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'near' zero volt bias for low level stages

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Posted on September 11, 2013 at 01:53:40
shane.
Audiophile

Posts: 394
Location: S.A.
Joined: March 27, 2004
 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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RE: 'near' zero volt bias for low level stages, posted on September 13, 2013 at 12:47:45
Thorsten
Manufacturer

Posts: 4209
Location: Somewhere nice on planet dirt
Joined: September 25, 1999
 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.

Sounds like something I may have suggested/used. It is important to check your tubes. Not all like this way.

> 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?)

Sounds like something playing in my system right now - I did test it on the AP2 at the shop,it measures no worse than cathode bias, but has less noise.

> 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.

There is no real "limit" - use what you like.

For me, personally, where applicable, I'd likely use a D3a Pentode wired as triode (not a dual) in this application.

That said, the 6072A (GE *****) I have in my DAC are awesome. No point using 6SL7 - the best cannot hold a candle...

> 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 principle - any high mu/high gm tube will be worse than low Mu/Gm tubes. In practice the real effects with such low source resistance are not worth loosing sleep over.

Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?

 

RE: 'near' zero volt bias for low level stages, posted on September 13, 2013 at 23:53:01
shane.
Audiophile

Posts: 394
Location: S.A.
Joined: March 27, 2004
 Hi,

It was one of your postings that lead me to try this, yes.

I had tried some SS I/V stages and also a cathode biased WE396A but with the 2mADC offset countered for by a CCS at the D/A output - this stage of yours sounds better.

Various euro 6922 all hold up well, EH 6072A is the only other that I've tried so far. I can't say that I have a real preference for either, both types sound good. I expect old stock 6072A to be better but they're not so cheap and not on hand.

I do have a pair of E280F here somewhere, not exactly D3A but perhaps close enough, I'll take your suggestion and try them.

Your comment about 6SL7 isn't all that encouraging, but only one way to find out.

In the end I wonder how far away any of these will take me from where I am now.

Thanks again,
Shane

 

RE: 'near' zero volt bias for low level stages, posted on September 14, 2013 at 00:15:07
Thorsten
Manufacturer

Posts: 4209
Location: Somewhere nice on planet dirt
Joined: September 25, 1999
 Hi,

> Various euro 6922 all hold up well, EH 6072A is the only other that
> I've tried so far. I can't say that I have a real preference for
> either, both types sound good. I expect old stock 6072A to be better
> but they're not so cheap and not on hand.

I have tried Mullard E88CC, NOS 1950's Tesla, Siemens 7308 and a host of others, theydo well, I like the 6072A better than any.

> I do have a pair of E280F here somewhere, not exactly D3A but
> perhaps close enough, I'll take your suggestion and try them.

Should do well enough I reckon.

> Your comment about 6SL7 isn't all that encouraging, but only
> one way to find out.

Please do try. Who knows, you may find the results greatly to your liking.

> In the end I wonder how far away any of these will take me from
> where I am now.

I would say nowhere fundamentally different, more towards a slightly different flavour...

Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?

 

thanks (nt), posted on September 14, 2013 at 00:59:28
shane.
Audiophile

Posts: 394
Location: S.A.
Joined: March 27, 2004
 Thanks Mr T.

 

RE: 'near' zero volt bias for low level stages, posted on September 11, 2013 at 09:50:33
Paul Joppa
Industry Professional

Posts: 7344
Location: Seattle, WA
Joined: April 23, 2001
 You must actually measure the grid current to know it - it's almost never specified, and most modern production tubes don't meet the same spec for grid current as the older versions.

I measured:
old Sovtek 6922 (6H23P) 1uA at -.2v bias, 6.4uA at 0v.
Gold pin Amperex 7308 1.7uA at -.2v, 11uA at 0v.
NIB EH 6922 4uA at -.2v, could not measure at lower bias in my setup
A couple Holland 6DJ8s were 15uA and climbing rapidly at -.6v and -.4v; I was not able to get closer to 0v.
GE 6BZ7/6BQ7A 6.4uA at -.2v, 14uA at -.1v

On the plus side, the effect of a few uA through a 33 ohm resistor is essentially nothing, and the nonlinear resistance of the current drawing grid is still so much larger that is has almost no effect.

 

RE: 'near' zero volt bias for low level stages, posted on September 11, 2013 at 15:09:09
shane.
Audiophile

Posts: 394
Location: S.A.
Joined: March 27, 2004
 Thanks, Paul.

Its encouraging and follows what I did measure with no measurable bias shift using 6922, after all even 10uA is 0.5% of 2mA Iout.

What I also find encouraging is it seems the tube will be operating in perhaps its most linear region.

Its a starter, thanks for your time.


Regards,
Shane

 

"more prone to grid current issues and also non-linearities when biased so low. " Yes, those are the issues., posted on September 11, 2013 at 05:27:09
Chip647
Audiophile

Posts: 2793
Location: The South
Joined: December 24, 2012
 A2 or grid current issues will kick in as soon as any part of the signal causes the grid to be equal or greater in voltage in respect to the cathode. For linearity, you need to look at the plate curves for the tube, they get pretty variable near the bottom of the chart. Don't fear the cathode resistor.

 

RE: "more prone to grid current issues and also non-linearities when biased so low. " Yes, those are the issues., posted on September 11, 2013 at 15:11:15
shane.
Audiophile

Posts: 394
Location: S.A.
Joined: March 27, 2004
 Thanks, Chip.

Well Away from the bottom of the curves here and with 33ohm grid resistor bias shift isnt an issue. Trying to avoid any much larger value of cathode resistance, I'm wanting no bypass C and close to full gain from the stage.

Regards,
Shane

 

RE: "more prone to grid current issues and also non-linearities when biased so low. " Yes, those are the issues., posted on September 12, 2013 at 06:58:14
Lew
Audiophile

Posts: 11011
Location: Bethesda, Maryland
Joined: December 11, 2000
 I suppose this is obvious to the cognoscenti, but you could use a red LED or a Schottky diode for a safer bias point and a cathode resistance much lower than 33 ohms, a win-win. Works for me and many others.

Credit to Naz for the idea of using a Schottky.

 

RE: "more prone to grid current issues and also non-linearities when biased so low. " Yes, those are the issues., posted on September 12, 2013 at 21:56:54
shane.
Audiophile

Posts: 394
Location: S.A.
Joined: March 27, 2004
 Thanks, Lew.

I hadnt thought of it. So its just the voltage drop of the diode (0.7V?).. that could be useful. Any idea what the AC impedance is for one of these, and how linear that impedance is wrt frequency?.

Regards,
Shane

 

RE: "more prone to grid current issues and also non-linearities when biased so low. " Yes, those are the issues., posted on September 13, 2013 at 08:56:07
Lew
Audiophile

Posts: 11011
Location: Bethesda, Maryland
Joined: December 11, 2000
 Those issues were discussed here a while back. You might search the archives. The impedance is low enough such that no bypass is needed; for sure it's way below 33R and not that reactive in nature at audio frequencies. Of that I am confident. I've used red LEDs that give 1.8V, and I've used Schottky's that give much lower voltage drop (less than 0.7), such that you may want to string a few together to get the bias where you want it. Both sound excellent. I actually robbed some red LEDs from an unused Bottlehead CCS kit to do this, because they were hand selected to give 1.8V.

 

Thanks, Lew. (nt), posted on September 13, 2013 at 23:34:12
shane.
Audiophile

Posts: 394
Location: S.A.
Joined: March 27, 2004
 Thanks, Lew.

 

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