My main concern Don is that giving out a calculated carrier level for a set
of tubes to a guy who does not have a scope and audio generator is not
going to give the amateur band another clean signal. As I mentioned in a
previous email, I have been unable to obtain the efficiencies you mention
and would like to hear from someone who can measure everything with test
equipment and tell me about it.
I have never worked much with AM linears or low level modulation in my own
station, but I have helped others set up their amplifiers. I do recall
getting a SB-220 working with a FT-301, running a KW DC input and about 350
watts carrier out, with good modulation on the scope and little downward
carrier shift in modulation. I also recall a Continental Electronics 250
watt broadcast transmitter used as a stand-by at a station where I once
worked, which used a pair of 4-250's in the final, running a combination of
screen and control grid modulation. It ran 250 watts output, with full 100%
modulation, and the DC input was exactly 750 watts input.
I recall back in the 60's, Walt WB4AOE running a pair of 833A's in class B
linear service and getting 350 watts carrier output, until he acquired a
modulation transformer to run plate modulation with another pair.
I hear a lot of guys on the air with AM linears, and most of the time they
claim to be running way too much carrier for the tubes they are using. I
often hear stuff like 200 watts out with a pair of 811A's or 250 watts out
with a pair of 813's in g-g. If so, they are cooking the tubes, or else the
efficiency is running too high, and they are flat-topping all over the
place.
To me, that's the disadvantage of low level AM. You have to fiddle with it
and get everything just exactly right: not too much grid drive, make sure
you have heavy enough antenna loading, and keep the audio level at the right
place. With plate modulation, the antenna loading and rf drive are not
critical. Just tune everything to resonance, and make sure you are not
overmodulating (and that the modulator is putting out clean audio), and as
long as the tube is running reasonably close to recommended parameters,
everything will be ok.
Unfortunately, very few hams use scopes as modulation monitors anymore.
They have become so unpopular that the major transceiver manufacturers no
longer include monitor scopes in their equipment line, and haven't done so
for years. I suppose the new breed of ham finds oscilloscope patterns too
complicated to understand. No wonder there are so many trashy signals on
the air.
Don K4KYV
_______________________________________________________________
This message was typed using the DVORAK keyboard layout. Try it - you'll
like it.
http://www.mwbrooks.com/dvorak/
http://gigliwood.com/abcd/
