Wow, great detail! Thanks You. I just got lost on the voltage drop used to calculate the current. But to dumb it down for me, say I have a single power tube fixed bios amp... If I were to measure the DC resistance from B+ to the Plate (anode) pin (say pin 3 on an EL34) with the amp off, then turn the amp on (as explained in the video) and measure the DC voltage across those same points (B+ and Plate pin) and plugged those numbers into the I=v/r, wouldn't that give me the current?
There is two camps of biasing push pull Class AB output tubes is either High Plate Voltage and low cathode Current = High Crossover distortion, compared VS to Lower plate voltage & High cathode Current = Low Crossover distortion. I'm not sure why higher plate voltage & low cathode current will cause HIGHER Crossover distortion? compared to biasing the amplifier using Lower plate voltage & high cathode current will cause LOWER crossover distortion, any reasons why?
Thanks for the informative video. What does it usually mean if the bias is drifting a lot? The bias on my two rock studio pro 35 amp test point varied from 0.054v to 0.074v during a gig after I set it at the recommended 0.064 initially! Also the test point feels soft with the multimeter probe almost like I’m stabbing it into play dough!! I’m so confused!! The tubes are new and quality matched tested ones so unlikely to be the cause.
So without know much about that amp, it looks like it’s a single test point shared across both tubes? So, when that’s the case, the accuracy isn’t going to be as good, because you have divide by 2 to get the average bias value for each tube. So, for example, if you’re see 0.074V on that single test point, it means that, on average, the cathode current is about 0.037V (or 0.037A) per tube. Bias drifting up or down before, during and after use isn’t uncommon by a small margin. But because it’s a single test point shared across the tubes, that could explain why it seems greater, because it’s the sum of a possible drift across both tubes. All that said, a couple additional things that could present higher-than-normal bias drift: 1. The tubes are new - which you said they are. Depending on the brand, they may need to be “broken in” for a few hours before they “settle down” into a more stable bias setting that requires less adjustment. 2. The amount of AC voltage at the wall can significantly change the amount of B+ DC voltage available to the amp. When that happens, the bias can drift as well and may any require an adjustment.
@@sonictoneamps thanks for taking the time to reply. It is a single test point for 2 tubes yes. On reflection I was thinking more along the lines of the bias pot / bias circuitry having a problem as I’ve never known the bias to drift anywhere near as severely as this in all the years I’ve owned and gigged the amp. Just worried I’ll burn out the tubes or cause a serious problem soon with it going above 0.070 which is the absolute maximum recommended current in the amp manual.
So if the tubes behave normally in other amps, then as you suggested- the bias circuit in the Two Rock amp may have some components that are failing and preventing a consistent negative DC bias voltage from getting to the grids. Probably best to have that checked out.
SONICTONE< you didn't show how you measured or calculated the plates voltage drop? once your got the plates voltage drop * bias current = tubes power dissipation, how do you compute to get the tubes power dissipation?
Apologies - in the interest of time, calculating the voltage drop was glossed over when going over the XLS file and calculator. Around 13:40 I talk about how I probed the amp and got the B+ voltage of 431.2 and the plate voltage of 428.8. From there a simple subtraction formula is plugged into the XLS file to subtract the plate voltage from the B+ voltage to get the voltage drop (431.2V - 428.8V = 2.40V) After that, Ohm's law is also plugged into the XLS file (as well the example done in the SonicTone calculator) to get the current by dividing the voltage drop by the resistance of the primary winding: V = IR, flipped to V / R = I, so 2.40V / 46R = 52.17 mA [or 0.05217A] From there, those values are applied to the power formula (P = IV), so 0.05217A * 428.8V = 22.37W (which is about 74.57% plate dissipation of a 30W 6L6GC tube.) Let me know if that helps clarify things. Thanks.
@@sonictoneamps yes thanks. I think VHT Fryette Amplifications will measure the plates voltage drop using a DVM volt meter while the amplifiers is LIVE and also measuring the phase inverters balance using the oscilloscope. Its a very different way of biasing amplifiers which I'm not sure what this biasing technique is called because its mostly based on the plates voltage drop to balance them both out with the phase inverter. The 50s and early 60s used germanium transistors which have a lot of leakage current. The old biasing technique were called "leakage biasing" which you have to subtract and apply the leakage current into the biasing formula. Maybe you can make lessons about these other biasing techniques.
