Measuring Tube Gm - Page 6

euro21 · 29th April 2012, 06:11 PM

Oh, I understand!
To use only one panel meter with limited (1Meg) resistance to metering bias and another voltage, not an easy task.
Switching panel meter (on/off) surely will modify bias point, unless driving resistance not enough low.

euro21 · 29th April 2012, 06:29 PM

Theoretically if you use (circa) 100mA current source (from 90V supply) and 500R potmeter, then moderately (high) voltage divider, 1Meg switching on/off only affect operating point below 1% (0.66).

mogliaa · 29th April 2012, 08:08 PM

Quote:

Originally Posted by euro21

Theoretically if you use (circa) 100mA current source (from 90V supply) and 500R potmeter, then moderately (high) voltage divider, 1Meg switching on/off only affect operating point below 1% (0.66).

Right, so we are back close to my original gyrator proposal from previous posts. A PMOS gyrator can sustain grid bias point when meter is switched on/off and also will offer high AC impedance, therefore will not load the source and provide good BW with a small Input capacitor, so why not using it?

Thanks for your help, I'm learning a lot with this!
Cheers,
ale

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29th April 2012, 06:11 PM	#51
euro21 diyAudio Member Join Date: Sep 2004 Location: Budapest	Oh, I understand! To use only one panel meter with limited (1Meg) resistance to metering bias and another voltage, not an easy task. Switching panel meter (on/off) surely will modify bias point, unless driving resistance not enough low.