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Old 5th July 2014, 10:08 AM   #1
atnaat is offline atnaat  Wales
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Default Valve Rectifier Testing

I desperately need to test some directly heated valve rectifiers. Would it be OK to use a bench power supply for the heater and connect the anodes to a variac and simply plot the current with a multimeter?

Would I need to add resistance on the cathode? I'm slightly worried that there's no isolation between the variac and the bench supply / multimeter.
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Old 5th July 2014, 10:27 AM   #2
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Bad move - always go through a transformer.

Do you have an amp that uses the rectifier?
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Old 5th July 2014, 10:47 AM   #3
atnaat is offline atnaat  Wales
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No amp, it's for an amp i'm in the process of restoring.
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Old 5th July 2014, 10:56 AM   #4
DF96 is offline DF96  England
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Bad idea to have test equipment directly attached to the mains.

What are you looking for? The valve datasheet will give either DC performance, or performance in an AC PSU with smoothing. Unlikely to give figures for half-wave rectification without smoothing so what will you compare against?
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Old 5th July 2014, 11:03 AM   #5
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It might be unsafe these days but olden days TVs used to use live chassis.

Tube diodes are easy to test, they normally either work or they dont. You will need a small load to test them and a smallish 10uF cap and a resistor will suffice.
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Old 5th July 2014, 12:16 PM   #6
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A few diodes did have the cathode connected to the heater, if this is the case then you do need to worry about potentials around the valve.

You can take away the Gnd connection of the heater PSU, it's not a safe practice but it was common with scopes with live chassis TVs.
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Old 5th July 2014, 01:24 PM   #7
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Rectifiers can be tested in a number of ways, the Funke tube testers put 50 or 100 vac directly across the rectifier and measure the current whit an analog meter.

Depending on where you live you could use a simple variac coupled to an insulation transformer, then use a DVM to monitor the AC voltage, and a DC ammeter to measure current. You can use the Funke test cards to determine what voltage you need, and what the minimum current is.

Beware that because your testing rectifiers its advisable to use centertapped AC heating, because otherwise one end of the fillament will be at a lower potential.
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Old 5th July 2014, 11:15 PM   #8
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99% of the time, as long as the flash gettering is good and filament
shows continuity on an ohm meter, then it will work...
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