I've seen quite a number of instrument tube amp designs that have a high mu gain stage driving a fixed bias pp stage using large grid leak resistors. Often times these resistors far exceed the max resistance indicated in the power tube data sheets.
Take for instance the Ampeg B15N. Using 6l6GC power tubes at fixed bias, the grid leak resistors should be no more than 50k, but the amps use 270k.
What are the implications of amplifiers using such high grid leak resistances? Could designs such as this be improved upon by inserting a cathode follower pair between the inverter and the power tubes and reducing the grid leak resistors? Were the power stages designed with the expectation of bias drift?
Take for instance the Ampeg B15N. Using 6l6GC power tubes at fixed bias, the grid leak resistors should be no more than 50k, but the amps use 270k.
What are the implications of amplifiers using such high grid leak resistances? Could designs such as this be improved upon by inserting a cathode follower pair between the inverter and the power tubes and reducing the grid leak resistors? Were the power stages designed with the expectation of bias drift?
I've seen lots of Dyna Stereo 70s (which have 6CA7/EL34 output tubes and 270k Ohm grid resistors), but never had a problem with this.
Tubes in these amps can last a very long time. I have seen a few leaky coupling capacitors in these that caused some bias drift until they were replaced.
Most Guitar Amps abuse the max. Rg1 (grid leak) value. They do that to keep the loading on the phase splitter to a reasonable value so as to not lose too much gain.
The datasheet max Rg1 value applies to a tube running at the maximum dissipation that is listed on the datasheet.
If you bias the output tubes for a more typical 70% of maximum dissipation (at idle) then you can safely use 2 to 2.5 times the max Rg1 value.
Dissipation in the output tubes with signal will be greater than that 70% and it will increase as you overdrive more and more, but will still be sufficiently less that 100% of max. to be reasonable safe with Rg1 at the X2 Rg1 max datasheet value.
For guidance see this datasheet:
http://www.triodeel.com/kt88p1.gif
You willl note that as well as the different max Rg1 values for fixed and cathode bias there are also different values for when Anode plus Screeen dissipation is above or below 35 Watts. (Max Anode + Screen Dissipation is 42 + 8 = 50 Watts, so 35/50 = 70%).
Cheers,
Ian
The datasheet max Rg1 value applies to a tube running at the maximum dissipation that is listed on the datasheet.
If you bias the output tubes for a more typical 70% of maximum dissipation (at idle) then you can safely use 2 to 2.5 times the max Rg1 value.
Dissipation in the output tubes with signal will be greater than that 70% and it will increase as you overdrive more and more, but will still be sufficiently less that 100% of max. to be reasonable safe with Rg1 at the X2 Rg1 max datasheet value.
For guidance see this datasheet:
http://www.triodeel.com/kt88p1.gif
You willl note that as well as the different max Rg1 values for fixed and cathode bias there are also different values for when Anode plus Screeen dissipation is above or below 35 Watts. (Max Anode + Screen Dissipation is 42 + 8 = 50 Watts, so 35/50 = 70%).
Cheers,
Ian
This looks like a good thread to ask.
Type 47 power pentode allows max 500k grid leak resistor when auto-biased while fixed-bias allows only awfully low 50k. Design constraints forces me using mixed bias: about 4.6v of auto-bias and a 9V grid battery for additional fixed bias, ending with a total of Vgk -13.6V. Now.. the driver will have Zo of approx 47k which means ideally the grid leak resistor should be 10x that to avoid loading down the driver. Can i use 470k in my case? If not, what would happen if i do use it especially in the long run?
Type 47 power pentode allows max 500k grid leak resistor when auto-biased while fixed-bias allows only awfully low 50k. Design constraints forces me using mixed bias: about 4.6v of auto-bias and a 9V grid battery for additional fixed bias, ending with a total of Vgk -13.6V. Now.. the driver will have Zo of approx 47k which means ideally the grid leak resistor should be 10x that to avoid loading down the driver. Can i use 470k in my case? If not, what would happen if i do use it especially in the long run?
