[AMPS] SB-220 TUNE Cap Arcing

Rich Measures measures@vc.net
Thu, 15 Oct 98 07:39:23 -0800

>Hi Jon,
>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 
20w max..  

>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
>here either.
>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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