[SOLVED] Light Bulb Fuse in a Tube Tester.

[Calambres]

Hi there!

I recently acquired an old Precise 111 Tube Tester, but it seems to be a problem somewhere as I get the "Fuse" light on when I adjust the line voltage level (no tubes in any socket). As I approach the "?" indicator in the panel, that is, the correct line voltage, the fuse light goes on and stays bright. If I reduce the line voltage pot, the light goes off.

This light bulb must be a GE-81 (6.5V 1.02A) but what's in there is as GE-67 (13.5V, 0.59A) instead. I have been told that using the wrong bulb may well be the problem and so I've ordered a couple of correct #81 bulbs. Unfortunately the mail service is greatly delayed due to COVID and a very strong snow storm we've suffered recently and I haven't received them yet. I have checked the tube tester wiring (a real nightmare!), recapped and corrected some resistors that have drifted in value and everything seems OK except for this bulb.

This is what the Precise 111 manual says about that light:

FUSE LAMP
This automobile type bulb is in series with the 110 volt line and serves as a fuse for over-loads. On certain tubes it may glow dimly, but under no circumstances should it glow brightly. If it does, turn instrument off at once and recheck all switch settings.

The construction manual for the Precise 111 indicates that this bulb is a "B81" lamp which I understand to be the same as a "#81" or "GE-81" automotive bulb.

I do not fully understand the rationale for using such a low voltage bulb (6.5V) in series with a 110V power line. Wouldn't it vaporize instantly?
Evidently not because this has been used for decades in many circuits, notably in tube testers like this. Please anyone can shed some light (pun) on this?

Thanks a lot in advance.

[Zero999]

The idea is the bulb acts both as an indicator, as well as a fuse. It won't vapourise, as long as there's something to limit the current, in the same manner an LED with a forward voltage drop of only 2V, can be run off a 12V battery, with a suitable series resistor and not blow up. Using a bulb with a lower current rating, as you've done, will cause it to glow brightly at a much lower current and is probably why you have this problem.

[drvtech]

As a temporary measure, have you got another of the 0.59A bulbs you could wire in parallel with the first. That would give you approximately the same characteristics as the correct bulb.

[Calambres]

Nope, sorry!.

I do not dare to use the tube tester just in case there's a real short somewhere. As I said I've checked the wiring but it is SO complicated and SO difficult to trace that I may be wrong. In case someone is interested, click here to see the schematic.

Still awaiting the proper #81 bulbs to arrive.

Below: a pic if the tester's guts as received, prior to recapping and "re-resistoring" 

[bob91343]

The problem is almost certainly the incorrect bulb.  Be patient and when you replace it you will find the unit works as expected.

[Jwillis]

That bulb is in series with the rest of the circuit. . Tubes draw very small amounts of current ,in the mA range , across the anode , cathode ,screens and suppressors at a high voltage . So as Ohms law dictates V *I =W . So the 6.5V 1.02A bulb parallel to a battery would be 6.6W. Close enough to be a 7W bulb .  The GE-67 is an 8W bulb.  At 120V the GE-81 would only need about 58mA  and the GE-67 would need around 67mA  for either to glow brightly . That's a difference of only 8mA approximately between either bulb . Not very significant  to be a problem .Theirs another  issue besides the bulb if it's glowing brightly without any tubes installed . Bad capacitors or a short some where else in the circuit is more likely the issue. Those old paper capacitors are terrible for being dried out or shorted. Old switches can be worn out . Check that none of the point to point wires are shorting together . The old carbon resistors may have drifted some but shouldn't cause shorting .Although unlikely a transformer may be shorting . Theirs also plenty of potentiometers and any one could cause problems .
Just remember to always discharge capacitors any time you do a service and always check voltages with one hand . A set of alligator test wires for your multi meter is best. Make sure your DMM can handle the voltages.  Be careful and good luck.   

[nanodocl]

You can replace the bulb with a equivalent resistor  value, while the "right" bulb arrives.

[Calambres]

Yes, but then the warning/fuse functionality of the bulb won't be there and I'm not sure if there's really a short somewhere. I may use a dim bulb tester.

[Calambres]

The correct #81 bulbs arrived at last and now everything works correctly:


If you look closely, now the LINE setting is correct (see the panel) and the FUSE light (below and left of the panel) is not lit 

Left: the wrong GE-67 bulb
Right: the correct GE-81 bulb


Thanks a lot for your help. Now on to calibration...