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Old 5th April 2012, 05:27 AM   #51
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I will save the never and always argument for when it is important, like with my wife. If there is an exception to a True False question the answer is always False.

DIYAUDIO is a good place. there is a world of knowledge and experience, much to be shared and learned. I worked with a Mechanical Engineer from Transilvania Romania. Paul always had a fresh view on how to solve a problem. We argued a lot. I think that those discussions produced better solutions.

In terms of SRPP I agree that there is likely a lot of distortion cancelation. I also agree that the performance is highly dependent on load. If the connected load is not considered in the design the balance between push and pull can be upset and what is thought to be equal and opposite cancelation is not true. This is the exception that makes the answer False.

People love to argue about SRPP.

DT
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Old 5th April 2012, 10:06 AM   #52
DF96 is offline DF96  England
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Quote:
Originally Posted by Wavebourn
The main advantage of SRPP is not cancelation. It is dynamic load of bottom tube and using the top one as cathode follower as well, i.e. as the result the bottom tube sees higher load resistance, that is the source of lower distortions.
In a balanced SRPP (i.e. top and bottom identical, no cathode bypass, only lightly loaded) cancellation is a major effect. In an unbalanced SRPP (e.g. bottom cathode bypassed, or mu-follower) there is little cancellation, so the higher load resistance is the main source of lower distortion. In both cases there is further advantage from the reduced output impedance due to the modified CF behaviour of the top valve. Most people use an unbalanced SRPP, so your statement is true for the most common case.
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Old 5th April 2012, 04:47 PM   #53
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I did not mean that cancellation do not happen. I mean, 2 devices don't fit like a glove on a hand, so canceling 2'nd order error such a way you add more audible distortions. However, when swing is quite small it is nothing to add, and nothing to cancel.
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Old 5th April 2012, 06:39 PM   #54
DF96 is offline DF96  England
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Yes, odd-order won't cancel and could be worse (open loop). However, if the stage is within a feedback loop then the reduction in even-order distortion could result in a reduction of odd-order from recycled distortion.
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Old 5th April 2012, 07:04 PM   #55
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Quote:
Originally Posted by DF96 View Post
Yes, odd-order won't cancel and could be worse (open loop). However, if the stage is within a feedback loop then the reduction in even-order distortion could result in a reduction of odd-order from recycled distortion.
Sure. When linearizing by feedback all possible means of linearization of open loop amp are valid, in terms of linearity. But in no feedback amp the only way to go is to put the tube into most privileged conditions.
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Old 6th April 2012, 06:20 AM   #56
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I see both cancelation (feedback included) and parameter restriction as design tools.

Speaking of parameter restriction we can increase the load impedance to the extreme of the active load of a mu-follower. When we do this the best we can do is approach the inherent distortion limit imposed by the 3/2ís law of the Ideal triode. There is a limited selection of ideal triodes!

When the SS JFET guys get to this point they add the cancelation of a differential pair.

I scratched the above to post half a day ago. Wavebourn you posted just as I was to pull trigger. I thought the topic was beaten to near death, I guess not.

When we reduce swing to limit distortion one trade off is to reduce the Signal to Noise Raito. Or put another way, say we reduce the output from 20 volts where it popular to test and report distortion data. At 20 volts the higher harmonics are measurable above the grass of the noise floor and within the ability of the test gear to detect. At 2 volts output which is greater than a line amplifier will be used or 10 db down the noise floor is still where it was, the fundamental and harmonics are 10 db less. The higher harmonics are likely lost down in the grass where no one sees or cares.

DT
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Old 8th April 2012, 10:53 PM   #57
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Getting back to the original question, the 6CG7 or 6FQ7 is an excellent B9A replacement for the 6SN7. Apart from plate dissipation, my tests have shown it is about as identical to a 6SN7 as you can get.

Cheers

Ian
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