There is no mystery about why is the cathode current higher than Plate current. As soo nas you have beaming quattrode or pentode you ill have this "phenomenon". In triodes they will have the same value. You got it already? Yes it is the screen current. Screen takes its own few mili Amperes current. So plus come from the plate from the output transformer and screen gets it's plus voltage from screen resistor or in the case of ultra linear tube amplifiers from output transformers screen tap. In both cases screens have usually smaller voltage than plate. On the tube you have only one connection to negative side and that is the cathode so that is the only place to go for both currents. So Cathode current is Plate current plus screen current. Usually screen currents are a few mili Amperes (3-5 for common output tubes). Also as soon as you do change bias current to new value you need to measure plate voltage that will correspond with new situation. You can't just measure palte voltage and think it is going to stay the same no matter what plate current we draw. It might be sort of logical to think like that because plate currents are mili ampers and that should not make much difference right? Wrong. 5 mA more will drop plate voltage considerably. It will lover it down. When you are lowering the plate current your palte voltage will go up. So you always have to measure both values. To be honest 60 percent is all you need nowadays with the fixed bias amplifiers. Your working currents will reach the maximum value with maximum signal even with 60 percent value. 60 percent is not sounding worse either. Especially not in cleaner amplifiers. Fixed bias amplifiers are more efficient and therefore it is easier and more clever to make a big amplifier with fixed bias power stage. Plus with NFB those big amplifiers will stay usable and not turn into the unplayable monsters. One thing is true we are talking about percentage of the dissipation power and not about the mili Amperes. Though we have to obey the tube datasheet and not overstep the maximum plate current. For example in some nice amplifiers designers use lover voltage overall. They do not care about how to use full power of the tube they are after the sound it self. So for example they do not go with 450V for 6L6GC but they opt to go with 320V. If that would be a cathode bias amplifier to reach 90 percent of maximum plate dissipation you might have to go more than maximum plate current. Then they do leave it at lower biassing current and do not achieve 90 percent. But if that sounds good who cares?? Who cares is it 20 or 30 or 40 watt amplifier? They are after the tone and not the power in such amplifiers. Plus if that turns out to be no NFB cathode bias amplifier with big output tubes and not that much power still it is going to sound bigger and louder than actual Watt numbers would make you think. Think VoxAC30. Not many people were complaining it is not loud enough for what it is. There is no shame in bringing your amplifier for tube change and inspection and biasing to proper tube amp tech. Amplifiers contain lethal dose of DC Voltage. Voltage that stays present in the filter caps even when the amplifier is off. Most of the amplifiers have discharging resistors but those take time to drop the voltage to safe levels. Plus there is a benefit to it. Proper tech will check everything and find if something is lose or broken and your amplifier will be back as new again. Ask me how I know? Well people brought me amplifiers with one issue and most of the times there are more issues. Like one amplifier came to me for tube change and biassing and checkup and I found out that presence cap have had broken hookup wire. Presence can't work without that cap. And that was in really good built boutique amplifier. Once owner got it back he called me after his first gig and asked WTH have I doen to it and how come it sounds so much better now :) Well new tubes, fixed presence and overall cleaning and check up and reflow of few spots does that. Just make sure that if you own really valuable amplifier you do not go to random guy. Often you see bad parts and parts that lower the value of the amplifier. Also if he will not call you after the initial checkup and before he fixes what he thinks needs to be done without consulting you about costs and what needs to be done I would avoid those people. Some people just assume you got money and you want everything done so they just go and do their stuff and charge you after it. You need good communication and pictures and even videos.
Don't forget about the infamous Fender hybrid fixed/cathode bias circuits, introduced around 1968 and quickly abandoned. I happen to own 2 of those beauties, a 1968 Bassman (AC568) and a 1969 Bandmaster Reverb (AA768). Both had the bias balance (output tube matching) pots, so I added a bias trim pot and use the output transformer resistance method to bias them.
I’m still learning so I have a question, isn’t the B+ voltage inclusive of the -voltage on the bias? As an example, the figures of the voltage be B+ and - bias voltage =512 volts in your example. I’m not saying I’m correct, genuinely interested to know. Thanks for the video and am learning a lot from them.
