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Old 8th May 2009, 04:07 AM   #1
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Default Valve Tester Guru Help

Guys,
I tried to measure grid current (gas test) on a batch of 6SL7 using my AVO MKIII. ( I wanted to use them in Guitar Amp with high Rg1 values).
I every case (all tubes checked) I get negative meter movement.

Does that just mean that I'm getting positive grid current due to the testing point used rather than negative grid current due to gas in the tube (in other words the tubes are all OK gas wise) or is there something wrong with my AVO. It appears to do everything else properly. Certainly all gm measurements make sense.

Cheers,
Ian
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Old 8th May 2009, 07:23 PM   #2
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Location: Taxland, New Jersey
Since I've never owned an AVO I'm not familiar with it. But I have used a New London 901A which may be similar. Also, based on how my transmitting rack tester operates, your meter moving down scale is probably normal. I say this because I've found that you must have some minimum negative grid voltage applied in order to show grid current from gas. Without some level of grid bias, especially in a high inpedance environment, some positive grid voltage will develope and deflect the meter backwards.

On my big tester any tube (that has emission) under filament power and nothing else, the 200uA gas meter (which is in series with the grid and negative source) will deflect backwards.
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Old 29th May 2009, 02:15 AM   #3
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Hi, I have tested a couple of randomly selected 6SL7s in a AVO Mk 4, with bias set to -2V, same result, reverse meter deflection on gas test. Upping bias to -5V or more seems to resolver this erroneous reading. Suggest this is normal, I agree with Hollowstate's explantion.
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