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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. I posted this to another forum in reply to someomes query but thought I'd copy it here for you possible interest.
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 increased 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, then boom 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.
Grid current is statistical in nature, that is to say that as well as developing a DC voltage across Rg1 it also develops a noise (hiss) voltage across Rg1 which is then amplified by the tube. Low Rg1 values not only give you a more stable bias point but also lower noise. You can think of this as the lower Rg1 shunting the grid noise to ground.
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,
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 increased 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, then boom 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.
Grid current is statistical in nature, that is to say that as well as developing a DC voltage across Rg1 it also develops a noise (hiss) voltage across Rg1 which is then amplified by the tube. Low Rg1 values not only give you a more stable bias point but also lower noise. You can think of this as the lower Rg1 shunting the grid noise to ground.
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,
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