tubes age 229
- 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@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.
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Date: Wed, 2 Mar 2011 01:19:57 -0500
Subject: Re: [R-390] OT: HP-410B Tubes
> This is one of the early models. I found the DC calibration pot is quite bad.
One trouble reported with the HP meters is that they used a wire wound (200
ohm, I think) pot for some of the cal or settings pots. These develop
intermittent or high resistance at the ends where the winding is crimped by the
solder tabs. Treatment with De-Oxit might help. you may have to re-crimp the
crimps if you can get to them. Or replacement pots would be a good idea. if
you find this is the case, let us know where you found the pots.
> I didn't check the 12AU7A's for gas. That also might be a problem.
You might also simply let them run for a week, then try calibrating/zeroing
again. There are many tubes similar to the 12AU7 that might work in it's place.
The 5963 is a "computer rated" tube built to avoid "cathode interface" from long
periods in the Off mode. The 6211 is another similar tube meant for long
periods of cutoff conditions and may be useful. Sometimes the A suffix on a
tube number indicates that it is selected for balance between units. Often the
military version of a dual tube has tighter specs for inter-unit balance. Note that
the 12AU7 filament is 150 mA on 12.6 volts and the 5814 is 175 ma. I doubt
that this difference is going to matter anywhere near as much as the likely
higher than used-to-be line voltage. I suggest you measure the filament voltage
and your line voltage to see what is really happening there.
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Date: Wed, 2 Mar 2011 08:15:59 -0500
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