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Old 1st November 2007, 06:37 PM   #1
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Default Grid Leak Bias on small signal pentodes.

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 ??


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Old 1st November 2007, 07:03 PM   #2
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Default Re: Grid Leak Bias on small signal pentodes.

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Originally posted by Shoog
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.
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Old 1st November 2007, 10:24 PM   #3
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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.

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Old 1st November 2007, 10:45 PM   #4
rdf is offline rdf  Canada
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Mullard used the technique with an EF86 on the 3-3 kit. That application was atypical for returning the output tube cathode to the EF86's screen.
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Old 1st November 2007, 10:54 PM   #5
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Quote:
Also the tail resistor can be significantly smaller because relative to zero cathode resistance any resistor is high
After reading the Schmitt and CCS thread, it seems that a CCS will be a superior solution. I was hoping to totally eliminate silicon from this design - alas its not to be.

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Old 2nd November 2007, 12:06 AM   #6
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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.
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Old 4th November 2007, 06:11 PM   #7
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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.

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Old 4th November 2007, 06:30 PM   #8
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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.
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Old 4th November 2007, 07:18 PM   #9
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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.

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Old 4th November 2007, 08:12 PM   #10
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Quote:
Originally posted by rdf
[B]If I recall correctly Thorsten eventually played with dumping grid-leak bias on his phono pre,
He used grid leak bias on a phono stage?? Was he mad?
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