I've been having trouble with my Swan Mark II amp for the last few months,
and would like to get the opinion of a few group members.
First I'll give you a bit of background info. I've had the amp for about
three years. When I bought it one of the Eimac tubes was missing, so I
replaced it with a new Westinghouse unit that I had. Power output was up to
specs with a little over 1000 watts output on ssb voice peaks, and this was
fine for a couple of years. I used the amp most of the time in the "tune"
position, which dropped EP to 1700 volts and the power output to +/- 700
watts. The amp runs a little cooler in this position, and produced more
than enough output for my day to day use.
One day when trying to have a DX qso I was looking for some more power so I
switched to the "SSB" position. which increases EP to 2900 volts. I know
I was running the amp hard but wanted to finish the qso badly, and the band
was going out. Power output on my meter was sitting at about 1250 watts
when it smoked. The plate cap resistors melted, and so did the hv choke.
(I know it was stupid so no need to comment!)
No big deal, I replaced the parts, and the Eimac 3-400Z with another new
Westinghouse. I tested each tube by itself, and power output was within 15
watts of each other. The power output total was down to about 500 watts
and 650 watts (tune and ssb positions) and when I talked the plates now
turned red in less than 10 seconds. Normal drive is 65 to 80 watts,
sometimes as high as 100 watts.
I figured the power had to be going somewhere, the tubes were good??, so
the amp was oscillating somewhere internally. I lifted the grid pins off
of ground, and inserted caps and a grid fuse resistor for each tube. I
installed new plate cap choke kits etc. from Rich, tryed the amp again and
am still having the same problem.
Heres my test results, using the internal volt/amp meter and an external
watt meter. These meters may not be 100% accurate however their readings
are relative. The dummy load is a Heathkit Cantenna.
-Amp in "tune" position, key up.
EP 1700V
IP 0 amps
IG 0 amps
-Amp in "tune" position, ptt ssb on Kenwood 940
EP 1650V
IP .14 amps
IG 0 amps
no power output (watt meter deflects slightly on relay changeover)
-Amp in "tune" postion, cw carrier, drive 80 watts.
EP 1525V
IP .6 amps
IG .22 amps
power output 500 watts
-Amp in "ssb" position, key up.
EP 2900V
IP 0 amps
IG 0 amps
-Amp in "ssb" position, ptt ssb on Kenwood 940
EP 2700V
IP .25 amps
IG 0 amps (watt meter defects slightly on relay changeover)
-Amp in "ssb" position, cw carrier, drive 90 watts.
EP 2400V
IP .7 amps
IG .2 amps
power output 750 watts
In the "ssb" position on the amp, with the exiciter in ssb, and with no
voice modulation, ptt closed, the plates still turn red in about 10
seconds.
The amp does not have a tuned input, and the Kenwood will only produce
80-90 watts now because of SWR. It used to be able to input 100+ watts.
The amp was designed to be driven by a Swan xcvr, so this wasnt' a concern.
73 Gary Smith VE4YH
gsmith@pcs.mb.ca
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