'near' zero volt bias for low level stages - shane. - Tube DIY Asylum

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'near' zero volt bias for low level stages

Hi,

I'm experimenting with an idea that I read about. It uses the output current of a D/A chip across an I/V resistor to bias the tube analog stage.

Its a combination biased stage, with -66mV provided by the -2mA current output across the 33R I/V resistor, supplemented by the plate current through the 12R current sense resistor in the cathode circuit.

Maximum signal voltage across the input of the tube is ~132mvAC p-p.

Using 6922 @ 12mA effective bias is -66mV - 144mV = -210mV no measurable grid current, every thing is fine.

Using 6072A at 3mA effective bias is -66mV - 36mV = -102mV, and all is ok, all though the 12AY7 curves suggest this one is ok even biased +ve at the grid to 1V ? (wheres the current go?)

What I'm wanting to use is a single dual triode with higher gain but I'm unsure as to what makes a candidate suitable for this type of stage.

I am considering 6SL7. Output impedance is not a concern as it would be loaded with a CCS and the output taken from the 'mu' node, perhaps a couple hundred ohms.

What I am concerned about is whether there are particular types of tubes that are more prone to grid current issues and also non-linearities when biased so low.

In this example, the 6SL7 would be operating at a constant 2mA plate current, biased at (-66mV grid - 24mV cathode = -90mV). Maximum signal half cycle would peak at (-90mV + 66mV) -24mVDC grid and should set up at between 100 and 120V plate to cathode.

Any hints, suggestions or comments are appreciated.

Thanks,
Shane

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Topic - 'near' zero volt bias for low level stages - shane. 01:53:40 09/11/13 (12)

RE: 'near' zero volt bias for low level stages - Thorsten 12:47:45 09/13/13 (3)

RE: 'near' zero volt bias for low level stages - shane. 23:53:01 09/13/13 (2)

RE: 'near' zero volt bias for low level stages - Thorsten 00:15:07 09/14/13 (1)

thanks (nt) - shane. 00:59:28 09/14/13 (0)

RE: 'near' zero volt bias for low level stages - Paul Joppa 09:50:33 09/11/13 (1)

RE: 'near' zero volt bias for low level stages - shane. 15:09:09 09/11/13 (0)

"more prone to grid current issues and also non-linearities when biased so low. " Yes, those are the issues. - Chip647 05:27:09 09/11/13 (5)

RE: "more prone to grid current issues and also non-linearities when biased so low. " Yes, those are the issues. - shane. 15:11:15 09/11/13 (4)

RE: "more prone to grid current issues and also non-linearities when biased so low. " Yes, those are the issues. - Lew 06:58:14 09/12/13 (3)

RE: "more prone to grid current issues and also non-linearities when biased so low. " Yes, those are the issues. - shane. 21:56:54 09/12/13 (2)

RE: "more prone to grid current issues and also non-linearities when biased so low. " Yes, those are the issues. - Lew 08:56:07 09/13/13 (1)

Thanks, Lew. (nt) - shane. 23:34:12 09/13/13 (0)

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