[R-390] HP410B rectifier tubes

[n4buq at knology.net]

On Tue, 21 Mar 2006 09:25:38 -0500, "Tim  Shoppa" <tshoppa at wmata.com> 
wrote :

> 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.

There may be other factors involved here, I agree.  I was measuring the 
voltage at the tube base.  I did notice that when I first checked the 
voltage, it was not at 5.0VAC, but had drifted downward.  I don't recall 
the exact value, but I think it was about 4.6VAC.  I don't have a very 
convenient way to check the current draw the filament is creating, though.  
I plan to go through it again, checking testpoints, tubes, and 
calibration.  I may find other things have gone downhill since I worked on 
it last.

Thanks,

Barry - N4BUQ