Rk vs Rg voltage

[EtherealWidow]

Thank you jjman. That's also another REALLY good point that it took me a couple of reads to figure out, although I feel like it's really under emphasized.

[tubeswell]

Its not just the difference in resistances between Rk and Rg that result in more tube current flowing through Rk. The cathode emits electrons because its heated to sufficient hotness to emit an electron space cloud, which then gets 'picked up' by the +ve voltage at the plate (and so the electrons whiz across the gap to the plate).

Whereas the grid isn't heated (in the same way that the cathode is heated - okay the grid gets a little warm due to being in relatively close proximity to the heated cathode, but typically not red hot - unless something is wrong and the tube is in thermal runaway or something).

Any positive charge that happens to accumulate on the grid (due to losing a few electrons because of its being in close physical proximity to the cathode) is replenished with electrons from the ground return via the grid leak resistor. But this is a comparatively tiny amount compared to what the cathode is able to emit.

If you make the grid leak resistance high enough (like 5M plus or so), this creates sufficient voltage on the grid that can be used to bias the tube.

But as the current in a 1M Rg is so tiny, the grid might as well be regarded as being at ground potential, because the voltage drop across the 1M is so tiny that both ends of the resistor are virtually at the same potential.

The grid leak resistor also functions as part of the stage's input impedance as well as also being part of the AC load on the previous stage.

[EtherealWidow]

But isn't the current that's at the top of the bias resistor coming from the ground that is connected to the plate of the diode? I can't imagine the electron cloud inside the tube flowing toward the plate to cause a voltage drop at the top of the cathode resistor. I pretty much got all the other functions of Rg, I just wasn't getting why it didn't cause a voltage drop like Rk would being that they're both connected to ground.

[gingertube]

Grid Current is one of the most poorly understood topic and so at the risk of teaching grandma to suck eggs I've written a little essay.

Hope its helpful - flag immediately if you think any of it is factually incorrect.

Grid Current and all that – remember that, due to historic limited knowledge, current flow is defined as from positive to negative whereas in actual fact electrons flow from negative to positive.

There are 3 types of grid current

Current flowing into the grid is known as POSITIVE grid current
• When the cathode is heated a cloud of electrons forms around the cathode known as a “space charge”
• Some of these electrons gather at the grid. These electrons then flow out of the grid which is the same as saying that current flows into the grid.
• This positive grid current generates a voltage across the grid leak resistor (Rg1)
• This voltage makes the grid more negative which ADDS to the bias
• If the grid leak resistor is large enough then this positive grid current can generate the entire required bias – This is known as “Grid Leak Bias”

POSITIVE grid current is a low level phenomenon and can easily be overshadowed by NEGATIVE grid current

Current flowing out of the grid is known as NEGATIVE (or REVERSE) grid current
• Negative grid current can be caused by
1. gas ioniszation current
2. leakage current (grid to cathode)
3. grid emission (from grid being heated by the cathode, screen or anode)
• gas ionization current dominates with the other 2 being low level effects
• As electrons accelerate “up” the tube from cathode toward the anode, some of them collide with residual gas atoms. This collision is energetic enough that it strips an outer orbit electron from the gas atom which turns it into a positively charged ion.
• The positively charge ion accelerates back “down” the tube toward the cathode
• Some of these positively charged ions collect at the grid (which is usually the most negative potential of any tube element). Electrons must flow into the grid to neutralize these ions which is the same as saying that current must flow out if the grid
• This current generates a voltage across the grid leak resistor.
• This voltage makes the grid more positive which SUBTRACTS from the bias and results in increase tube current.
• This effect is proportional to tube (anode) current and so is worse in power tubes
• This effect also is worse in old “gassy” tubes.
• This is why there are always 2 specifications for maximum Rg1 values. One value for cathode (auto) bias where the increased current is opposed by an increased bias due to uncreased voltage drop across the cathode resistor, and another smaller value for fixed bias where there is no action to oppose the current increase.

The mechanism of NEGATIVE grid current, reducing the bias, increasing the current, increasing the negative grid current, reducing the bias etc. etc. round and round is called thermal run away and is what causes a lot of power tubes to self destruct.

This is made worse by the fact that Rg1 values in most guitar amps ignore the recommended maximum Rg1 values. This is done so as to not load down the output of the phase splitter too much. This is usually compensated to some degree by biasing the output tubes at 70% of rated maximum dissipation, that is reduce the tube idle current by 30%. That allows use of an Rg1 value of about double the recommended maximum which is based upon running the tube at 100% of its dissipation rating. This helps at idle but does not help much when running the amp with full signal.

So NEGATIVE or REVERSE grid current is something you really need to watch in power tubes.

It can be a problem in small signal tubes as well, particularly high mu triodes which led to the RDH "Rule of Thumb" that for high mu triodes (like 12AX7, 6SL7 etc.) that Rg1 should be no more than 3 times the anode load resistor for cathode bias and no more than twice the anode load resistor for fixed bias.

