How about a small signal silicon diode (the small copper and black colored ones) for biasing a 12AX7? red LEDs have too much voltage drop in most cases. Is there an advantage or disadvantage to small signal v.s. rectifiers?
mike
mike
A 12AX7 typically needs over 1V of bias in order to prevent grid current. You would need two ordinary silicon diodes for this, so why not use one red LED instead?
On a related note can one used fixed or diode bias along with a currecnt source plate load or do they fight each other for control?
That works well -one is constant current, the other is constant voltage. No worries.
The anode voltage will go down util the voltage/current condition matches, for that tube, the -Vgk from the diode.
Mona
What became of 3V coin cells and 1.56V dry cells for small-signal tube bias? At one time these were fairly popular, or at least often-discussed, among DIY enthusiasts.
For a 12AX7, you may be able to use 1.5V bias. It may take an adjustment of the plate load resistor, and an adjustment of the B+. Draw load lines on the tube curves to see how to optimize that.
How to implement the battery bias:
Connect the input signal to a grid return resistor to ground. Connect the junction of the signal input and the grid return resistor to the positive end of a 1.5V N cell. connect the negative end of the N cell to the grid stopper resistor, and the grid stopper resistor to the grid.
The only time there is grid current is when the signal goes more positive than 1.5V. That is called clipping, which is not desired and does not sound good, so reduce the signal to the input slightly.
If it is in a clipping mode, that will actually try to very slightly charge the battery. It will not discharge the battery. So the battery will last for a long time.
How to implement the battery bias:
Connect the input signal to a grid return resistor to ground. Connect the junction of the signal input and the grid return resistor to the positive end of a 1.5V N cell. connect the negative end of the N cell to the grid stopper resistor, and the grid stopper resistor to the grid.
The only time there is grid current is when the signal goes more positive than 1.5V. That is called clipping, which is not desired and does not sound good, so reduce the signal to the input slightly.
If it is in a clipping mode, that will actually try to very slightly charge the battery. It will not discharge the battery. So the battery will last for a long time.
This will work very well for triodes, not so well for pentodes, and not at all for LTPs that already have a CCS in the tail (seen in a book of a well-known author, though...)
Consider that there's cathode current flowing through the battery, which a dry cell might not tolerate.
Best regards!
There are several methods of battery bias, I recommend 3 and 4:
1. Battery in the cathode circuit (usually not good, see #10 above).
2. Battery between ground and the return end of the grid resistor (the problem is
that All signal is charging and discharging the battery).
3. Battery between ground and the return of an interstage secondary. Good, the only "charging" occurs when you draw grid current, which is distorted, so you automatically turn the volume down (no more grid current).
4. Battery between the signal to the grid, and the grid. Good, the only "charging" occurs when you draw grid current, which is distorted, so you automatically turn the volume down (no more grid current).
1. Battery in the cathode circuit (usually not good, see #10 above).
2. Battery between ground and the return end of the grid resistor (the problem is
that All signal is charging and discharging the battery).
3. Battery between ground and the return of an interstage secondary. Good, the only "charging" occurs when you draw grid current, which is distorted, so you automatically turn the volume down (no more grid current).
4. Battery between the signal to the grid, and the grid. Good, the only "charging" occurs when you draw grid current, which is distorted, so you automatically turn the volume down (no more grid current).
In the grid circuit, with no grid current, the battery voltage is constant with changing signal levels.
In the cathode circuit, I am not sure how constant the battery voltage is, versus changing signal levels. It might need a bypass cap, it might not.
What is the dynamic impedance of a NiCad battery?
How does that compare to the impedance of 1/transconductance (Ohms) that the cathode presents to the battery?
In the cathode circuit, I am not sure how constant the battery voltage is, versus changing signal levels. It might need a bypass cap, it might not.
What is the dynamic impedance of a NiCad battery?
How does that compare to the impedance of 1/transconductance (Ohms) that the cathode presents to the battery?
> What is the dynamic impedance of a NiCad battery?
Very low. Much lower than cathode impedance. Before they were good batteries they were used as voltage regulators in low-volt work. Forget about it.
Any dry-cell will tolerate 12AX7 cathode current.
Life will be self-rot shelf life. Common cells don't last for many years no matter what you do.
In the 1930s, it was common to use a Grid Battery, soldered-in. Because the grid current is essentially zero, and it should never supply "power", they used less-aggressive chemicals and had better shelf life. Some worked until the 1960s. In the 1980s some restorers discovered that drilling a micro-hole and adding a micro-drop of water brought the voltage back near normal for a while.
Red or IR LED in the cathode does about the same thing, unless you are semiconductor-phobic.
Very low. Much lower than cathode impedance. Before they were good batteries they were used as voltage regulators in low-volt work. Forget about it.
Any dry-cell will tolerate 12AX7 cathode current.
Life will be self-rot shelf life. Common cells don't last for many years no matter what you do.
In the 1930s, it was common to use a Grid Battery, soldered-in. Because the grid current is essentially zero, and it should never supply "power", they used less-aggressive chemicals and had better shelf life. Some worked until the 1960s. In the 1980s some restorers discovered that drilling a micro-hole and adding a micro-drop of water brought the voltage back near normal for a while.
Red or IR LED in the cathode does about the same thing, unless you are semiconductor-phobic.
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