[Amps] Arcing 8877

[Steve Thompson]

I had arcing a bit like that fixing a friend's W6PO 2m 8877 amp. 
It didn't have the big bang take out the fuses side but we could 
see arcing somewhere around the fins when the power was wound up. 
It turned out to be damage to the doorknob caps used for the dc 
blockers - mechanical stress from moving things when the tube was 
changed eariler had fractured the metallisation on the caps and 
all the rf volts were showing up across a tiny air gap which then 
arced across. With so much metal around the light was reflected 
everywhere and it took a while to pinpoint where it was coming from.

Steve

> Hi All,
>  
> I have a 2nd hand 8877  based 144 Mhz amp, built like a rock.  Has worked
> fine for over a year, then bang. I saw a flash and of course the fuse did
> its job.  The tube in it was maybe 20 years old.
>  
> It took out the shunt( resistor and diode)   in the Grid and Meter which I
> replaced.  I  figure I would replace the tube since I had a newer used tube.
> I " burned" it in for about 18 hours with the filament and fan running and
> another 6 hours with the HV on, just for good luck.
>  
> I keyed it and slowly brought the power up, worked  fine at 5 W and at 10W
> in I measured 230W on the bird meter and it flashed again, fuse again.   I
> replaced the fuse  and powered it up and again at 10 Watts showing 230W, but
> the metering was gone and I looked through the vent screen and saw it arcing
> intermittently  at the age of the fins, above the Anode strap that goes
> around it.
>  
> I am looking for advise. Easy to fix the meter shunts, but is it a problem
> with the tube(s),  the anode strap is as tight as it goes.... any guesses. 
>  
> 73s
> Les