Those familiar with 12AX7 circuits used in guitar amps will note that Rg1 is often ten times the anode load resistor value. This is because one of the defining characteristics of a 12AX7 which actually makes it ideal in guitar amps is unusually low NEGATIVE grid current.

The above also explains why grid leak bias does not work with older gassy tubes. The negative grid current from the gas ionization opposes the positive grid current needed to establish the grid leak bias.

There is one more type of grid current, GRID RECTIFICATION Current. When the grid is taken positive with respect to the cathode the grid to cathode circuit starts to look like a forward biased diode. Current into the grid increases with more positive voltage and usually this has the effect of clamping the positive going signal at the grid. The current also charges up any interstage coupling (DC Blocking) capacitor and this is the root cause of blocking distortion.

Cheers,
Ian

[tubeswell]

But isn't the current that's at the top of the bias resistor coming from the ground that is connected to the plate of the diode?

The electrons 'boil off' from the super heated cathode into the electron cloud space charge that's above the surface of the super-heated cathode, and these 'free electrons' are then whisked away by the positive voltage at the plate - creating tube current. We say that 'conventional current' flows from positive to negative - so it goes in the opposite direction to electron flow. Nevertheless, when the electrons are 'stolen' from the space charge by the positively charged plate, that leaves the cathode at a +ve voltage with respect to the ground return at the other end of Rk.

I pretty much got all the other functions of Rg, I just wasn't getting why it didn't cause a voltage drop like Rk would being that they're both connected to ground.

The grid doesn't experience the space charge in the same way that the cathode does, because its the cathode thats being super-heated, not the grid.

[pdf64]

Ian, that's really helpful, thanks.
Pete

[martin manning]

Cheers, Ian!

[EtherealWidow]

Are you saying current doesn't flow up from ground, through Rk, through the tube and the rest of the circuit eventually getting back to the rectifier cathode?

[tubeswell]

Are you saying current doesn't flow up from ground, through Rk, through the tube and the rest of the circuit eventually getting back to the rectifier cathode?

Electrons 'flow' that way. Conventional current 'flows' in the opposite direction.

But the main reason that effectively zero current flows through Rg compared to Rk, is that the catalyst for electron emission from the cathode is heat. (Ok so resistance would account for a certain degree of difference - in as far as when you increase resistance you decrease current, all other things being equal, which in this case they aren't quite, because of the heat being applied to the cathode, and not to the grid)

[EtherealWidow]

Alright. I guess now I just have to wrap my head around why the cathode would cause more of a voltage drop than the grid. I get that the cathode is heated and electrons are spewing off of it, which isn't the case with the grid. But tome it's all the same because that electron current is flowing all around the circuit across all resistances regardless of temperature. I don't understand anything anymore.

[Structo]

You know, I can understand current as in the flow of electrons moving one way, and then the "holes" that cause the electrons to be shifting to be moving in the opposite way.

That's it. "Hole current" is what the conventional current is. It's also the most difficult concept to grasp in "Introduction to Semiconductors" class.

Yeah, I remember that from my semiconductor theory class.

Holes move one way and electrons the other.

I believe it refers to valence electrons.

[Tillydog]

Alright. I guess now I just have to wrap my head around why the cathode would cause more of a voltage drop than the grid ... I don't understand anything anymore.

OK, a quiz for you....

I get that the cathode is heated and electrons are spewing off of it...

Every electron that spews off the cathode needs to be replaced.

Q1: Where are the electrons coming from to replace those that spew off the cathode?

..., which isn't the case with the grid.

Q2: How many electrons need to fight their way through the grid leak resistor to get to the grid, then?

You know that electrons flowing is an electrical current, yes?
You know that the voltage drop across a resistor is current multiplied by resistance, right?

Q3: Why is there a voltage drop across the cathode resistor, but not the grid leak resistor?






vvvvvvvvvvvvvvvvv

A1: They're fighting their way through the cathode resistor.
A2: None
A3: Because there is a flow of electrons (i.e a current) through Rk, but not through Rg

[EtherealWidow]

Haha. Nice pop quiz. But see, each tube is a current source. The current is going through the entire circuit and dropping voltages across all resistances regardless of where it originates from. If a resistor is in the path of the circuit the electrons have to fight their way through it regardless of where that current is coming from.

[vibratoking]

Your statement indicates that you are not putting together the difference between grid current and plate to cathode current. The grid is like the handle on a water valve. It contols how much current flows between the plate and cathode. Make the grid voltage sufficiently negative with respect to the cathode voltage and almost no current flows between plate and cathode. Make it close to the cathode voltage and more current flows between plate and cathode. The grid to cathode voltage is what controls the flow of current between plate and cathode, but the grid current is zero for a first order understanding. You should recognize this first. Then you can begin to understand the subleties of grid current that have been discussed in this thread.

Edit: corrected per Martin's post below. Thanks Martin.

[martin manning]

:^)