[R-390] RF IMD Problem SOLVED!

[Barry Hauser]

Phil wrote:

> What I DON'T understand is this.  These two tubes checked IDENTICALLY in
> the B& K emission tester, both checked IDENTICALLY for gain in my
> receiver at 21MHz.  Neither test shorted or gassy!  RF GAIN CONTROL had
> no affect on the IMD.  What really messed up my thinking was that since
> the RF gain control didn't affect the IMD the RF amplifier must NOT be
> the culprit.
>
> YOUR THOUGHTS ???

I'm no thermionic emission engineer, but I can tell you there are toobs and
there are tooobs.  They have various mechanical characteristics and
foibles -- not to mention a few nuances thrown in.  I don't know if a mutual
transconductance tester would have shown this up -- probably not.  But I
have a hunch.

Put the IMD 6DC6 back in and let things warm up.  Tune around the subject
frequencies and gently tap the tube.  There's some chance that this tube has
also gone microphonic, or you may find some fluctuation in the IMD action.
Most of the military tube tester manuals have a version of YMMV
disclaimers -- "the only true test is in the actual equipment" or words to
that effect and offer the suggestion to try another tube if a circuit is
suspect.

If this "works" or sheds any light on the situation, credit for tube
swapping goes to the tube jockeys of yesteryear -- the ones who made house
calls with tube caddies.  If the tap test works, credit goes to early
Neanderthal "technicians" who developed the "if in doubt, hit it with a
stick" repair and diagnostic method. (Hey, it worked with fire ;-)

Barry