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Old 6th July 2012, 01:19 PM   #1
Magz is offline Magz  United States
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Default Testing my Tube Tester

I have a Heathkit emission tester that has seemed to work well over the years for telling if a tube is good or worn out, but I suspect there is something amiss when testing rectifier diodes for section match. In all cases, the side that is activated with the "D" lever tests significantly lower than the one activated by the "F" lever; statistics say that at least some times it should be the other way around, no?

The ideal would be to have a known tube with perfectly matched sections to check the unit out. I was thinking that I should be able to make a "dummy" tube to do this test. My idea is to use a blank tube base and attach a known resistor and diode in series from pin #4 to pin #8 for one section, and a matching resistor and diode from pin #6 to pin #8 for the other section. This should work for all rectifiers that use pin #8 for the B+.

I'm thinking ~ 120 ohms as the resistor value.

I guess I'm looking for a sanity check - any issues with this idea?


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Last edited by Magz; 6th July 2012 at 01:58 PM.
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Old 6th July 2012, 03:06 PM   #2
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I suggest to make a bifilar secondary winding trafo, of say, 100V and load the rectifier with a pure resistive load. Then with an o´scope measure the height of the two half sine wave rectified, so you can test in a reality work, notin a tester.
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Old 6th July 2012, 04:18 PM   #3
Magz is offline Magz  United States
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OK. I appreciate the idea for a more accurate test, but if I want to test my tube tester's performance, my scheme should work, right?
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Old 6th July 2012, 04:30 PM   #4
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Sounds reasonable. Check for dirty or worn rotary switches and all the resistor values. Back then they used cheap 20% tolerance resistors and by now they're probably drifted to 50%. Had a 5 watt resistor in my Sencore tester read bad because of corrosion building up. It stopped being a resistor and started being a battery! Measured a good 50 millivolts across it.
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Old 6th July 2012, 04:48 PM   #5
Magz is offline Magz  United States
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OK. Thanks!
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Old 6th July 2012, 09:00 PM   #6
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Normal. One side has 5V more voltage... the chart SHOULD adjust for this, most don't. Use the other heater pin and see if the difference doesn't swap sections. Wouldn't occur with a separate cathode (bet 6X4s, 6X5s don't shown this difference unless they're unevenly worn).
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Old 6th July 2012, 11:49 PM   #7
Magz is offline Magz  United States
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Hmmm. Good Point, Tom.

After I build my dummy tube I'll check that out - too much wine in me now...

Wonder if I could just up the plate voltage by 5V to compensate when testing the low side.
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Old 7th July 2012, 07:25 AM   #8
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Rectifier tubes should be checked for forward voltage by a CC PSU

you only need a 50V CC psu at 300Ma to test 95% of audio rectfiers.

Tube testers measure in an unknown parameter in a unknown way.


when a tube exceeds a certain percentage over design center value its bad. Its that simple

(offtopic mode) TV dampers Like PY88 are as good as a GZ34 . whit higher PIv

cheers

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Old 7th July 2012, 06:57 PM   #9
Magz is offline Magz  United States
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It looks like Tom got it right. I made a dummy tube base with diodes and matched 110R resistors, and the meter read 85/85 when set to 26V on the plate and filament off, so it seems there is not an issue with the meter.

I re-read some of the previously tested tubes but added 5V to the plate V when testing on the "D" setting, and the readings all matched up within 1-2 units.

Thanks to all!
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