Cathode Bias questions

[rongon]

I'm new here, so hello there, I'm glad I found this great forum.

I'm hoping someone can help me with a question about cathode biasing.

I've seen three simple ways of biasing a grounded grid, plate-loaded triode stage. They're in this illustration...



1) Of course the standard way is to put a suitable value resistor in the cathode circuilt so that the cathode is positive in regards to the grid. I showed the cathode resistor unbypassed, for simplicity.

2) Putting a rechargeable AAA or AA battery in the cathode can also put the tube's cathode 1.2V above the grid. I tried it and it worked. I'm not sure how much I liked the sound, but you know how that is...

3) I picked this up from a Van Alstine SuperPAS 3 I built from a kit, back in the '80s. A 1N4148 fast switching diode goes in the tube's cathode, although I never understood why the diode's anode would be sent to ground.

Anyone have experience with battery bias, or diode bias? Opinions?

I've got a phono preamp where the first two stages are diode-biased, and it seems to work well. Any horrible downsides to this that I don't know about? I wonder why I never see this...

Thanks for any info.
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[Sheldon]

Look up LED bias here. An LED, other than emitting light, is just a diode. You'll find lots of examples. An LED or an ordinary diode has to be oriented as shown, or no current will flow. In this case the forward diode drop provides the bias voltage. For higher voltage differential, a zener could be used in the opposite orientation.

LED's are often preferred, because their change in resistance with current variation is small (low dynamic resistance - or impedance). Dynamic resistance will generate distortion. An LED, in it's linear range has a dynamic resistance of 10R or sometimes less. Putting an unbypassed resistor of, say, 100R in series with the diode will reduce the distortion in this case (10R impedance) by a factor of 10.

Sheldon

[SY]

More than that, LEDs have instantaneous recovery from overload- none of the blocking that RC bias can have.

The single diode bias has one major problem- it's too low. Most tubes are starting to draw grid current with only 0.7V grid-to-cathode, reflected in increased distortion.

[athos56]

Someone on Audioasylum tested some fancy new Cree diodes and had good results.

LINK 1
LINK 2

[rdf]

He reports interesting measured dynamic impedance numbers. Does 1.6 ohm sound reasonable?

[dsavitsk]

Maybe add this to your list as well.

[SY]

Originally posted by rdf
He reports interesting measured dynamic impedance numbers. Does 1.6 ohm sound reasonable?

Yes. But the real figure of merit is Z/Vf- one needs to series them to get a useful voltage value. With two in series, the forward voltage is about the same as a red LED and so is the impedance. How does their price compare to (say) one of the Agilent red LEDs?

[rongon]

dsavitsk: Maybe add this to your list as well.

As I see it, you're putting one leg of the heater to the cathode of the tube. So one leg would be lifted above ground by the voltage developed across the cathode bias resistor. With AC heaters, wouldn't that put 6.3VAC at the cathode? Or if with DC heaters, wouldn't you have the heater DC voltage at the cathode? I can't see how that makes sense. But you know, I'm not any kind of expert at this.

Is there some reason why an LED would be preferable to something like a 1N4148?

(Possible trolling question) - Any preference for LED color? Red? Green? Amber? Blue? Infrared? Other?

I've seen cathode bias as low as- 0.5V in the first stage of a phono preamp, using 12AX7 or 6DJ8. Are these drawing grid current with the 47kOhm load of an MM cart? What about 1kOhm for a high-output MC cart?

[jonusgrumby]

The advantage of the diode or LED over a resistor is that the stage will have higher gain. The resistor, assuming it is not bypassed with a capacitor, reduces the stage gain, but also provides feedback to reduce the distortion of the stage. With a small voltage swing in a preamplifer, the distortion reduction is likely to be insignificant (as is the gain reduction too). The diode or LED would be lower noise than a carbon composition resistor, but at line levels it too would be insignificant.

The reason to use one LED over another is for the voltage drop, with red typically being the lowest and blue the highest. You can accomplish the same effect by placing diodes in series. I would not use LEDs in low current stages (less than 5 mA).

The LED or diodes will set the cathode/grid voltage which will determine the operating point for the tube. You need to consult the average plate characteristic curves for the tube you wish to use to pick a plate voltage and current that correspond to the cathode/grid voltage you selected. For example, using the curves for a 12AX7, and three 1N4148 diode in series (for a 2 volt bias voltage) you have 0.5 mA bias at a 200 volt plate voltage and 1.5 mA at a 275 volt plate voltage. You choose your palte resistor based on your supply voltage. The higher the plate resistor, the higher the gain (but you have to consider the loading from the following stage if there is another stage).

If you use a 6SN7, the cathode/grid voltage tends to be 5 to 8 volts so you'll need a lot of diodes or LEDs. Unless you really need the higher gain, I think most times the resistor is the better choice.

Actually, I prefer current sourced long tailed pair stages to resistor or diode bias. The current source will handel the bias voltage for you automatically and I have found it gives MUCH less variation from tube to tube in operating point. I'm sure that statement is not supported universally, but it has been my experience.

[SY]

No troll at all! Different color LEDs have different voltage drops and different internal impedances. So they each have their place. In Morgan Jones's "Valve Amplifiers," 3rd edition, there's some data for a variety of LEDs and diodes. It's somewhat dated (I await the 4th edition!), but a very useful guide to this circuit trick.

As to the distortion issue, you will indeed start seeing some not-good changes in the distortion spectrum as you drop the bias voltage. How much and at what point depends on the particular tube and the circuit operating points. Having a 0.5V bias is asking for trouble, IMO. However, schemes that increase distortion are often popular, for what reason I do not understand.