[Hammarlund] 6C4 issues - progress ! (was dead on 15m HQ-170a)

[Glen Zook]

The problem with tube testers, emission, transconductance, etc., is that they cannot test tubes under all circumstances.  For example, there was on manufacturer (I am "blank" on which one right now) that made a batch of 6146W tubes for the military.  That particular batch checks fine in any tube tester yet they will not function much above audio frequencies.  Now this manufacturer got paid for the tubes and then went out of business before the tubes were actually used.  When those tubes started to be put into service it was found that they would not work even at 2 MHz.

Over the years I have "run into" a number of tubes that were well into the "green" on an emission tester yet when put into a radio there were all sorts of problems.  Sometimes the tubes were extremely noisy, sometimes the circuit oscillated (i.e. "squealing" in an audio circuit), sometimes the tube had very low amplification (i.e. when used in an r.f. or i.f. stage in a receiver), and so forth.

Then I have run into tubes that checked basically as almost dead that were working fine.  For example, years ago, when tube type commercial FM two-way receivers were the norm, I have checked the performance on various Motorola receivers that had not been "touched" for many years (like 15 to 20 or more).  The receiver still met factory specifications yet when the tubes were tested a number of them were, according to the tube tester, almost completely dead.

>From what I have read, in the 1950s General Electric designed and built a tube tester that could test a tube under all known (at least at the time) conditions including frequency, load, and so forth.  Reportedly the tube tester cost in excess of $3,000,000 in 1950s dollars.  Pretty expensive, especially for the period.  I believe that it was constructed so that General Electric could verify the performance of tubes that were being supplied for critical performance applications.

In the relatively few applications where a truly "matched set" of tubes is required (and none of the amateur radio equipment manufacturers shipped transmitters with "matched" tubes from the factory) use of a tube tester is handy.  However, one also has to realize that many tubes do not "age" at exactly the same rate and therefore, after several hours of operation, they start becoming mismatched anyway. 

I own 3 or 4 tube testers.  But, it has been years since I have even turned them on.  Now of couple of these were actually made before World War II and I keep them basically as examples of test equipment from the 1930s.  If I need to check the filament/heater it is much quicker to use a VOM or DMM to verify that it is still good.

Now a tube checker is useful in checking for shorted elements.  But the performance in the unit being serviced is usually a good indication of the usefulness of the tube.

Now I don't criticize someone who uses a tube tester to check the condition of their tubes.  But, at least for me, I haven't found a tube tester to be that important.

Glen, K9STH

Website:  http://k9sth.com


--- On Mon, 1/26/09, Ken Kaplan <krkaplan at cox.net> wrote:

I have heard this about using the unit itself as the tube checker and I understand why. But are you suggesting that a tube that measures low in an emissions test may work just as well as a tube that measures high in the same test? Even more odd, it sounds like you may be saying that the opposite may sometimes be true (high emissions = poor performance and low emissions = good performance).

If such is the case, then it seems to me that the emissions type testers are not testing enough. Actually, I think we probably all agree on that <g>. But I'm guessing that even a transconductance tester is not totally sufficient? Maybe that Tektronix 570 Curve Tracer I passed up a few years ago would be the ticket. Then again, I could buy a lot of tubes for the price of that beast.
 
What do you think? Is emissions and/or transconductance testing a waste of time?