# How is the grid biased on a triode vacuum tube amplifier?

1. Sep 7, 2014

### BeautifulLight

Here is my single triode vacuum tube amplifier circuit

I understand there needs to be a bias on the grid otherwise the signal will be rectified -not amplified. Is biasing the grid as simple as running a resistor in-between the grid and the cathode on the power supply like this (in red)?

From my understanding, the ideal bias voltage on the grid would be -150V. This would allow the waveform to equally fluctuate from 0V to 300V within your rail voltage.

So?

So the cathode from the supplied voltage is 0V. How is wiring a resistor in series from the grid to the cathode giving you -150V on the grid? There is something about relative potential that I am not seeing.

Last edited: Sep 7, 2014
2. Sep 7, 2014

### jim hardy

Where'd you get that 150 volt number ?

Take a look at a triode datasheet. Here's one.
http://www.wooaudio.com/docs/tube_data/12AU7.pdf

Have you a copy of the RCA Receiving Tube Manual ? It's a marvelous resource and well worth the few bucks on Ebay. First chapters are a course on vacuum tubes and basic electronics.

http://www.ebay.com/sch/i.html?_kw=RCA+Tube+Manual

3. Sep 7, 2014

### Staff: Mentor

Valve radios have two independent power supplies: low voltage AC to heat the cathodes (e.g., 6.3 VAC), and a smooth high voltage DC known as B+ (e.g., 300 VDC) supplying the plates (anodes). There is no negative supply.

4. Sep 7, 2014

### TurtleMeister

Your bias resistor should be in the cathode circuit and bypassed for ac with a capacitor (for class A linear operation). Do an image search for common cathode triode amplifier.

5. Sep 7, 2014

### AlephZero

With a 300V supply voltage, aiming for a 300V peak-to-peak output isn't realistic. 200V would be a a lot. The other factor you forgot is that the tube is a voltage amplifier. So for a voltage gain of say 20, a 200Vpp output would only need about 10Vpp on the grid.

As TurtleMeister said, you would put a "small" resistor between the power supply ground and the cathode, to raise the cathode voltage to about 10V above ground, and the grid voltage would then swing a few volts either side of -10V relative to the cathode.

6. Sep 7, 2014

### jim hardy

Grid leakage is sometimes used to provide bias when cathode is grounded and there's no negative supply.
That led to youthful tomfoolery - "I fixed your radio. All it needed was the grid leak drip pans emptied."
I guess nowadays it'd be more like "Anybody know where to connect Arduino filament voltage?"

The datasheet linked gives values for Rgrid both with and without cathode bias.

7. Sep 7, 2014

### BeautifulLight

150 is half of 300. And I arbitrarily chose 300. I was going to choose 9V, but 4.5 is not an even number. And I just remembered some guy on Youtube choosing 300V to explain how a tube worked.

Figure A is the input signal we're trying to amplify.

Figure B is a representation of what would happen if you didn't bias the grid -you'd only get half the waveform. *Please keep in mind the signal here is TRUE AC (not varying DC).

Figure C is what happens when you bias the grid with -150V. Now look at where the waveform has started in figure C. It's already at +150V or "half throttle" as some people like to put it. This is going to allow the waveform to fluctuate both upwards and downwards. In other words, biasing allows the tube to "capture" the whole waveform being inputted.

I just didn't quite/don't understand how the -150V is impressed on the grid.

Last edited: Sep 8, 2014
8. Sep 8, 2014

### BeautifulLight

Got it!

Put a small resistor in-between the PSU's GND and the cathode and the valve should idle (forgot about this term) somewhere in-between 0 and 300V.

There's the video if anyone wants to watch it. First minute he explains how you need to impress a negative voltage on the grid relative to the cathode...

Also, there's no way I can make it so that my tube will only "see" a varying DC signal? ...doing that should eliminate the need for a bias, correct?

Last edited by a moderator: Sep 25, 2014
9. Sep 8, 2014

### AlephZero

I don't think that will work. The basic source of the current through the tube is electrons emitted from the hot cathode. The number of electrons emitted per second is constant. The purpose of the grid is to stop them all getting to the anode. If you imagine there is a linear electric field between the anode and cathode, the grid needs to be at a negative voltage compared with the voltage at that point in the electric field. The grid is usually physically close to the cathode, so in practice it needs to be a few volts negative compared with the cathode.

10. Sep 8, 2014

### jim hardy

i think you should go to a real source of information about tubes.

Everything you ask is explained quite well in the first few pages of RCA tube manual.

Did you even look at that 12AU7 datasheet ?

11. Sep 8, 2014