[TMC] FRR receivers and testing 4CX tubes

[Sheldon Daitch]

Sometimes you have to do what yo have to do, to test.

I was reading an article in one of the broadcast trade magazines
just the other day, about a situation a tube rebuilder had, in
testing tubes.

Point is, the tube rebuilder had a 20 kW Harris FM transmitter they used
to test each tube.  No tube tester, just one real transmitter.

73
Sheldon
WA4MZZ

H. L. wrote:

> Hi Roy, K1LKY:
>
> Many thanks for your reply. This issue still bugs me.  I have several 
> old ceramic tubes of unknown condition, and it would be a big hassle 
> to open up my heavy, rack-mounted amplifier, remove working output 
> tubes, replace them with the ones to be tested, and then stand back, 
> cross my fingers, and hope no sparks fly!! There has to be an easier 
> way for a quick, simple test before the REAL test.
>
> I guess my first question is: were any of the 1960s tube 
> testers--military or commercial--equipped with the proper socket and 
> parameters for testing these tubes? If so, which models?
>
> If not, I still feel it should be possible to make crude checks and 
> comparisons of tubes for matching purposes.  An outboard socket could 
> be wired to an external filament transformer that can deliver 6.0V at 
> 2.6A. The grid/plate/cathode/screen leads would be plugged into the 
> tester's regular octal socket and its parameters set up for the 
> closest matching standard tube, (6146?) just to get some kind of 
> reading. Once a reading is established for a known good tube, that 
> reading can serve as a benchmark for unknown tubes.
>
> It has been suggested that to save the bother of an external filament 
> transformer, why not simply use the 5.0V setting or use a variac on 
> the tester to reduce its 6.3V setting to 6.0V?  I am reluctant to do 
> either because the 2.6A filament draw may damage the tester's 
> multi-tap power transformer.  I would hope that for a brief test, the 
> plate and screen draw will not damage the tester.
>
> What do you think?  Which standard tube listed in most tube testers 
> best matches a 4CX250? I am surprised that no one has ever published 
> in an older QST or CQ or "Hints & Kinks" a "workaround" means of 
> testing ceramic tubes such as the 4CX250.
>
> Happy holidays to all TMC fans.  And a special thanks to John Poulton, 
> K4OZY, for maintaining and expanding his invaluable TMC reference 
> website to include the SBT-1K.
> 73, Hal
>
> P.S. Help! Still need the TMC FFRD-5 plug-in tuner.
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