[Amps] Old amps are fun! (SB-221 smoke)

[R L Measures]

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 at k6rix.com
>
>
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R L MEASURES, AG6K. 805-386-3734
r at somis.org