[TMC] FRR receivers and testing 4CX tubes

[Meir-WF2U]

When I was still making my living as an RF design engineer and I was
involved with the design and manufacturing of wide-band, high power
industrial RF amplifiers, we fabricated a very simple test jig to match
4CX250's by emission.
All you need is a suitable tube socket, a power supply for plate voltage (it
can be equal to your actual amplifier plate voltage) with a fuse or current
limiter at the maximum plate current, the same with the screen current, and
an adjustable bias supply.
If you look at the tube characteristics curves, you can see what bias
voltage will give you a certain plate current. By turning the bias voltage
control (measure it or put a dedicated voltmeter in the circuit) to achieve
the typical plate current, you can determine whether the tube's emission is
still within specs. If you have to reduce the bias voltage below what it
should be for a certain plate current, it means that the tube is weak (weak
emission). Make sure the test jig has the proper air flow according to the
manufacturers' spec (CFM at a certain pressure) through the tube
socket/chimney. 
Also, the screen current should be monitored. Typically a weak tube will
draw more screen current than normal, sometimes overdissipating the maximum
screen dissipation, blowing the screen to smithereens...

73, Meir WF2U 
Landrum, SC

-----Original Message-----
From: tmc-bounces at mailman.qth.net [mailto:tmc-bounces at mailman.qth.net] On
Behalf Of H. L.
Sent: Monday, December 17, 2007 4:13 PM
To: tmc at mailman.qth.net
Subject: Re: [TMC] FRR receivers and testing 4CX tubes

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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