To me, bias balance only applies to Quiescent current.
And, your push pull output transformers will 'love you' for having balanced push and pull quiescent current.
For an example of un-balanced current, see the distortion graphs of the Heathkit W5M (25 Watt?) amplifier at the bottom page of the instruction manual.
Even with global negative feedback, and at only 1 Watt, you can see the additional harmonic distortion when the currents are slightly un-balanced, and it shows the results for 100Hz, and 20Hz too.
AC (signal balance), is totally different than bias balance.
A constant current sink for the parallel push and pull cathodes, will ensure that the Total current of the tube that is increasing plus the current of the tube that is decreasing is a constant (and always equal to the quiescent current, all the way to the point where one tube is completely cut off). But that kind of amplifier operates in class A, not in Class AB.
AC balance needs to take account of not only the output tubes, it needs to take account of the driver tubes AC balance too.
Or, if you want, you can let the AC balance sum of the driver and output stages be less than perfect, and cause 2nd Harmonic distortion.
If you do not use negative feedback (such as schade or global feedback), the 2nd harmonic will not be removed by the negative feedback. But Ultra Linear feedback is OK, it is local to the output tubes and output tube primary, then with un-balanced AC you will have some 2nd harmonic distortion, and it just might sound a little like a single ended amplifier that does not have negative feedback.
Did I miss anything?
Just my opinions.
And, your push pull output transformers will 'love you' for having balanced push and pull quiescent current.
For an example of un-balanced current, see the distortion graphs of the Heathkit W5M (25 Watt?) amplifier at the bottom page of the instruction manual.
Even with global negative feedback, and at only 1 Watt, you can see the additional harmonic distortion when the currents are slightly un-balanced, and it shows the results for 100Hz, and 20Hz too.
AC (signal balance), is totally different than bias balance.
A constant current sink for the parallel push and pull cathodes, will ensure that the Total current of the tube that is increasing plus the current of the tube that is decreasing is a constant (and always equal to the quiescent current, all the way to the point where one tube is completely cut off). But that kind of amplifier operates in class A, not in Class AB.
AC balance needs to take account of not only the output tubes, it needs to take account of the driver tubes AC balance too.
Or, if you want, you can let the AC balance sum of the driver and output stages be less than perfect, and cause 2nd Harmonic distortion.
If you do not use negative feedback (such as schade or global feedback), the 2nd harmonic will not be removed by the negative feedback. But Ultra Linear feedback is OK, it is local to the output tubes and output tube primary, then with un-balanced AC you will have some 2nd harmonic distortion, and it just might sound a little like a single ended amplifier that does not have negative feedback.
Did I miss anything?
Just my opinions.
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When setting bias on torrid PP outputs I've found that the best way is to play a LF tone and look at the wave form on a scope. Then adj for lowest distortion. Better results than measuring at idle.
Sounds like then,
That idle and dynamic biases are independent setpoints. I'm inclined to think that idle bias balance is mostly academic, but dynamic bias is where the distortion can be minimized....
although which point is just a snapshot of the total frequency envelope.
Jim
That idle and dynamic biases are independent setpoints. I'm inclined to think that idle bias balance is mostly academic, but dynamic bias is where the distortion can be minimized....
although which point is just a snapshot of the total frequency envelope.
Jim
It is all a tradeoff.
Setting push pull amplifiers AC balance for minimum distortion of a sine wave at a particular power out . . .
Is a combination of the driver AC balance, the output stage AC balance, output stage DC quiescent balance, and all of this is also affected by negative feedback if present.
Then, after all of that, it may change the distortion mix (2nd and 3rd) at different power levels, low, medium, high.
I re-edited my Post # 22.
Go back and look at that Heathkit W5M instruction manual bottom page.
You may be surprised.
Distortion caused by saturation not only affects low frequencies, it affects LF to HF Intermodulation distortion, even at 1 Watt levels.
Ideally, for a push pull amplifier, there should be a separate quiescent current balance, and AC balance.
My push pull amplifiers use very well matched output tubes both for DC and AC; and I use matched Self Bias RC networks, and can easily measure the bias voltage (and therefor calculate the tube current).
For the phase splitter/driver, I use a constant current sink for the parallel cathodes, and matched plate load resistors, that gives intrinsic AC balance to that phase splitter/driver stage.
Setting push pull amplifiers AC balance for minimum distortion of a sine wave at a particular power out . . .
Is a combination of the driver AC balance, the output stage AC balance, output stage DC quiescent balance, and all of this is also affected by negative feedback if present.