Thanks - appreciate the question. No, the bias voltage is its own separate, independent voltage supply. Typically, it's half-wave rectified in the negative direction to give you the negative voltage. Also, the value of the bias supply voltage is much lower, typically between -50 and -60 volts (it's not a high voltage like the B+ at 400 or 450 volts.) Around the 12:28 I probe the separate bias supply and show that the available bias voltage is about -62.8 VDC. The actually bias voltage at the grid is -41.6 VDC, which is what's needed to put the current flow through the tubes where we want it. Hope that helps.
Ahh ok that makes sense now. Appreciate the time you spent replying. I have a Mesa that needs work on it (all the time), local techs won’t work on the brand…. So I’m learning myself, have worked through two JTM style kit builds to get some experience…. And read several of Gerald Weber’s books, More for fun really. I find your type of content very helpful!
@@braddenyar3641Im actually taking his tube amp course right now (Gerald Weber). Really great stuff with a ton of info plus you can ask him questions as you go through it. Highly recommend
The power supply and rectifier portion of the schematic weren't included for space reasons. It was simplified to the basics for the purposes of this video. The rectification would be "off-screen" sending the B+ DC voltage into the primary center tap of the output transformer (on the left side of the transformer diagram.) The output transformer secondary is on the right side of the transformer diagram, sending the speaker load to the speaker jacks (with an impedance selector switch.)
I don’t think your calculations for cathode bias are correct. Plate voltage is with respect to cathode, not to ground, so plate dissipation Pd = (Va-Vk) * Ia.
Only an armature would trust an amps factory bias test points. Real pro's use a bias meter taking the results directly from the tube. I have proven time and time again the bias testing circuits build in amps are GROSSLY incorrect.
Excellent. I'm learning and this has been very helpful.
Excellent and very concise video. Thank you
Wow, great detail! Thanks You. I just got lost on the voltage drop used to calculate the current. But to dumb it down for me, say I have a single power tube fixed bios amp... If I were to measure the DC resistance from B+ to the Plate (anode) pin (say pin 3 on an EL34) with the amp off, then turn the amp on (as explained in the video) and measure the DC voltage across those same points (B+ and Plate pin) and plugged those numbers into the I=v/r, wouldn't that give me the current?
There is two camps of biasing push pull Class AB output tubes is either High Plate Voltage and low cathode Current = High Crossover distortion, compared VS to Lower plate voltage & High cathode Current = Low Crossover distortion. I'm not sure why higher plate voltage & low cathode current will cause HIGHER Crossover distortion? compared to biasing the amplifier using Lower plate voltage & high cathode current will cause LOWER crossover distortion, any reasons why?
Thanks for the informative video.
What does it usually mean if the bias is drifting a lot? The bias on my two rock studio pro 35 amp test point varied from 0.054v to 0.074v during a gig after I set it at the recommended 0.064 initially! Also the test point feels soft with the multimeter probe almost like I’m stabbing it into play dough!! I’m so confused!! The tubes are new and quality matched tested ones so unlikely to be the cause.
So without know much about that amp, it looks like it’s a single test point shared across both tubes? So, when that’s the case, the accuracy isn’t going to be as good, because you have divide by 2 to get the average bias value for each tube. So, for example, if you’re see 0.074V on that single test point, it means that, on average, the cathode current is about 0.037V (or 0.037A) per tube. Bias drifting up or down before, during and after use isn’t uncommon by a small margin. But because it’s a single test point shared across the tubes, that could explain why it seems greater, because it’s the sum of a possible drift across both tubes.
All that said, a couple additional things that could present higher-than-normal bias drift:
1. The tubes are new - which you said they are. Depending on the brand, they may need to be “broken in” for a few hours before they “settle down” into a more stable bias setting that requires less adjustment.
2. The amount of AC voltage at the wall can significantly change the amount of B+ DC voltage available to the amp. When that happens, the bias can drift as well and may any require an adjustment.
