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Grid Leak Bias on small signal pentodes.

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Are there any issues to be aware of when using grid leak bias on small signal pentodes.
The way I intend to implement things is with a phase splitting input transformer driving a 6AU6 LTP. The CT of the input transformer will have a an appropriate resistor (say 10meg) to earth - to bias up the grids. Noise through the grid leak resistor will be common mode so should be effectively cancelled in the LTP.

I have done a similar thing with a triode input stage to good effect. The question is are there any particular issues associated with this approach and pentodes ??


Shoog
 
Shoog said:
Are there any issues to be aware of when using grid leak bias on small signal pentodes.

Other than that it's a bad design? "Contact potential" bias was used with a 6SJ7 in This Design. I don't see why it shouldn't work other than the fact that the resulting grid bias, and thus the Q-Point, becomes very dependent on the characteristics of the individual tube(s) you stick in that hole.
 
It was not a question about the wisdom of the idea - that would be discovered by experience. It seems from your link that it is technically possible, which is what I needed to know.

As I said I have implemented the self same design with a PP ECL82 headphone amp, but with the LTP been made up of high Mu triodes. Its very early days yet, and its difficult to tell whether it will present major problems as the valves age. However at the moment it works very well with unmatched valves, and with minimal power supply filtering it is nearly hum free. Infact it is one of the best sounding amps I have yet built. This is why I want to use it on a bigger amp. There are numerous technical reasons why it presents advantages over a more typical LTP phase splitter, not least of which is the total elimination of the cathode resistor (and hence the improvement in the differential performance) and the removal of the need for a negative supply rail. Since the bias resistor is common, it presents little noise issues, the main concern with high grid reference resistors. Also the tail resistor can be significantly smaller because relative to zero cathode resistance any resistor is high. Additionally the LTP will tend to correct any imbalance in the input transformer without sacrificing any of the gain of the 6AU6. remember that the screen is the dominant controlling grid in terms of plate current.


Theorsten used grid leak on his simple Phono preamp, which I have built and it sounds very good indeed.

Shoog
 
I was hoping to totally eliminate silicon from this design - alas its not to be.


You can put a constant current sink in the tail of a differential pair, without resorting to SS. Use a pentode! Candidates for your situation include the 12BY7 and EL822.

As for grid leak (AKA contact) bias, it works well in situations where the I/P signal is tiny. IMO, that limits contact bias to service in phono sections, in so far as HIFI is concerned.
 
Hi all,
I have decided on further reflection not to go with this scheme. I did some extensive searching on grid leak bias.
I came up with the statement that the reason it is extensively used on the inputs of guitar amps, is because when driven hard in distorts badly.
It does work in both Theorstens phono amp and in my headphone amp. I can only conclude that in the case of my headphone amp the differential nature of the input does a lot to keep things linear, also there is the fact that it will rarely if ever receive an input voltage greater than about 0.2V, a small proportion of the bias voltage and well within its linear range.

So my conclusion - appropriate in its place, and that would be with signals that represent a small percentage of the bias voltage.

Thanks for your inputs.

I will go with a common CCS and a negative rail, a little more hassle but not much.

Shoog
 
If I recall correctly Thorsten eventually played with dumping grid-leak bias on his phono pre, reasoning the grid currents and potential distortion running A2 at cartridge output levels didn't warrant it. While my pref for front ends is still LED bias, tests a while back with grid leak on a trioded E180F were very promising too. From memory Vg-k settled around 1 VDC.
 
Thorsten uses it on his second stage, so its dealing with probably about 0.1V input to a ECC88 at about -1V bias.

As I said, I am well impressed with how it sounds on my headphone amp so i definately think its worth a try if the circumstances seem appropriate.

Shoog
 
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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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.
 
Eli Duttman said:


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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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.
 
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.

Shoog
 
kevinkr said:
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.
 
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.

Shoog
 
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