Value of grid-leak resistor

[hkoetz]

For an EL84 the maximum value of the grid leak resistor in cathode bias mode is quoted as 1 M and for fixed bias as 0.3 M (Philips, 1969). Why are these values different and what determines the maximum value in general?

[SY]

Fixed bias values are generally lower.

Although an ideal tube draws no current through the control grid, real-world assemblages of glass and metal are not ideal. As grid current is drawn, it creates a voltage drop across the grid leak; this upsets the tube bias, increasing idle current, and thus increasing grid current. That's positive feedback at DC and the tube will run away.

Now, a tube with cathode bias has a built-in negative feedback mechanism- as idle current increases, the positive voltage on the cathode increases, partially offsetting the decreased grid bias. So cathode biased tubes can tolerate a higher grid-leak value.

[hkoetz]

SY,

thanks for your reply.

Now, a tube with cathode bias has a built-in negative feedback mechanism- as idle current increases, the positive voltage on the cathode increases, partially offsetting the decreased grid bias. So cathode biased tubes can tolerate a higher grid-leak value.

This I understand, but

Although an ideal tube draws no current through the control grid, real-world assemblages of glass and metal are not ideal. As grid current is drawn, it creates a voltage drop across the grid leak; this upsets the tube bias, increasing idle current, and thus increasing grid current. That's positive feedback at DC and the tube will run away.

The lower the grid-leak is resistor is, the higher the tendency to counter the bias, because the negative charge on the grid will leak away to ground via the grid-leak resistor. Thus the current will have a tendency to increase. Now what happens then: Will more current flow through the resistor trying to offset further the negative bias? With cathode bias this will be counteracted by the mechanism described in the first quote. with fixed bias the is no such mechanism, except the 'power' of the fixed bypass.



Now suppose the current through the valve increases for some season. In cathode bias this will result in a more positive cathode relative to the grid. This will counteract the current increase.

[hkoetz]

SY,

thanks for your reply.

Now, a tube with cathode bias has a built-in negative feedback mechanism- as idle current increases, the positive voltage on the cathode increases, partially offsetting the decreased grid bias. So cathode biased tubes can tolerate a higher grid-leak value.

This I understand, but

Although an ideal tube draws no current through the control grid, real-world assemblages of glass and metal are not ideal. As grid current is drawn, it creates a voltage drop across the grid leak; this upsets the tube bias, increasing idle current, and thus increasing grid current. That's positive feedback at DC and the tube will run away.

The lower the grid-leak is resistor is, the higher the tendency to counter the bias, because the negative charge on the grid will leak away to ground via the grid-leak resistor. Thus the current will have a tendency to increase. Now what happens then: Will more current flow through the resistor trying to offset further the negative bias? With cathode bias this will be counteracted by the mechanism described in the first quote. with fixed bias the is no such mechanism, except the 'power' of the fixed bypass.

[SY]

You might be conflating two different grid current mechanisms. For small signal tubes, there is a tendency for the grid to get charged negatively because of the impingement of electrons from the cathode-to-plate electron stream. So they tend to drift negative. This is the basis for so-called grid leak bias- the cathode is returned directly to ground and the grid is biased via a very high value resistor (typically 10M) to ground.

In power tubes, where the grids are not as "tight" and the currents are higher, the dominant mechanisms of grid current are ion currents due to gas and grid emission due to grid heating. Both tend to drive the grid positive.

If you can get a copy of Tomer's book (I think there's a pdf on Pete Millett's website), he goes into this in great detail.