[volt-nuts] Neon bulbs for HP 3420B or 419A chopper

[Charles Steinmetz]

David wrote:

> Then an A3C (NE-2U) shouldn't work.  The drive voltage is 125V.

Correct.

> The operating current is 1.5mA (.15V/102R) in the 3420B, so I would expect this to be the A2B (NE-2V) part.

The NE-2V is another 700uA design current lamp, just like the CML A2B 
and A2B-T.  Indeed, if you look at p.117 of the 1965 GE glow lamp manual 
I linked to yesterday, you will see that it lists the NE-2V as an "A2B" 
lamp, with "NE-2V" in parentheses.  So, the primary designation for this 
lamp was "A2B" at least as far back as the mid-'60s.

Note that 700uA is the "design current," not the maximum safe current. 
GE explicitly sanctioned using neon glow lamps at up to 2x the design 
current, and they were routinely used at even higher currents back in 
the day.  Some of GE's published application circuits used them at 
2-3mA.  Mostly, the design current was the current at which the lifetime 
was rated (higher current leads to shorter life).

It seems easy enough for you to obtain CML A2B or A2B-T lamps from 
either Mouser or Digi-Key in the UK, so why don't you just get some and 
see how they work?

> Interestingly the operating current for the 3420A is 2.45mA which you would think might suggest the A3C (NE-2U), but the drive voltage isn't high enough!

I would not be surprised to find that the original neon lamps in the 
3420A were 700uA "design current" parts that HP determined had 
sufficient lifetime at the 3420A operating current.  If this is correct, 
presumably the 3420A choppers fail with lower operating hours than the 
419 and 3420B choppers.  (Perhaps this is why HP reduced the neon 
currents in the "B" version?)

Note that 700uA was the highest "design current" rating for wire-leaded, 
standard-brightness neons back in the day.

In a related vein, neon glow lamps produce less light as they age (both 
from envelope darkening and because less of the cathode [DC operation] 
is illuminated).  This may mean that the HP optical choppers degrade 
significantly before they outright fail, so the chopper amps may start 
drifting more than specified some time before the choppers fail entirely.

Best regards,

Charles