I would like advice on a new 2200/630m amplifier that I have built. It works
but exhibits two problems.
It uses a pair of APT5023 mosfets, 48Vdc supply, output transformer 3T:6T on a
pair of 26mm type 75 tube cores to make a big binocular core, DC feed via a
separate center tapped choke, biased for AB (or maybe closer to B as the bias
is only 0.3A). On 2200 and 630 my SoftRock RXTX with 1W output provides
sufficient drive to drive it to the start of flat-topping at about 200W output.
I have been running it at about 100W output.
I have two questions that I would like advice on.
1) When PTT is asserted (with no RF drive), which results in a relay applying
the gate bias and doing the antenna change over, there is a turn on transient
visible on an oscilloscope. There are two short bursts of oscillation (at max
power) each at about 100 kHz. This is over by about 1 msec after which it seems
stable.
Should I worry about this? If so, can you suggest a cure?
2) The output waveform is close to a sinewave (with no LPF) when driving a 50
Ohm dummy load. However, when driving my antenna, especially on 137 kHz, the
waveform is distorted. On 137 kHz the antenna load is 50 Ohm and close to
resistive (at 137 kHz) as best as I can measure. I tried inserting a 1:1 balun
at the output of the output transformer, which had no effect. A LPF cleans up
what comes out of the filter nicely. However, the input to the filter (amp
output) still looks distorted in the same way. I am using a T format LPF which
I believe is the correct configuration for the amp output stage which is
(correct me if I am wrong) voltage feed b/c of the center tapped choke DC
connection. The input drive to the amplifier looks like a nice sinewave in all
conditions.
Do you see a problem operating the amp into a T (inductor input and output) LPF
with the distorted output (voltage) waveform? It seems to be running reasonably
cool so I don’t see an overheating problem. I would prefer to see a clean
output from the amplifier and would like to operate with linear modes
occasionally so the distorted waveform leads me worry about IMD (I have not run
IMD tests).
Oscilloscope screenshot attached.
BTW, I tried using a center tap on the output transformer instead of the
separate choke feed. The result was a badly distorted waveform into the 50 Ohm
dummy load on all bands. (It works on 160 also, but the Softrock can only drive
it to 50W.) I don’t understand why the center tap method resulted in such a
different output.
Thanks for your help.
73,
Roger
Output waveform when connected to antenna matched to 50 Ohm at 137.4 kHz.
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|