[TMC] FRR receivers and testing 4CX tubes

[Roy Morgan]

At 10:34 AM 12/18/2007, WA5CAB at cs.com wrote:
>Hal,
>
>Unfortunately it was written too early to show the tubes you mentioned in
>your original.  But GE Ham News, May-June 1951, had a piece on Transmitter 
>Tube Testing.

Hal, Robert and others,

That is a good GE Ham News issue. The article describes three or four 
methods to test transmitting tubes (this is from weak memory):

  - Emission: tie all elements but the cathode together, apply 50 volts and 
measure current.

  - Filament activity: I think a similar test, but reduce the filament 
voltage till a set decrease in current occurs.  This is especially good for 
thoriated tungsten filaments.

  - Special test circuit: A self-excited oscillator is shown that runs the 
tube at more or less full power.  Plate current or some other parameter 
(like output power) is measured as the indication of tube health.  Normal 
operating voltages are applied and loading is adjusted to reach typical 
operation. The thing runs at 14 or 17 mc, I think.  (I suspect that this is 
because many tubes of the day had to be de-rated above such frequencies.) 
It could be this circuit that's used for the filament reduced-voltage test.

>  Possibly it could be adapted if you can figure out the correct
>numbers.  The GE piece covers only tubes that GE was making at that time
>(naturally).  They ran from 2E26 up to 4-250A, 35T, 100TH, 211, 203A and 
>592.  And 805
>through 838 plus 8000 and 8005.
>
>In a message dated 12/18/2007 7:38:10 AM Central Standard Time,
>sdaitch at mor.ibb.gov writes:
>...
> > >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?

I don't know. I suspect that 6146's would be found in the charts of most 
tube checkers.  The 811/811A is in the TV-7 chart, BUT, the test done, if I 
remember right, is an emission test.  It certainly is not a 
transconductance test at anything like normal operating voltages or power.

> > >If not, I still feel it should be possible to make crude checks and
> > >comparisons of tubes for matching purposes.

Yes, it seems as if this could be done.  The question is, what should you 
expect from a 4CX250 with 200 volts on the plate and 100 volts on the 
screen?  I think that an adapter with a tester such as the TV-2 might be 
good. The TV-2 has individually adjustable *DC* supplies and meters for 
current.

>  An outboard socket could be wired to an external filament transformer 
> that can deliver 6.0V at 2.6A.

It's quite possible that normal testers can deliver 2.6 amps at  the 6.0 
volt filament tap.  I once tried to test a 6336 which draws 4.75 amps at 
6.3 volts, once warmed up!  The TV-7 tester complained a lot.  (The 6336 is 
a 6080 on steroids and one can replace 2 or 3 of them)

> > >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?

If the 4X and 4CX family of tubes is not run at very close to 6.0 volts, 
they do not perform right.  I would make sure that the filament voltage is 
right and that you can trust your voltmeter.

>  I am reluctant to do
> > >either because the 2.6A filament draw may damage the tester's
> > >multi-tap power transformer.

The tester manual may well have info on this, OR, you can just see what the 
highest filament current tubes listed in the tables are/is.  The larger 
compactrons are candidates.

>  ... Which standard tube listed in most tube testers
> > >best matches a 4CX250?

Hard to answer.  The 4CX250 is meant as a transmitting or audio modulator 
tube with total power output on the order of a kilowatt.  No normal tube is 
anything like that.  And most importantly, most tube testers have plate 
supplies much lower in voltage than even audio power tubes use in service.

>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.

Maybe someone has, and we have not uncovered it yet.  there has been lots 
of information about testing and keeping these tubes happy and well.  Gas 
leakage through the envelope in the glass versions, reverse grid currents, 
effects of internal gas, maximum control and screen grid currents, 
necessary cooling for certain power levels what happens when you store them 
in the cold - all these topics have had lots of discussion.

Roy



- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing
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