[sdiy] calling tube gurus...this old rare one has themostpeculiar filament problem

[Ken Stone]

Some of these oldies had multi segment filaments with them wired
series/parallel. A segment may break and short, or simply just go OC. In the
case of one I had, it went short, so the filiment would measure, but not
light. Cutting off the shorted pin allowed a single section of the 8 stage
filiment to light - the part that was in parallel with the broken stage. The
paralleling of the others made the resistance too low for them to light.
This subtle resistance change you have suggests you may have a similar issue.

Ken

>Yeah I don't get heater on the new tube.  It's dc measurement on the fat 
>pins (this is an old style 6pin.  I think it is very similar to a type 
>75 tube)  The old tube that works is 3.5ohms. Bad one 3.8.  I'm just 
>saying that it appears from DC that it would have a working filament.  
>But maybe Paul's theory is correct..that as soon as you apply current 
>something goes open.  Reminds me of the water that that I built an early 
>attempt at a PCR machine with.  The guy who worked on it before me wired 
>the heating element backwards.  And sometimes..as it cooled...the 
>element apparently decided to break and touch somehow against the shield 
>on an end that instead of blowing a breaker..would send the message 
>"Hotter, hotter you fool you fool!" to the thing :-)
>
>Sometime during the weekend end this prototype which I'd begun testing 
>using straight anti-freeze.... hits flash point.  Melted the silver 
>solder right out of the edges, burned down the plexiglass apparatus I'd 
>built above it and melted some light fixture covers above...could barely 
>tell there's been a fire other than that :-) -Bob
>
>Roy J. Tellason wrote:
>
>>On Wednesday 23 January 2008 05:20, Bob Weigel wrote:
>>  
>>
>>>What could be going on here.   I've got a Teisco amp ..a very old one
>>>that I'm repairing for a guy who does the Teisco amp page.  It uses a
>>>6Z-DH3A  and yup that is the actual tube number.
>>>    
>>>
>>
>>I sure don't run into those too often.  :-)
>>
>>  
>>
>>>It's a preamp tube.  I fortunately have a working one but it measure real
>>>low on quality check.  But at least it lights up.  This one does not
>>>HOWEVER...both filements measure good on dc test. Both a little under 4ohms. 
>>>3.5 and 3.8 on the bad one..which..is actually a brand new tube I believe.
>>>    
>>>
>>
>>That might be a bit of manufacturing variance there.  Maybe.
>>
>>  
>>
>>>Resoldered the bottom but did no good but now I see that it obviously
>>>wouldn't.
>>>    
>>>
>>
>>Are we talking about an octal base here?  I don't seem to have a datasheet on 
>>that one.
>>
>>  
>>
>>>That filament is stone cold but theo ther lights in my Jackson tester.  I
>>>get no measure on it of course at all since the filament..really really
>>>isn't lighting at all.  I can shield the tube from light and there is no
>>>glow at all. -Bob 
>>>    
>>>
>>
>>I'm not sure what you're saying here,  you get no heat on which one?  But you 
>>measure good DC resistance on both of them?  Please elaborate...
>>
>>  
>>
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_______________________________________________________________________
Ken Stone   sasami at hotkey.net.au
Modular Synth PCBs for sale <http://www.cgs.synth.net/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>