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valve bias

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the cracken

New Member
im making a distortion pedal with a miniature valve and was wondering does it damage the valve to have the grid go positive?
thanks
 

Boncuk

New Member
I think it's the most reliable way to destroy an electronic tube. The grid is supposed to control electron flow between cathode and anode and no current should across except for the control current. So it must be negative.

Boncuk
 

the cracken

New Member
how and why.
i know that if an air particle becomes an ion and then accelerates towards the cathode it will damage it. does the negative bias give the particle an electron stopping bombardment of the cathode?
or is it something else???
 

Boncuk

New Member
how and why.
i know that if an air particle becomes an ion and then accelerates towards the cathode it will damage it. does the negative bias give the particle an electron stopping bombardment of the cathode?
or is it something else???

Hi the cracken,

I 'm afraid you don't know anything about electronic tubes.

These are the principles of electronic tubes:

The (heated) cathode emits a cloud of electrons searching their way to neutralize. The anode is (normally) positively charged attracting the electrons.

So far so good. Hope you understand this.

An electronic tube does not contain air particles, but is a vacuum tube with almost not containing air, but argon, a gas isolating electrons as also used to weld stainless steel.

If there is no grid (charged with a fixed or variable negative voltage) the electrons will flow directly toward the anode (charged positive) and take care of an unlimited current flow between cathode and anode, depending on the cathode to produce free electrons.

This is good for a rectifier tube, but not for a controlled AF-amplifier.

To control the electron flow between cathode and anode the grid must be charged with a negative voltage to slow down the flow of electrons, hence controlling amplification, like working as a brake.

The grid is also commonly known as "control grid" or "steering grid".

Each electronic tube has its own characteristics and the best bias voltage for its application should be selected carefully for best performance according to data sheets, may it be for a class-A class-B or class-C amplifier.

This is the best information I can provide for you. If you're not satisfied you better google for principles of electronic vacuum valves.

Regards

Boncuk

P.S. Are you Norwegian?
 
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the cracken

New Member
no im english and i know the principles of a vacuum tube. there is no such thing as a complete vacuum and an ionized particle's whether it be nitrogen or argon it will still bombard the anode given the chance. i dont want to make a perfect amplifier i want distortion. my question was simple what is the reason for a negative bias. maybe i didn't make clear what i meant but ill explain a little better. if i have a bias voltage smaller than the input voltage which makes the output distort will it damage the valve and how!
 

crutschow

Well-Known Member
Most Helpful Member
I believe damage could only occur if the grid voltage causes the tube current to exceed its ratings. There are not enough ions in a normal vacuum tube to cause any damage.

Boncuk, normal vacuum tubes do not contain argon. They would get in the way of the electrons. The ideal is a total vacuum, so they typically include a getter material in the tube to absorb any stray molecules of gas left after the vacuum pump evacuates the tube.
 
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the cracken

New Member
thanks for the reply! i was reading this Doubts on cathode stripping in tubes - diyAudio but i think its slightly different but still interesting. so really the only reason they say valves need a negative bias is the same reason why (npn) transistors need a positive bias to get a clean undistorted sign wave in class a amplification. so as long as a huge current doesn't run between the grid and cathode it will not damage the valve?
 

BobW

Active Member
Positive grid bias can be harmful in an audio output tube and other power tubes, but if you're just using a small signal tube like a 12AX7 or similar, a bit of positive bias shouldn't hurt. Just make sure that you don't exceed the plate current rating, and also make sure that you don't draw excessive grid current (sufficient resistance in the grid circuit will prevent this).
 

the cracken

New Member
thanks for the reply
im using a sub sub miniature tube thats smaller than the 12ax7 :)
but what will the positive bias do??? i like to know how and why, would it just be the current going though the grid :? i can't see why it would damage the tube at all.
thanks :)
 

spuffock

Member
So long as you limit the grid current, it will do no harm. As the grid voltage becomes less negative, more anode current flows. Assuming a resistive anode load, this will cause the anode voltage to fall, until it becomes so low it can no longer pull more current through the tube. you are now into square waves for your distortion. As the grid becomes positive, it will begin to draw current of its own. If steps are not taken to limit the current, we run into problems with the grid getting hot and cathde damage.
If you choose to limit the current by running the heater on less power we get the interesting situation where the total supply of current is limited by the cool cathode, and, as the grid goes positive, it robs electrons from the anode, whose voltage will rise again. This is not good for the valve, since the cathode will eventually degrade due to the wrong temperature, but it could make for some interesting distortion characteristics with a bit of experimentation!
 

the cracken

New Member
here is the circuit im going to be using. i don't know what size grid 2 resistor i should use for this though any suggestions????
the anode resistor needs to be a minimum of 36Kohms or higher. for some reason i get better gain with about 100 and something Kohms. its running off 2x9v batteries 18v and 1x1.5v battery for the filament.
thanks for any help :)


**broken link removed**
 

spuffock

Member
I'm not much used to running valves at low voltages, I've usually got a couple or three hundred volts to play with, but that screen resistor strikes me as being a bit low. Maybe 10 to 100 k?
The valve will develop its own negative bias if you drive it with a strong signal. the grid will draw current on the positive bits and charge ite input capacitor.
Remember that the output will be a high impedance and a transistor to follow might load it heavily
 

the cracken

New Member
i though it might need to be a bit higher its just i get higher gain with a lower value.
it does produce its own negative bias, but if i overdrive it the top part of the signal on the grid it starts to go positive. this produces (alot of) distortion that i actually want but what i want to know is if it will damage the valve??? its confusing because some people have said yes and some no. i can't see why it would with this circuit!
thanks :)
 
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