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[AMPS] Mixing Tubes in an SB-221

To: <amps@contesting.com>
Subject: [AMPS] Mixing Tubes in an SB-221
From: measures@vc.net (Rich Measures)
Date: Fri, 3 Apr 98 07:22:23 -0800
>In a message dated 4/2/98 6:29:36 PM Central Standard Time, measures@vc.net
>writes:
>
><< Zero emission meaning that the filament is not lit?  If the filament is 
> not lit, the solder on pin 1 or pin 5 may have melted and dripped out of 
> the pin.  Resoldering the pins using rosin flux usually fixes the 
> problem.  Oiling the fan helps.  Sometimes, one needs to tighten the 
> helper-spring tension on contacts 1 and 5 on the sockets. 
>  >>
>Rich,
>
>Okay, well perhaps not exactly zero.  The tube seems to be in excellent
>condition other than the emission is significantly less than the other tube.
>Here are my observations:
>
>1.  When transmitting the plate of the "good" tube glows a nice dull red. The
>"bad" tube doesn't light up at all.  If I swap tubes, the dull red stays with
>the "good" tube.
>
>2.  When I operate the amplifier with only one tube in place (for short
>periods of time) the idling current is much less on the "bad" tube.  

What are the numbers?

>Once again, socket position does not matter.  I get output from the "bad" 
tube, but
>it is much less than the "good" tube.
>
ok

>3.  The tubes look identical from a physical examination.  I can see no
>evidence of internal damage.  With power applied, the filaments look
>identical. (Try to do that with your 8877 :-)  
>
>From this, I have concluded that for some reason, the thorium in the filament
>of the bad tube has been depleted.  

The thorium content memains c. 1.5% during the emissive life of the 
cathode.  The thing that depletes is the tungsten-carbide layer.  
However, this depletation takes place slowly, not suddenly.  

>Perhaps there may be other explanations?
>
My guesses are that either the filament helices are bent in one of the 
3-500Zs, or that the tubes just happen to have very different 
characteristics.  .  One test for a bent filament can be done with a 
C-meter, by measuring the grid/fil.  C using very short test leads.  
Normal C is roughly 8.5pF.  If the C measures much more, the filament 
could be bent.  .  However, the best test is with a high-pot.  Normal 
grid/fil. withstanding ability is 7 to 9kV -  provided that the tube has 
a good vacuum.  

cheers
Rich...

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


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