[AMPS] My SB-220 smoked today too!

[Rich Measures]

>I've been following the thread this week regarding the SB-220 saga, and it
>must have cursed mine.
>
>First of all, the amps been working 100% up to now.  No mods have been done
>and the 3-500Z's were replaced a few years ago with a pair of Eimac's.  
>I reached over and threw on the power switch, with the amp in the tune (low
>voltage) position as I usually do.  The exciter was in receive mode, as I
>scanned around 20m to see who was around.  I smelt the smoke at first,
>looked over at the plate current meter, and it was sitting at .3 amps.  

I vote for: grid/filament short.


>I didnt' get a chance to look at the grid current before I powered off the
>Heathkit, but there was a faint sizzle noise also.

Good thing you switched it off, Gary.  With the stock V-cutoff bias 
config., a shorted tube will summarily destroy the unfused filament 
transformer.  Ditto for the TL-922 amplifier.  

>One of the tubes has a dark black line down the crease inside the tube, and
>a light brown mark on the inside of the glass accross from it.  The high
>voltage wires that pass the filter caps inside a clear plastic tube, appear
>OK, but the plastic is burnt as it passes one of the shorting resistors. 

What 'shorting resistors'?

>The rectifier metering board has a large burn mark under one of the diodes,
>plus it looks a little black at the other end where the high voltage lead
>departs.
>I'll replace the tube, and was thinking of new caps and metering / rect.
>board anyway.
>What got it?  Inrush  current?  Tube failure? Filter cap? 
>Just curious on the chain of events.
>
>Gary Smith VE4YH
>
It appears to be a rather curious chain of events.  An article about the 
SB-220 appears in the 11/90 and 12/90 issues of *QST*.
Rich---

R. L. Measures, 805-386-3734, AG6K   


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