[Hallicrafters] Re: Tube 50L6-GT / S-38D

[Ken Kaplan]

Ya know, I often read the below about a tube test in circuit being more accurate than in a 
tester in response to tube testing questions. I totally agree about old tubes lasting a long 
time. Unless they leak or have been abused, they seem to really last (especially compared 
to passives like capacitors & resistors). I'm not an expert but as far as I know, most tube 
testers are either simple emission or transconductance (mutual conductance) types. I have 
an Eico 667 which is supposed to be some kind of blend (ha). It even measures 
heather/cathode resistance.

I wonder what specifically is being missed if a tube passes in a tester but performs poorly in 
circuit? Is it transconductance? If so, shouldn't a decent transconductance tester should 
eliminate the tester/in circuit disparity? If not, just what is being missed in the testing 
process? In other words, if it fails in circuit but passes in a tester, what is the tester missing? 
Don't tell me it is transparency, articulation, focus, resolution, weight or impact <g>.

I also wonder what is being missed when tube swapping is used to determine tube quality. I 
could imagine a couple of 6AB7's (or whatever) working fine as an RF amp. but one might 
actually perform better. The increased performance of one might not be immediately 
noticeable but perhaps might consist of lower noise, higher gain, etc. In other words, the 
difference should be measureable. Short of testing the performance of the tube in circuit, a 
decent tube tester should help with this task.

I know that drug store testers are useless for this task (even if they are collectible). Are 
there testers out there that are up to the task? I'm starting to regret not getting that Tektronix 
570 curve tracer on eBay a few years ago (couldn't afford it anyway).

btw - I just learned that the mho has been replaced with the siemens. Heh, they lost me 
when hertz replaced cycles.

73 Ken kb7rgg

> In a message dated 7/6/2006 11:49:18 AM Central Standard Time,  
> kgoodwin at houston.rr.com writes:
>
> > I have a Hickok 6000A tube tester but can't find the settings for the  
> > audio output tube in the S-38D; specifically, a 50L6.  Any  
> > recommendations on how one tests this tube other than just seeing if  
> > it plays in the S-38D?
>
> Actually, that's really the best way. With old radios, one of the most  
> reliable parts are the tubes. If they have lasted 40-50 years, chances are they  
> are still serviceable. There are a lot of opinions on tube testers, but unless  
> you have a VERY good one, only very basic good/bad tests are possible. The 
> best  test is in the radio itself. This is particularly true of RF amplifier 
> and/or  converter tubes. They may test fine but perform poorly in the radio. In a 
> series  set like the S-38D, if the tubes light up but the radio doesn't play, 
> it's rare  that a tube is the problem.
>
> If you are restoring an S-38 and need additional help, feel free to post  
> here or write me direct, whichever is more convenient for you.
>
> Regards,
>
> Doug Moore