[R-390] OT: HP-410B Tubes

Barry n4buq at knology.net
Wed Mar 2 08:15:59 EST 2011


> Sounds like you got some lousy 12AU7s.  And you've had a valuable 
> lesson: that tube testers -- even "good" ones -- only tell you a 
> little about how well a tube will work in a circuit.  So, it was 
> money well spent even if the tubes are junk.

I don't think the tubes are exactly "junk", but just not quite good enough for this circuit.  The manual is quite detailed about matching, gas, etc., for these tubes so it appears these problems were there from the start.  I figured, though, I'd be able to find a pair of 12AU7[A]'s that would work.

> I have run 5814s in everything I own that calls for 12AU7s since the 
> '60s, and have never had any problems with the slightly higher heater 
> current.  The 410B has way more transformer than it needs (unlike the 
> Heath VTVMs, for example -- but even these are perfectly reliable 
> with 5814s, IME), so there is absolutely no reason for concern.

Transformer specs for this model are something I haven't been able to find.  I was just trying to err on the safe(er) side.

> Actually, you've had a second valuable lesson: the tubes available 
> today are mostly cr at p.  Unfortunately, this tends to be true whether 
> you buy newly-manufactured tubes or new-old-stock tubes.  The former 
> aren't built correctly, and the latter have suffered miniscule leaks 
> over the decades they have been unused.  If they had been operated 
> periodically, the getter flashing would have scavenged this gas and 
> the tubes would still be fine.  But the getter can't scavenge 50 
> years worth of gas all at once, fast enough to prevent irreversible 
> damage to an oxide-coated cathode -- so by the time NOS tubes are 
> burned in, many of them are burned out.  Most broadcasters rotate 
> their spare high-power tubes through their transmitters every six 
> months or so for exactly this reason.  (The poisoning mechanisms are 
> not the same for filamentary tubes, but the end result is.)

I well remember some of the discussions that Nolan Lee had about using a lighter to de-gass tubes.  I never tried it but I think others did and it apparantly works in some cases.  I need to test these 12AU7A's for gas (although it probably won't be conclusive as I've never found a gassy tube with the Hickok 752A).

> Very often, the best tubes you can get for your boatanchor are the 
> ones that are already in it -- even if they register "weak" on your 
> tube tester.
> 
> So put the 5814s back in the 410B and find something else to worry
> about.
> Best regards,
> 
> Don

Already found the "worry" point.  DC calibration pot is bad.  It's been replace once in its lifetime so the pot may not be quite the grade it had originally.

Fun stuff...

Thanks Don,

Barry - N4BUQ



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