Then, after all of that, it may change the distortion mix (2nd and 3rd) at different power levels, low, medium, high.
I re-edited my Post # 22.
Go back and look at that Heathkit W5M instruction manual bottom page.
You may be surprised.
Distortion caused by saturation not only affects low frequencies, it affects LF to HF Intermodulation distortion, even at 1 Watt levels.
Ideally, for a push pull amplifier, there should be a separate quiescent current balance, and AC balance.
My push pull amplifiers use very well matched output tubes both for DC and AC; and I use matched Self Bias RC networks, and can easily measure the bias voltage (and therefor calculate the tube current).
For the phase splitter/driver, I use a constant current sink for the parallel cathodes, and matched plate load resistors, that gives intrinsic AC balance to that phase splitter/driver stage.
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With toroids in PP I can get better idle balance by setting using LF sine wave too though. But this is with my limited experience testing toroid outputs.
There are different types of output transformers.
Some EI have no air gap, some have very small air gap, some have moderate air gap.
I have no experience with Toroids.
But I expect that with output tubes that are well matched for both DC and AC; matched self bias RC networks; and very importantly with matched AC signals from the driver; you will get very good performance with the toroid, and very nearly the same result, as you get with your adjustment procedure.
AC balance adjustment of an amplifier that has global negative feedback will include all the stages of the amplifier, not just the output stage.
The better the balance and performance of each stage, the less correction that negative feedback needs to make.
If for any reason there is saturation distortion of the output transformer, then global negative feed back will make it worse when the low frequency signal saturates the output transformer.
Some EI have no air gap, some have very small air gap, some have moderate air gap.
I have no experience with Toroids.
But I expect that with output tubes that are well matched for both DC and AC; matched self bias RC networks; and very importantly with matched AC signals from the driver; you will get very good performance with the toroid, and very nearly the same result, as you get with your adjustment procedure.
AC balance adjustment of an amplifier that has global negative feedback will include all the stages of the amplifier, not just the output stage.
The better the balance and performance of each stage, the less correction that negative feedback needs to make.
If for any reason there is saturation distortion of the output transformer, then global negative feed back will make it worse when the low frequency signal saturates the output transformer.
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Oh, the reason you can set good DC bias with a AC tone with Toroids is because they saturate easily and so any DC will result in LF distortion. So really you’re not adjusting AC balance in this way. Just lowering saturation distortion to a minimum.
Didn't mean to hi-jack your thread, Doug-
Thanks for being cool w/ it... But I should start dedicated thread to this topic...
Jim
Thanks for being cool w/ it... But I should start dedicated thread to this topic...
Jim
Setting the distortion when negative feedback is present is a little like . . .
Focusing a manual focus lens by looking at the screen/LCD/Viewfinder, when the lens aperture is set to F16.
Just my opinion.
Focusing a manual focus lens by looking at the screen/LCD/Viewfinder, when the lens aperture is set to F16.
Just my opinion.
It’s a bit like that. Should really get the open loop distortion as low as possible before adding FB.
Here's part one of a two part video I'm making regarding this amp. Part two will have detailed measurements.
6S19P Push Pull Circuit Walk through (Part 1) - YouTube
6S19P Push Pull Circuit Walk through (Part 1) - YouTube
Here's part 2 with all the measurements. 6S19P Push Pull Circuit TESTING (Part 2) - YouTube
Here's a ling to the frequency sweeps 6S19P Push Pull Sweeps - Google Photos
Here's a ling to the frequency sweeps 6S19P Push Pull Sweeps - Google Photos
Hi Doug De Young, thinking to get tubes for your design; how happy were you with the sound of your amp. I would like to increase number of tubes to 3 on each side; would that require complete redesign or it could still be driven by 6H8C with some minor modification? Regards Tomo
I got the output transformers here. I did pot them in the cans you can see in the photos of the amp though.
https://a.aliexpress.com/_mOi3SDU
https://a.aliexpress.com/_mOi3SDU
You’d need an even number of output tubes per channel. Probably the 6H8C would drive them but at a reduced gain.
Hello Doug:
It has been a while since you put an audio video up on Youtube. I learned a lot from the ones you have up, any chance of another soon?
It has been a while since you put an audio video up on Youtube. I learned a lot from the ones you have up, any chance of another soon?
I have been a bit busy tbh. But if you want to see the most recent projects and circuits I recommend following the BUG AMPS FB page. I post all my incremental progress there. For example I’m developing a A2 EL34 SE triode cascode output amp right now.