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

[2002tii]

Barry wrote:

>I have an HP-410B that I have cleaned, recapped and trying to get 
>working properly again.  The problem is with the tubes.
>
>It came with 5814's in place of the 12AU7's.  These work great, but 
>they draw extra heater current and I would like to get 12AU7's in 
>there again.  I ordered a set of 12AU7A's but the meter will not 
>work well at all with them.  It won't stabilize, zero, or do 
>anything else correctly (well, the ON lamp does work, but...) with 
>them in place.

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 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.

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.)

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