On May 7, 2006, at 1:52 PM, Dino Darling wrote:
> After cleaning the dust out of an old SB-221, I pulled out the trusty
> POWERSTAT to slowly bring up the amp.
>
> I checked the POWERSTAT with the VOM so I knew where 55 volts were
> and especially where 110 volts was. (Amp was received wired for
> 120V. I have not done anything to the amp other than to dust it
> out. It has been sitting for probably 20 years.)
>
> I'm not saying I did this right, but I started at ZERO, turned on the
> amp, and slowly brought up the POWERSTAT until the light came on, the
> tubes began to glow, and the fan was spinning. I let it sit there
> for a couple of minutes and brought it up a little further. The
> lights got brighter, the fan spun faster and the tubes glowed nicely.
>
> At about 75%, I got smoke! A whole lot of smoke! POWERSTAT to zero
> and I flipped the switch off!
>
> After removing the outer cover, I see where the 1mH choke on pin 4 of
> the inside tube (socket N) smoked.
>
> #1 Where can I get a replacement?
You need a new tube because that one that ate the choke has a grid-
fil short which caused the +110V PS - that is applied on Rx to the
fil xfmr CT- to short to gnd through the RFC, and that opened the
choke. Also, it is safer to replace the grid-to-gnd RFC with a 1/4A
fuse or a fusing R because if the RFC fails to open, the unfused fil
xfmr will melt down unless the amplifier is promptly switched off. .
> #2 What caused the failure (or where do I start looking)?
Such shorts are caused by the perpendicular EMF created during the
large pulse of cathode-current concurrent with an intermittant
parasite c. 110MHz. Basically, the 1830ºK tungsten filament can be
bent laterally when it is hot.
>
> Dino - K6RIX
> dino@k6rix.com
>
>
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R L MEASURES, AG6K. 805-386-3734
r@somis.org
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