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Old 11th September 2013, 06:36 AM   #1
Ceglar is offline Ceglar  Australia
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Default Question about '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

Last edited by Ceglar; 11th September 2013 at 06:40 AM. Reason: -/+
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Old 11th September 2013, 09:43 AM   #2
DF96 is offline DF96  England
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High gain valves are more likely to suffer from grid current problems near zero bias. For some this can start from as far away as -1V bias. The voltage linearity of the valve does not suffer; on the contrary it can be very good. The problem is the grid current (which will be fairly non-linear with voltage) develops a voltage drop across the source resistance and this adds distortion. Fortunately you are driving the valve from a very low impedance (33 ohms) so this will not be problem.

What you may have a problem with is the low anode voltage. This can create distortion if the valve is being 'squashed' almost into anode bottoming. The best way is to try it and see. My guess is that most simulations can't be relied on too much in these unusual regions of the valve characteristic.
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Old 11th September 2013, 07:14 PM   #3
Ketje is offline Ketje  Belgium
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Have a look at the CK5755 tube.It can be used with zero grid voltage and still very little grid current.
Mona
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Old 11th September 2013, 10:29 PM   #4
Ceglar is offline Ceglar  Australia
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Quote:
Originally Posted by DF96 View Post
What you may have a problem with is the low anode voltage. This can create distortion if the valve is being 'squashed' almost into anode bottoming. The best way is to try it and see. My guess is that most simulations can't be relied on too much in these unusual regions of the valve characteristic.
Thanks for this, things are looking encouraging enough to make a start.

I'm interested to learn what you mean by 'anode bottoming', I'm assuming its not to do with signal headroom as it would be swinging approx 10vAC p-p across the plate load.

Regards,
Shane

Last edited by Ceglar; 11th September 2013 at 10:38 PM.
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Old 11th September 2013, 10:43 PM   #5
Ceglar is offline Ceglar  Australia
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Quote:
Originally Posted by Ketje View Post
Have a look at the CK5755 tube.It can be used with zero grid voltage and still very little grid current.
Mona
Thanks, Mona.

Interesting tube, but doesnt seem to be quite so linear as 6SL7 (for what that matters in this instance, who can be sure). I'll try with what I have on hand firstly, and if that doesnt work out, a quick look shows that these CK5755 are quite affordable.

Regards,
Shane
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Old 12th September 2013, 09:44 AM   #6
DF96 is offline DF96  England
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By 'anode bottoming' I mean operating the valve down in the bottom left-hand corner of the mutual characteristics where linearity is constrained by the ability of the valve to sink current at low anode voltages.
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Old 12th September 2013, 11:53 AM   #7
Merlinb is offline Merlinb  United Kingdom
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The 6922 has some of the highest grid current of the common audio types, so the fact that your circuit work satisfactorily with that tube means it will probably work just as well or better with a 6SL7 (which is likely to have less grid current).
Nevertheless, it sound slike you are using an anode voltage around 50V, and nothing is certain at such low voltages.
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Old 13th September 2013, 05:18 AM   #8
Ceglar is offline Ceglar  Australia
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Quote:
Originally Posted by DF96 View Post
By 'anode bottoming' I mean operating the valve down in the bottom left-hand corner of the mutual characteristics where linearity is constrained by the ability of the valve to sink current at low anode voltages.
I can push up a little to 2.5mA with the same grid bias for closer to 140V plate. Is there anyway to tell at which point the anode bottoming would likely occur?.

Thanks,
Shane
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Old 13th September 2013, 05:29 AM   #9
Ceglar is offline Ceglar  Australia
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Quote:
Originally Posted by Merlinb View Post
The 6922 has some of the highest grid current of the common audio types, so the fact that your circuit work satisfactorily with that tube means it will probably work just as well or better with a 6SL7 (which is likely to have less grid current).
Nevertheless, it sound slike you are using an anode voltage around 50V, and nothing is certain at such low voltages.
Yes, the 6922 sets up close to 60V in this circuit. Grid 'leak' resistor is only 33R so any grid current issues are limited in their actual effect toward the operation of the circuit.

Focusing on the 6SL7 for now, the comments have been encouraging with the curiosity of anode bottoming which is something I need to learn more about. Its time to build something..

Thanks,
Shane
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Old 13th September 2013, 05:29 AM   #10
Ceglar is offline Ceglar  Australia
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Quote:
Originally Posted by DF96 View Post
By 'anode bottoming' I mean operating the valve down in the bottom left-hand corner of the mutual characteristics where linearity is constrained by the ability of the valve to sink current at low anode voltages.
I can push up a little to 2.5mA with the same grid bias for closer to 140V plate. Is there anyway to tell at which point the anode bottoming would likely occur?.

Thanks,
Shane
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