Top Side of Chassis
Design: This is the standard plain vanilla grounded grid circuit. The
meter is in series with the filament center tap to ground. The
tank circuit is a modified AirDux tank coil. I found the taps for each
band by detaching both plate leads from the tubes and fastening one
2800 ohm resistor from the top of the plate choke to ground. The 2800
ohms was chosen because the plate impendance of a 4-400 is 5600 ohms so
two in parallel is 2800 ohms. I then took my MFJ analyzer and attached
that to the rf output connector while bridging the antenna relay with a
jumper to complete the circuit. For each tap just start with a quick
and dirty calculation for each band and zero it in by trial and
error looking for a 1:1 match on the analyzer. I will put a schematic
online as soon as I make one using a good schematic drawing software
amp tunes and runs about like you'd expect. About 700W-800W output which is on
the low side but as you read further you can see why..
standing voltage is 3000 vdc but under keying it dips all the way down
to about 2150 vdc. Even under idle, at 120 ma, it dips down to about
2400 vdc so output would benefit with better voltage regulation. The
filter is only 24 uf so undoubtedly that has something to do with it. I
plan on putting on an external filter cap which will raise the overall
capacity and hopefully the drop won't be so prevelant.
This puppy puts out a lot of heat. Not only are the 4-400's putting out
an awful lot of heat but also the microwave transformer is getting VERY
hot. The muffin fan below the 4-400's seals pretty well when the case
is put on and lined up with the hole in the case bottom. I plan to put
a small computer fan to blow air across the transformer in an
effort to cool it down. After all, it is providing 2000vdc output
from the secondary but also 2- 5volt/14amp filament windings. I cut
about a dozen louvered holes in the top of the case which did a lot to
let some of the heat out.
I fulfilled my goal of making a totally enclosed RF Amplifier, 160M-10M, all enclosed in one cabinet.
ADDENDUM - PLEASE READ !
the heat became so much of an issue, I decided to take out one of the
4-400's and only run with one. What a difference ! The amp now runs
just warm and the output is almost as much as it was with two tubes.
The poor microwave transformer was just over saturated with running the
filaments of two 4-400's at 14amps each as well as the high voltage. My BAD !