Hickok 800A Question

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Hickok 800A Question

Postby skidave » Sun Dec 04, 2011 7:29 pm

I pulled out my Hickok 800A today to test some 12AX7s and I'm a bit confused. The chart settings for a 12AX7 indicates setting the English knob to 58. However, reading the instructions, it refers to the three dots on the English knob that point to 3000, 6000, and 15000 which correspond to the matching Mircomhos scales on the meter. The 3000, 6000 and 15,000 indexing dots are approx 74, 87, and 94 on the knob). If I set the English knob to 58 like the chart indicates...what scale do I follow?

The chart does say that the 12AX7 should measure 1250 Micromhos for a good tube. If I have the English knob at the 3000 mark, a good tube is around 1200. If I turn the English knob to 58, then the measurement increases to 2000 Micromhos.

I know the English knob is a scaling factor, but why does the info say to set the English knob to a non-scale represented number?

I did test several 'new' 12AX7s and both sections measured close to 1200 (when the English knob is set to the 3000 scale and referencing the 3000 scale on the meter). Then I tested two old junk test 12AX7s with much lower measurements (500 for one section and 800 for the other). For my circuit, it was obvious which was the bad tube or tubes. So I am confident I am measuring something correctly with the Hickok.

Any info would help. I also posted this at the Antique Radio Forum (http://antiqueradios.com/forums/viewtop ... 8&t=180489)

Thanks,

Dave
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Postby Thermion » Mon Dec 05, 2011 7:04 am

Dave,

The scale on the Hickok has numbers for mutual conductance and also has zones for good, bad, and ?. When you set the English dial to 58 for a 12AX7, you are using the "zones" and it will only tell you if the tube is good or bad. This is used for quick screening. If you want to know the actual value of mutual conductance, then set the knob to the appropriate color dot to take a direct reading.

FWIW, when I use my Hickok 6000A for tube measurements, I like to let the tubes warm up for 10-15 minutes before taking a measurement.

JT
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Postby skidave » Mon Dec 05, 2011 11:43 am

JT,

Thank you for the reply. This makes sense (now). I have to look again at the meter and test agin tonight. I was only focusing 3000, 6000 and 15,000 scales. There is a lot going on with the meter. Funny, I did not see where the operation instructions reference good / bad with the number on the roll chart.

I'll test again tonight. I usually let the tubes warm up a good bit and check my line voltage setting a minute or two before I check the value of gain.

Again, I knew I had some real plunker tubes by the way they were acting in the ciruit. But ideally, I wanted to match the good tubes I had for the project. Now I'm on the correct path!

Thanks, Dave
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Postby skidave » Tue Dec 06, 2011 12:10 pm

Yep, seems as though the when indexing the English knob to the number it puts the my various test tubes in the 'BAD-?-GOOD' scale. Definitely using the indexed dots for the scales to actually test tubes for a relative value.

Thanks,

Dave
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