[R-390] HP410B rectifier tubes

[Tim Shoppa]

N4BUQ wrote:

> Some of the meters use a 2-01C diode, some use an EA53, and others may
have 
> a probe where either type can be used (there are supposedly slight
physical 
> differences between the two tubes making them not completely 
> interchangeable).  Mine has an EA53 and I checked the specs for it.  I

> noticed that the 2-01C uses a 5.0VAC filament while the EA53 runs on 
> 6.3VAC.  Checking in my manual, it states to set the filament voltage
to 
> 5.0VAC and that was what I did when I went through it a few years ago.

It's also possible that a lower filament voltage makes the tube last
longer and
makes emission be more stable/predictable/slower-drifting. Not that I'm
saying you or the internet or HP is right or wrong!

> There is a variable resistor used with a ballast tube to set the
heater 
> voltage.  I set the heater voltage at the probe to 6.3VAC and
reinserted 
> the EA53.  I let it warm up for about an hour and, sure enough, the
probe 
> began to warm up again.  It is again working in the AC modes; however,
it 
> reads quite low (about 30VAC where it should be seeing 120VAC).  I
think 
> this is just a calibration thing and I can fix that.

I would've expected that boosting filament voltage would've improved
emission and resulted in too-high of a reading compared to your
old 5VAC calibration.

It is very odd for a filament to fail in a way that it "doesn't get hot"
when 
it gets the same voltage that it always did, but that boosting the
voltage
gets it to some emission (but not as much as it used to). Is there any
way you can measure the voltage really close to the tube, or measure
the current through the filament circuit compared to spec? I'd be
very suspicious of bad contacts/cables/etc.

Tim.