Thanks, everybody, for weighing in on comments, re my request for advice
about 6m parasitic suppressors. The near-consensus viewpoint was that no
suppressors were needed for the 3CX800A7 in the amplifier. Good advice.
As it turned out, my "power supply problem" mentioned below turned out
to be a nightmare, with an interesting twist. The plate voltage in the
amp normally runs about 2400V. The problem is that it intermittently
dropped down to about 1950V, with no discernible pattern. My first
thought was that I had a bad diode block (the amp used four 3kV/500mA
blocks in a bridge rectifier). I upgraded the diodes to 3kV/1Amp
modules, but it made no difference. I checked the six 330uF/450V
electrolytics in the HV capacitor bank, and they all tested good. I
measured the voltage drop across the six 220K bleeder resistors and
everything checked fine.
Next, I checked the primary side of the supply -- the switches, power
relay, and step-start circuit and everything worked perfectly. Now I was
beginning to worry about the plate transformer, which is a 35-year-old
transformer from an Alpha 76 amplifier with five secondary windings
(made by Berkshire Transformer Company, now defunct). I still use a
couple of BTC transformers and they're excellent quality. I was hoping I
didn't have a shorted winding somewhere, but all symptoms were pointing
toward the transformer.
Crossing my fingers I measured the RMS AC voltage from the transformer
HV secondary across the diode bridge, and it was perfect - exactly what
one would expect for 2400VDC. Whew. But the DC voltage across the
capacitor bank showed 1950VDC. Now I was really confused, since I'd
checked all the components in the rectifier/filter caps and they all
checked good. How could unrectified AC voltage be good, but rectified
and filtered DC voltage be low by 450VDC. That was a head-scratcher.
By then, it was late at night and time to shut off the amplifier. I
unplugged it, watched the HV decay on the panel meter, then grounded the
filter bank with clip leads to fully eliminate any residual charge. I
turned off my workshop light and was about to walk out the room when, in
the dark, I saw a weak violet glow coming from the amplifier in region
near the capacitor bank. It was a dim pinpoint of light, almost like a
reflection from something. Of course, with the capacitor bank discharged
(clipleads were still in place), and the amplifier unplugged, there was
no way anything could be glowing inside. Encouraged that I was on the
path to a correct diagnoseis I just went to bed. The next day I found
the problem.
A one inch long wire jumper connected one of the 330uF electrolytics to
an adjacent capacitor in the string. The jumper was 20AWG solid tinned
wire with teflon sleeving around it. The glow was coming from the middle
of the wire, through the sleeving! Evidently, when I wired up the
capacitor, I must have nicked the solid wire before I slipped the teflon
sleeving on it. Over time, the wire burned through at the weakened
point, eventually opening up. Once that happened, that particular
capacitor was electrically removed from the capacitor bank and no longer
connected to its bleeder, The stored charge in it was enough to create a
tiny corona discharge that emitted violet light. Interestingly, the next
day the capacitor still held 300VDC of charge and the corona was still
glowing. The fix of course was to replace the wire. That took five
minutes. Identifying the problem took about seven hours. A very strange
but interesting experience.
73,
Jim W8ZR
On Sunday, July 15, 2018, Jim Garland <4cx250b@miamioh.edu> wrote:
About fifteen years ago, I rebuilt a junker Alpha 76 (no tubes,
flashed-over bandswitch, burned plates on tuning cap, burned plate
choke,
other problems) and converted it to a six meter monoband amplifier. I
replaced the pair of 3CX400s with a single 3CX800A7, installed
vacuum T/R
relays, and redid the metering and front panel switching. You can
see it at
http://www.w8zr.net/homebrew/hb6mamp.htm.
I've used the amp for years, with about 750W output, but recently
ran into a
power supply problem and had to tear into it. In doing so, I
noticed the
parasitic suppressor was burned. I believe it's probably the original
suppressor, and it consists of 1.5 turns of 12AWG tinned wire
shunted by
two 51 ohm two watt resistors, one of which was burned in half.
The amp has
never had parasitic oscillations to my knowledge, so I'm guessing
there was
probably a bit too much inductance in the parasitic inductor. I'm
thinking
about replacing the two burned resistors with a single 22 ohm 5
Watt metal
film resistor, and trimming the inductor to a single turn. Does
this seem a
reasonable change? Does the tube require any parasitic suppressor
at all?
73,
Jim W8ZR
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