Aging new tube for a VTVM

[Paul P]

I'm putting an old RCA Senior VoltOhmyst VTVM back into service
and came across the following recommendation in the manual
when replacing the 12AU7A bridge tube :

"A suggested preaging method for the 12AU7A is to operate the
plates at 115 volts dc with the grids and cathodes connected
to the B- source. Heaters should be operated at 6.3 volts, 60cps.
The tube should be aged for approximately 12 hours before it is
installed in the instrument."

There is no mention as to what the "B- source" might be. I'm
aware of B+ and C- but this is the first time I see B-. The power
supply of the VTVM (rectified through a single diode) supplies +88v dc
and -40v dc. I'm not sure how this is accomplished since there's
no center tap on the transformer. There is a 27k resistor between
-40v and common, then a 1 meg resistor between common and
chassis.

Is the B- source the -40v dc rail ? (but the aging is to be performed
before installing the tube ?) I have a Heathkit power supply that
I could use to age the tube by setting it to 115 volts dc for the B+
but there is no B- supply to tie the grids and cathodes to. Also, what
should I do to limit the current ?

Thanks in advance for any clarification to the above.

[drewl]

why not just install the tube, power up VTVM for a day and then calibrate it?
I was in charge of a bunch of HP VTVM's when I ran a calibration lab for an electronics co. back in the mid 80's......I wish I would have scavenged a buch of the 12ax7's they used!

[SlidePicker]

Yeah, I'd either let it run in the VTVM or put it in an amp, radio or some such and let it run. Operating it in a standard Class A amp situation for 12 hours would age it just fine.

[Enzo]

B- just means the B supply return, in most cases that means ground. They are just talking in engineerese.

They also are not thinking about a guy in his basement. They want tubes ready to be installed and instantly calibrated for efficiency. They don't want a meter to sit on the bench for half a day just cooking, so they preage the tubes elsewhere.

Screw that, the whole point is they want the tube past the drifting they all do at birth. So just stick it in the machine and let it sit there cooking for a day before you calibrate it. You could calibrate it first thing, and then tomorrow calibrate it again to compensate for the drift - but why waste the effort.

It is really not much different than letting an amp warm up for 20 minutes before adjusting the bias.