24-07-2017, 11:25 AM
(This post was last modified: 24-07-2017, 11:26 AM by Terry.
Edit Reason: Typo corrected
)
For once, your American book is right - it is physically impossible to put a positive bias on a (control) grid.
However, it is important to measure it correctly. The bias is the voltage between the grid and cathode - not earth - although, in some circumstances it doesn't matter which you measure but this will only be when the cathode itself is connected to earth.
If you attempt to apply a positive bias to the grid, the grid effectively becomes the anode of a diode. This grid/cathode diode will conduct, as all diodes do, effectively clamping the grid and cathode together. If you attempt to increase this 'bias', all that will happen is that the current will increase. Continue with this action and you can expect to see the grid starting to get red hot - in fact, you could end up stripping the emissive materials from the cathode. Your valve will have a short life and not a happy one.
You can be fooled into thinking that a valve is positively biased if you don't examine the operating conditions very carefully. This is the well known Mullard 5-10 amplifier:
![[Image: 5-10-b1.jpg]](http://www.r-type.org/articles/5-10-b1.jpg)
When you study it you will see that the anode of the EF86 is connected directly to the grids of the ECC83 without a DC blocking capacitor and, as the EF86 obviously has a positive voltage on its anode you could be fooled into thinking that the ECC83, therefore, is being positively biased. But you would be wrong!
If you look at this voltage table:
![[Image: 5-10-x.jpg]](http://www.r-type.org/articles/5-10-x.jpg)
you will see that the voltage at the anode of the EF86 is 61V and, as expected, the voltage on the grids of the ECC83 is also 61V. However, if you look at the cathodes of the ECC83 you will see that they measure 63.5V, 2.5V higher than the grid voltage. This means that the grid bias is actually 2.5V negative wrt to the cathodes.
The ability of the grid and cathode to be used as a diode can be used very effectively in the right circumstances. The leaky grid detector has been used virtually from the beginning of receiver design.
![[Image: circ237.gif]](http://mysite.du.edu/~etuttle/electron/circ237.gif)
Here the RF signal is applied directly to the unbiased grid of the valve so the valve promptly clamps the positive going half cycles of the incoming signal to the cathode, thus it functions as the detector diode, passing the modulation onto the anode where the 1nF capacitor shunts the remaining RF to ground so that only the recovered modulation appears at the output.
However, this isn't positive bias! In fact, if you apply a very high impedance meter to the grid you will see that it has a negative voltage on it which varies as you tune through different transmissions being more negative on stronger stations than weaker ones. If employed later in a multi-stage receiver, this varying negative voltage can be fed back to earlier stages as AGC.
However, it is important to measure it correctly. The bias is the voltage between the grid and cathode - not earth - although, in some circumstances it doesn't matter which you measure but this will only be when the cathode itself is connected to earth.
If you attempt to apply a positive bias to the grid, the grid effectively becomes the anode of a diode. This grid/cathode diode will conduct, as all diodes do, effectively clamping the grid and cathode together. If you attempt to increase this 'bias', all that will happen is that the current will increase. Continue with this action and you can expect to see the grid starting to get red hot - in fact, you could end up stripping the emissive materials from the cathode. Your valve will have a short life and not a happy one.
You can be fooled into thinking that a valve is positively biased if you don't examine the operating conditions very carefully. This is the well known Mullard 5-10 amplifier:
When you study it you will see that the anode of the EF86 is connected directly to the grids of the ECC83 without a DC blocking capacitor and, as the EF86 obviously has a positive voltage on its anode you could be fooled into thinking that the ECC83, therefore, is being positively biased. But you would be wrong!
If you look at this voltage table:
you will see that the voltage at the anode of the EF86 is 61V and, as expected, the voltage on the grids of the ECC83 is also 61V. However, if you look at the cathodes of the ECC83 you will see that they measure 63.5V, 2.5V higher than the grid voltage. This means that the grid bias is actually 2.5V negative wrt to the cathodes.
The ability of the grid and cathode to be used as a diode can be used very effectively in the right circumstances. The leaky grid detector has been used virtually from the beginning of receiver design.
Here the RF signal is applied directly to the unbiased grid of the valve so the valve promptly clamps the positive going half cycles of the incoming signal to the cathode, thus it functions as the detector diode, passing the modulation onto the anode where the 1nF capacitor shunts the remaining RF to ground so that only the recovered modulation appears at the output.
However, this isn't positive bias! In fact, if you apply a very high impedance meter to the grid you will see that it has a negative voltage on it which varies as you tune through different transmissions being more negative on stronger stations than weaker ones. If employed later in a multi-stage receiver, this varying negative voltage can be fed back to earlier stages as AGC.
