[Amps] parasitic suppressor for six meters... and more

[Jim Garland]

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 at 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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