What sets quiescent grid voltage at -ve ground potential in cathode bias?

[Deke609]

In a cathode biased tube, I understand how the cathode resistor raises the cathode voltage above ground reference, and assuming that the grid is at ground reference this results in the grid being negatively biased relative to the cathode.  But where/how is the grid set at -ve ground voltage?  It must be from the signal wiring, but I don't see how/where.  This has confused me for months.


Many thanks in advance, Derek

 

[Paul Birkeland]

There is a resistor called the grid leak resistor that goes from grid to ground which grounds the grid (in terms of DC voltage at least).

In a directly coupled amp where the output stage is cathode biased and directly coupled to the preceding stage, then the grid will be at some positive voltage and not grounded (with a suitable increase in the DCR of the cathode bias resistor of the output stage).

 

[Deke609]

Aha! Many thanks PB. I totally misundertood how the grid leak resistor worked.


So, if I now have this correctly, it's actually grid leak current from grid to ground across Rg that keeps the grid at ground potential.  When current flows from cathode to anode, some of the electrons are captured by the grid, driving the grid negative. But as soon as the grid becomes more negative than ground the resulting difference in voltage potential between grid and ground generates a current across Rg and keeps the grid more or less stable at ground reference voltage. Do I have that right?


cheers and thanks, Derek

 

[Paul Birkeland]

So, if I now have this correctly, it's actually grid leak current from grid to ground across Rg that keeps the grid at ground potential. 

It's the absence of DC voltage across the grid leak resistor that keeps the grid at ground potential.  If you take a Kaiju and substitute the 249K grid leak resistor for 20M grid leak resistors, the grid will no longer be at DC ground potential (don't try this BTW).  The grid leak resistor maintains 0V DC at the grid.

When current flows from cathode to anode, some of the electrons are captured by the grid, driving the grid negative. But as soon as the grid becomes more negative than ground the resulting difference in voltage potential between grid and ground generates a current across Rg and keeps the grid more or less stable at ground reference voltage. Do I have that right?

Yeah, that about sums it up.  For most of our products, the grid current is quite small and the grid leak not particularly large in comparison, so the grid is simply at 0V.  This is not necessarily true for all tube amps, however, as some can have positive grids that draw current on their own. 

This is all in the realm of DC of course, and not particularly applicable to AC.

 

[Deke609]

It's the absence of DC voltage across the grid leak resistor that keeps the grid at ground potential. 

Second "Aha!" moment. Right, of course! Any difference in voltage potential between grid and cathode results in a balancing current across Rg. Not just from grid to ground, but also from ground to grid, depending on the polarity of the difference.

If you take a Kaiju and substitute the 249K grid leak resistor for 20M grid leak resistors, the grid will no longer be at DC ground potential (don't try this BTW).

Is that because, the resistance of Rg being so high, the resulting balancing current through Rg would be too low/slow to keep pace with the rate at which the difference in voltage potential between grid and ground is being generated?


Many thanks again, Derek

 

[Paul Birkeland]

Second "Aha!" moment. Right, of course! Any difference in voltage potential between grid and cathode results in a balancing current across Rg. Not just from grid to ground, but also from ground to grid, depending on the polarity of the difference.

It's more that the grid collects a few electrons, and they will head back toward ground through the grid leak resistor.  If the grid leak resistor value is made really high, those electrons will cause a negative voltage to appear between the grid and ground.

 

[Deke609]

Many thanks PB. I'm going to have to read up on the basic physics of electricity. Probably not necessary for making sense of a schematic, but I'm as fascinated by the physics and materials science of all this as I am by the more "technical" rules for coaxing electricity and wire to make great sound - perhaps more so.


cheers and thanks, Derek