[AMPS] SB-220 TUNE Cap Arcing
Thu, 15 Oct 98 07:39:23 -0800
>Just a followup and maybe the epilogue....
>I checked the resistors in the (resistor-nichrome wire) plate
>suppressors and they each read about 100 ohms as indicated.
>I cleaned the TUNE-C, which had a couple of specs (I mean real
>small specs) of solder sticking to a couple of fins. Also had
>slightly sanded a couple of TUNE C fins that had black or rough-
>ness where there had been some of the arc spitting.
>The TUNE-C was quite rough to turn, so I retensioned the nuts
>for smooth but not loose condition.
>Here is the present condition:
>Into a dummy load, using a Bird wattmeter and on amp meters:
>No apparent arcs on any band, at any power. Temporary key-down:
>band Grid(ma) Plate(A) HV(SSB pos.) POut
>10m 300 .88 2300 900w
>15m 250 .75 2400 1000w
>20m 240 .70 2500 1100w
>40m 225 .68 2450 1100w
>80m 235 .725 2450 1159w
>Of course the meter readings on the amp are not precise, but
>is the Plate Amps or Grid amps too high on 10m?
150 grid mA per tube is roughly 10w of grid diss. The Eimac rating is
>I may have read on the
>reflector that max Plate current is 400ma on a single 3-500.
>Do these average readings look average for this amp, given that
>the tubes may or may not be showing their age?
They look ok to me. However, a better test would be sending 50wpm dits
with a keyer and measuring pep watts output.
>Other than a little foreign material on the TUNE C, I cannot
>explain the original arcing.
>After doing the above tests, I did the same connected to my
>outside antenna (SWR was below 1.7:1 on all bands). No arcing
>Any comments before we close the subject?
My guess is that amplifier is not perfectly stable. There's a 1000v
safety margin in the breadown V of the Tune-C. When the Tune-C arcs,
chances are that it ain't due to teensy fruit flies or gnats.
- later, Bennett
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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