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