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Grid Leak Bias on small signal pentodes.
Grid Leak Bias on small signal pentodes.
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Old 4th November 2007, 08:46 PM   #11
Shoog is offline Shoog
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Quote:
He used grid leak bias on a phono stage?? Was he mad?
Obviously not, and that is why its wise to keep an open mind.

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Old 4th November 2007, 09:35 PM   #12
Eli Duttman is offline Eli Duttman  United States
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He used grid leak bias on a phono stage?? Was he mad?
Yes, Thorsten is crazy. Crazy like a fox!! There is good reason to use grid leak bias in the 2nd gain block of a RCA style passive EQ phono preamp. The lighter load on the EQ network improves bass extension.

I've attached a schematic for a tweaked RCA circuit, which employs contact bias in the 2nd gain block.

BTW, GE style phono preamps use the 6SC7. The common connection for both cathodes lead inexoribly to the use of grid leak bias in both gain blocks.
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Old 4th November 2007, 09:38 PM   #13
Eli Duttman is offline Eli Duttman  United States
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As mentioned above, GE style phono preamps are built around 6SC7s. I've attached the schematic for a 1950s vintage Fisher unit which is illustrative.
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Old 5th November 2007, 08:32 PM   #14
Jeb-D. is offline Jeb-D.  United States
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Shoog, I second your findings,

I've worked on guitar amps having an input stage that is grid leak biased pentode. The first time coming across it made me think WTF? They sound really clean lightly driven and produce a unique sounding distortion when driven hard.

Lately I've been experimenting with some low voltage, line-level circuits, and have had some surprisingly good results with grid leak bias.
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Old 7th November 2007, 07:09 PM   #15
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by Eli Duttman


There is good reason to use grid leak bias in the 2nd gain block of a RCA style passive EQ phono preamp. The lighter load on the EQ network improves bass extension.

I've attached a schematic for a tweaked RCA circuit, which employs contact bias in the 2nd gain block.
I guess you'd need the extra bass to overcome the simply vast amounts of Johnson noise from the grid leak, and flicker noise in the pentode!
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Old 7th November 2007, 10:53 PM   #16
kevinkr is offline kevinkr  United States
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Grid Leak Bias on small signal pentodes.
With careful network design the passive eq source impedance looking into the grid of the succeeding stage should be orders of magnitude lower except very close to dc. i.e. the johnson voltage noise will be that of the source impedance of the network looking into the grid, not that of the grid leak resistor itself.

There are of course other noise mechanisms in resistors (current noise, etc.) that might be a problem in some instances with large values of grid leak resistance.

I have no idea whether flicker noise in the tube itself would be a function of how the bias is derived so I can't weigh in on that comment.
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Old 8th November 2007, 07:31 AM   #17
Shoog is offline Shoog
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I guess you'd need the extra bass to overcome the simply vast amounts of Johnson noise from the grid leak, and flicker noise in the pentode!
Build it and report back on the Johnson noise.

I have had similar discussions on the use of Interstage Transformers and Toroidal Output Transformers, with people weighing in with plenty of theoretical advise as to why it can never works as well. Yet anyone who has experiemented with these "alternative" approaches has generally been won over by the empirical evidence.
So if you want me to take your speculation a bit more seriously, bring some empirical evidence to the table.

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Old 9th November 2007, 10:36 AM   #18
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by kevinkr
i.e. the johnson voltage noise will be that of the source impedance of the network looking into the grid,
True, the source impedance will shunt the Johnson noise, but it's ability to do that will usually get worse at very low freq's.

Even so, it just seems like very bad design to pick the noisiest possible circuit design, for a phono stage of all things. For a preamp, sure. go for it. But this is the most noise-susceptible part of audio!

Firstly, pentodes are noisier than triodes. They usually have large anode resistors = more Johnson noise, which won't even be shunted due to the very high ra of the pentode! Add grid-leak bias and away you go. Heck, if that's the way you like it then you may as well use CarbonComp resistors to boot.
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Old 9th November 2007, 11:48 AM   #19
Shoog is offline Shoog
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As I said previously, Thorsten used it on his second stage output tube, and so he would have been dealing with a respectable voltage - and not the voltage coming off the cartridge. Your concerns in this case seem unfounded.

I also said that this bias type should be considered in a place where the circumstances seemed to warrent it. I have had great success in the input stage of a two stage headphone amp, where noise doesn't seem to be much of issue, and the things it did bring to the table made it a great solution.

I would not advocate using it if you feel uncomfortable with the concept - but it is wise to keep an open mind and consider that at some stage in the future it might just present the perfect solution to a particular problem.

I am not going to go ahead with the idea i first proposed because its overload behaviour could have seriously bad effects on the sound, and for absolutely no other reason. If I could guarantee that the input signal would only ever represent 20% of the bias voltage i would not have these reservations.



I have been having a slight debate with another forum member about why he prefers SE over PP. I could muster any number of theoretical reasons why the PP will perform better, but this person maintains that the SE wins out. Theory loses out to experience again. His argument is largely that in overload situations the SE amp injects large amounts of 2nd harmonic distortion, where as the PP injects higher orders of distortion. I have to admit that in my personal experience I have never met anyone who doesn't prefer an amp with an element of 2nd harmonic over one without.
My point is - the build proves or disproves the design philosphy, and those who design purely by the book often end up with sterile results that fail to impress.

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