@@sonictoneamps thanks for taking the time to reply. It is a single test point for 2 tubes yes. On reflection I was thinking more along the lines of the bias pot / bias circuitry having a problem as I’ve never known the bias to drift anywhere near as severely as this in all the years I’ve owned and gigged the amp. Just worried I’ll burn out the tubes or cause a serious problem soon with it going above 0.070 which is the absolute maximum recommended current in the amp manual.
As a comparison, I’ve also replaced the output tubes an old fender bassman head with the same ones and the set bias on that amp is rock solid.
So if the tubes behave normally in other amps, then as you suggested- the bias circuit in the Two Rock amp may have some components that are failing and preventing a consistent negative DC bias voltage from getting to the grids. Probably best to have that checked out.
Great vid thanks.
SONICTONE< you didn't show how you measured or calculated the plates voltage drop? once your got the plates voltage drop * bias current = tubes power dissipation, how do you compute to get the tubes power dissipation?
Apologies - in the interest of time, calculating the voltage drop was glossed over when going over the XLS file and calculator. Around 13:40 I talk about how I probed the amp and got the B+ voltage of 431.2 and the plate voltage of 428.8. From there a simple subtraction formula is plugged into the XLS file to subtract the plate voltage from the B+ voltage to get the voltage drop (431.2V - 428.8V = 2.40V) After that, Ohm's law is also plugged into the XLS file (as well the example done in the SonicTone calculator) to get the current by dividing the voltage drop by the resistance of the primary winding: V = IR, flipped to V / R = I, so 2.40V / 46R = 52.17 mA [or 0.05217A] From there, those values are applied to the power formula (P = IV), so 0.05217A * 428.8V = 22.37W (which is about 74.57% plate dissipation of a 30W 6L6GC tube.)
Let me know if that helps clarify things. Thanks.
@@sonictoneamps yes thanks. I think VHT Fryette Amplifications will measure the plates voltage drop using a DVM volt meter while the amplifiers is LIVE and also measuring the phase inverters balance using the oscilloscope. Its a very different way of biasing amplifiers which I'm not sure what this biasing technique is called because its mostly based on the plates voltage drop to balance them both out with the phase inverter. The 50s and early 60s used germanium transistors which have a lot of leakage current. The old biasing technique were called "leakage biasing" which you have to subtract and apply the leakage current into the biasing formula. Maybe you can make lessons about these other biasing techniques.
There is no mystery about why is the cathode current higher than Plate current. As soo nas you have beaming quattrode or pentode you ill have this "phenomenon". In triodes they will have the same value. You got it already? Yes it is the screen current. Screen takes its own few mili Amperes current. So plus come from the plate from the output transformer and screen gets it's plus voltage from screen resistor or in the case of ultra linear tube amplifiers from output transformers screen tap. In both cases screens have usually smaller voltage than plate. On the tube you have only one connection to negative side and that is the cathode so that is the only place to go for both currents. So Cathode current is Plate current plus screen current. Usually screen currents are a few mili Amperes (3-5 for common output tubes).
Also as soon as you do change bias current to new value you need to measure plate voltage that will correspond with new situation.
You can't just measure palte voltage and think it is going to stay the same no matter what plate current we draw. It might be sort of logical to think like that because plate currents are mili ampers and that should not make much difference right? Wrong. 5 mA more will drop plate voltage considerably. It will lover it down. When you are lowering the plate current your palte voltage will go up. So you always have to measure both values.
To be honest 60 percent is all you need nowadays with the fixed bias amplifiers. Your working currents will reach the maximum value with maximum signal even with 60 percent value.
60 percent is not sounding worse either. Especially not in cleaner amplifiers.
Fixed bias amplifiers are more efficient and therefore it is easier and more clever to make a big amplifier with fixed bias power stage. Plus with NFB those big amplifiers will stay usable and not turn into the unplayable monsters.
One thing is true we are talking about percentage of the dissipation power and not about the mili Amperes. Though we have to obey the tube datasheet and not overstep the maximum plate current.
For example in some nice amplifiers designers use lover voltage overall. They do not care about how to use full power of the tube they are after the sound it self. So for example they do not go with 450V for 6L6GC but they opt to go with 320V. If that would be a cathode bias amplifier to reach 90 percent of maximum plate dissipation you might have to go more than maximum plate current. Then they do leave it at lower biassing current and do not achieve 90 percent. But if that sounds good who cares?? Who cares is it 20 or 30 or 40 watt amplifier? They are after the tone and not the power in such amplifiers. Plus if that turns out to be no NFB cathode bias amplifier with big output tubes and not that much power still it is going to sound bigger and louder than actual Watt numbers would make you think. Think VoxAC30. Not many people were complaining it is not loud enough for what it is.