The higher max grid leak resistance for auto-bias is because the local negative feedback reduces the possibility of the plate current running away. With mixed bias you are only getting a proportion of that feedback, so I would estimate a 'safe' value (in proportion) as:
50k + (500k-50k)x4.6/(9+4.6) = 200k approx.
You could try 470k - if your tubes last long enough, it must be OK.
50k + (500k-50k)x4.6/(9+4.6) = 200k approx.
You could try 470k - if your tubes last long enough, it must be OK.
Hi Guys
It is truly ironic that Rg is allowed to be higher in cathode bias than with fixed biasing!
Self-biased amps typically idle the tube at full dissipation, so one would imagine that this is where the greatest protection for the tube would be desired - and one would be correct. The screen grid is easily over-heated in self-bias and this will reduce the internal impedance of the tube causing runaway. The fix is simply to use a screen-stop with a minimum value of 1k; for EL-84 2k is even better.
You have to remember that the data sheets for tubes originated in a time when the only audio use was for hifi and that a certain amount of engineering "idealism" is assumed, including perfect layouts, perfect environments and no ageing issues. A guitar amp is none of these, rather it is a harsh environment and "abusive" operation is the norm. In MI grid-stops and screen-stops are mandatory if reliability is expected.
For example, the Ampeg SVT uses 22R screen-stops on the output tubes. These are bypassed by a diode to protect the PCB if the tube shorts and would otherwise cause the 22R to burst into flames. By using poorly considered screen-stop values, then adding a bandaid to protect the wrong component, Ampeg allowed the SVT to earn a reputation for eating tubes. Replacing the 22R+diode with a 1k-5W cures the issue and the amp never eats tubes. In this circuit, the output tubes are driven by a cathode-follower so their grids are protected quite well.
Hybrid bias does allow an in-between Rg value, but it is always safest to use the lowest value possible.
Have fun
It is truly ironic that Rg is allowed to be higher in cathode bias than with fixed biasing!
Self-biased amps typically idle the tube at full dissipation, so one would imagine that this is where the greatest protection for the tube would be desired - and one would be correct. The screen grid is easily over-heated in self-bias and this will reduce the internal impedance of the tube causing runaway. The fix is simply to use a screen-stop with a minimum value of 1k; for EL-84 2k is even better.
You have to remember that the data sheets for tubes originated in a time when the only audio use was for hifi and that a certain amount of engineering "idealism" is assumed, including perfect layouts, perfect environments and no ageing issues. A guitar amp is none of these, rather it is a harsh environment and "abusive" operation is the norm. In MI grid-stops and screen-stops are mandatory if reliability is expected.
For example, the Ampeg SVT uses 22R screen-stops on the output tubes. These are bypassed by a diode to protect the PCB if the tube shorts and would otherwise cause the 22R to burst into flames. By using poorly considered screen-stop values, then adding a bandaid to protect the wrong component, Ampeg allowed the SVT to earn a reputation for eating tubes. Replacing the 22R+diode with a 1k-5W cures the issue and the amp never eats tubes. In this circuit, the output tubes are driven by a cathode-follower so their grids are protected quite well.
Hybrid bias does allow an in-between Rg value, but it is always safest to use the lowest value possible.
Have fun
Copied a post I did to another tube amp forum for your possible interset:
The Max Rg1 specs are as a result of NEGATIVE or REVERSE Grid Current.
Copied Material:
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.
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,
Ian
The Max Rg1 specs are as a result of NEGATIVE or REVERSE Grid Current.
Copied Material:
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.
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,
Ian
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Not surprising: cathode biased has self regulation or (DC) NFB built in; higher currents, even if stray or loss or instability caused, will increase negative bias and tend to self limit.
While fixed bias is that, fixed, not varying at all with varying tube current, so no self correction, so you must be more careful/conservative with design, in this case smaller grid reference resistor values, called by some "grid leak".
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