Grid-Leak-Biasing

Tube or solid-state, this section goes to eleven!
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KippeKiller
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Post by KippeKiller »

Hello,

I'm looking for information on "Grid-Leak-Biasing".
I searched the web and found these links:

http://www.tpub.com/content/neets/14178 ... 178_46.htm

http://www.angelfire.com/electronic/fun ... mp-RC.html

http://www.w8ji.com/fusing_and_floating_grids.htm

they're interesting and educating, but very theorertical aswell.
Has anyone a bit more practical information on the topic?
Is it really that unpredictable, or am I missing something?

I need to calculate the amplification-factor of a parallel grid-leak-biased 6N1P.
As the nominal amplification-factor of the 6N1P is stated with 33 i'm not even sure if it could be done with this tube, but where do i get to know that?

Thanks in advance

greetings Mathias

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soulsonic
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Post by soulsonic »

Very informative reading. I'm not sure if I can help you calculate the gain of that circuit, but it does state that Contact Potential bias has a tendency not to work well with low-mu tubes because of the amount of positive ions produced from electron collisions with the tube's internal gases will cancel out the negative charge that normally builds up on the grid. I've only ever done contact bias with low-voltage pentodes, and it gave better results with that sort of circuit than the usual cathode bias methods, but I can't give any predictions about their behavior with a low-mu high voltage triode.
Based on the little experience I've had experimenting with different bias methods, it would appear that Contact Potential bias is the ideal for low voltage circuits with high gain tubes and low-level input signals, and Cathode Bias is the preferred method for high voltage circuits with a variety of different tubes. It looks like cathode bias is the way to go for the 6N1P.
"Analog electronics in music is dead. Analog effects pedal design is a dead art." - Fran

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KippeKiller
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Post by KippeKiller »

Hello,

i'll give the 6N1P a try, but I'll also try the 6SL7 as i have the same worries on the 6N1P and its amplification.
It's all about a little practice amp i'm planning on, in which at least the input stage should be "leakage-biased", just to hear how it sounds.

The gain calculation isn't that difficult, a member of the tubetown forum helped me out there.
As you know the Supply-Voltage and should know the Anode-Load you can draw a load line.
From the load line you should be able to estimate amplification, the Gate-Voltage isn't that critical there, as it won't go that negative under quiescent conditions.
Changes to the sound, amplification and bias could be done best and easiest with variations of the Anode-Load.
Cin should be kept small for as it defines a time constant with Rg for the bias-voltage to decharge, too large a value of Cin avoids proper biasing.

For german readers theres a thread in the tubetown-forum called "Bassverstärker mit wenig Watt", untill now it's mostly about the power-amp section as it is quite obvious that i'm still working on the preamp ;D

Thanks to you and to Martin for your help

regards Mathias

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