high voltage output transistors and have significantly
cleaned up the drivers - which means now we should concentrate on the
amplifiers and get good strong signals instead of bad strong signals.
I listen on the dx window and hear a lot of signals that could be better. I
believe the newer Alpha (and other amplifiers) have contributed a great deal
to cleaning up the bands because of two factors: first is the use of superb
triodes (3CX800A7), and secondly with the use of peak reading led measuring
devices to replace the analog meter. The second factor may be more
important than the first. Using my older Alpha with analog meters, I find
that my scope indicates full power when the meter only registers about 10%
of key down reading. There are a few souls who yell into the mike when
excited and trying to work dx and peg the meter (I have been in the shack of
a couple), and with the analog indicators, hams can make the meters pin
without quite as disasterous consequences.
Colin K7FM
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|