There is no shame in bringing your amplifier for tube change and inspection and biasing to proper tube amp tech. Amplifiers contain lethal dose of DC Voltage. Voltage that stays present in the filter caps even when the amplifier is off. Most of the amplifiers have discharging resistors but those take time to drop the voltage to safe levels.
Plus there is a benefit to it. Proper tech will check everything and find if something is lose or broken and your amplifier will be back as new again.
Ask me how I know? Well people brought me amplifiers with one issue and most of the times there are more issues. Like one amplifier came to me for tube change and biassing and checkup and I found out that presence cap have had broken hookup wire. Presence can't work without that cap.
And that was in really good built boutique amplifier.
Once owner got it back he called me after his first gig and asked WTH have I doen to it and how come it sounds so much better now :)
Well new tubes, fixed presence and overall cleaning and check up and reflow of few spots does that.
Just make sure that if you own really valuable amplifier you do not go to random guy. Often you see bad parts and parts that lower the value of the amplifier.
Also if he will not call you after the initial checkup and before he fixes what he thinks needs to be done without consulting you about costs and what needs to be done I would avoid those people. Some people just assume you got money and you want everything done so they just go and do their stuff and charge you after it. You need good communication and pictures and even videos.
Don't forget about the infamous Fender hybrid fixed/cathode bias circuits, introduced around 1968 and quickly abandoned. I happen to own 2 of those beauties, a 1968 Bassman (AC568) and a 1969 Bandmaster Reverb (AA768). Both had the bias balance (output tube matching) pots, so I added a bias trim pot and use the output transformer resistance method to bias them.
I’m still learning so I have a question, isn’t the B+ voltage inclusive of the -voltage on the bias? As an example, the figures of the voltage be B+ and - bias voltage =512 volts in your example. I’m not saying I’m correct, genuinely interested to know. Thanks for the video and am learning a lot from them.
Thanks - appreciate the question. No, the bias voltage is its own separate, independent voltage supply. Typically, it's half-wave rectified in the negative direction to give you the negative voltage. Also, the value of the bias supply voltage is much lower, typically between -50 and -60 volts (it's not a high voltage like the B+ at 400 or 450 volts.) Around the 12:28 I probe the separate bias supply and show that the available bias voltage is about -62.8 VDC. The actually bias voltage at the grid is -41.6 VDC, which is what's needed to put the current flow through the tubes where we want it. Hope that helps.
Ahh ok that makes sense now. Appreciate the time you spent replying. I have a Mesa that needs work on it (all the time), local techs won’t work on the brand…. So I’m learning myself, have worked through two JTM style kit builds to get some experience…. And read several of Gerald Weber’s books, More for fun really. I find your type of content very helpful!
Thanks for the feedback. Glad you find it helpful. Good luck with it!
@@braddenyar3641Im actually taking his tube amp course right now (Gerald Weber). Really great stuff with a ton of info plus you can ask him questions as you go through it. Highly recommend
Nice, is it online or in person? I don’t live in the US.
How about this -20 volt....and I need -13 volts...show me the circuits to get negative voltage.
At 10:11 your schematic shows trans secondary output as DC voltage? No rectifier?? Educate me please :)
The power supply and rectifier portion of the schematic weren't included for space reasons. It was simplified to the basics for the purposes of this video. The rectification would be "off-screen" sending the B+ DC voltage into the primary center tap of the output transformer (on the left side of the transformer diagram.) The output transformer secondary is on the right side of the transformer diagram, sending the speaker load to the speaker jacks (with an impedance selector switch.)
I don’t think your calculations for cathode bias are correct. Plate voltage is with respect to cathode, not to ground, so plate dissipation Pd = (Va-Vk) * Ia.
Only an armature would trust an amps factory bias test points. Real pro's use a bias meter taking the results directly from the tube. I have proven time and time again the bias testing circuits build in amps are GROSSLY incorrect.