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Old 21st February 2012, 02:25 AM   #31
roger57 is offline roger57  Canada
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Hi,
Have a question, and hoping to get some insight and guidance!

I'm modeling a 6SN7 front end gain stage for an amplifier, and it has an LTP driver for the power tubes. I have 2 models; one uses just a 15K resistor to set the cathode bias, and the other, using the CCS shown here (or a variant of it)
However....I see no difference in the distortion, gain or power over a range of input voltages from 0.25V to 2V. So the question is simply, what does (should) the CCS do for a differential pair amplifier? By the way, I should also mention that I've set the plate resistors equal for the CCS test, something I assume I can now due when both cathodes have a CCS. Otherwise, with a resistor bias, one triode uses a 33K plate resistor and the other a 39K resistor. I'm using Norman Koren's tube model library. I have the Morgan Jones book, unfortunately I cannot see in there where the exact benefits and advantages of a CCS are detailed.
Thanks for any insight you can provide - even pointing me at a good treatise on the subject would be useful.
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Old 13th March 2013, 05:17 PM   #32
frapa02 is offline frapa02  United Kingdom
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Hi, I just noticed this thread and an old outstanding question. The CCS will simply place the LTP in balance. This avoids potentially severe 2nd harmonic distortion from the imbalance caused by tube gain variance or un-compensated anode loads. So, this results in a low distortion phase splitter and amplifier.
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Old 13th March 2013, 07:10 PM   #33
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CCS will improve the linearity and balance of LTP a lot. I see that the biggest advantage with CCS is reduced distortion.
However, my opinion is that the balance of output signals is overestimated feature.
I have noticed in practice that the best overall AC-balance is achieved when one of the anode resistors of LTP is adjustable.
Just like fixed bias voltage should be individually adjustable for output tubes.
Equal driving voltage for both output tubes is useless if the gain of the tubes are unequal.
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Old 13th March 2013, 07:18 PM   #34
roger57 is offline roger57  Canada
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Quote:
Originally Posted by frapa02 View Post
Hi, I just noticed this thread and an old outstanding question. The CCS will simply place the LTP in balance. This avoids potentially severe 2nd harmonic distortion from the imbalance caused by tube gain variance or un-compensated anode loads. So, this results in a low distortion phase splitter and amplifier.
Hi!
Thanks for taking the time to respond on this. Since I wrote this, I have done some reading and experimenting.

My original circuit was a simple LTP resistor. It's performance was good but not stellar. After trying a few flavors of CCS, I realized as good as it sounded, it didn't. It seemed to "dry out" the sound (for lack of better words), and although I could measure just a marginal difference, I was hearing something I didn't like.
I went back to playing with LT Spice. Knowing that between the LTP and push-pull Class A amplifier I was getting high(er) levels of 3rd harmonics, I set out to tackle that by some tuning. At the end, I was able to get the 3rd harmonic to around -59dB, a 3dB improvement. But there was no 2nd harmonic to be seen. Although I don't necessarily love distortion, I also appreciate the fact that a certain amount of 2nd order brings about a pleasing sound - but I wanted just enough to be higher than the 3rd. So I "de-tuned" the LTP by making small adjustments to the anode resistors to bring up the second harmonic about 1-2dB above the third. This without affecting the level of the 3rd. Measured performance with a scope's Spec-A was fairly close to simulated (although I have yet to test thoroughly with ARTA)

It now sounds exactly what I wanted. So, perhaps this is a weird approach, but just one way to get what you want. It worked!

Gary
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Old 13th March 2013, 07:20 PM   #35
roger57 is offline roger57  Canada
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Quote:
Originally Posted by artosalo View Post
CCS will improve the linearity and balance of LTP a lot. I see that the biggest advantage with CCS is reduced distortion.
However, my opinion is that the balance of output signals is overestimated feature.
I have noticed in practice that the best overall AC-balance is achieved when one of the anode resistors of LTP is adjustable.
Just like fixed bias voltage should be individually adjustable for output tubes.
Equal driving voltage for both output tubes is useless if the gain of the tubes are unequal.
Agreed. I discovered by trial and error. The CCS is great where they have a place, but it wasn't for me in my application.
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Old 13th March 2013, 07:32 PM   #36
DF96 is offline DF96  England
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If you want balanced drive to the output stage, use a CCS and equal anode resistors. Other things being equal, this will minimise even-order distortion. Provided, of course, that the CCS is a good CCS with high and linear output impedance.

If you want to avoid this, then drop the CCS or use unequal resistors.
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Old 13th March 2013, 09:12 PM   #37
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If you want to minimise total harmonic distortion, use CCS and one adjustable anode resistor at the CCS. Output tubes are seldom fully equal.
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Old 13th March 2013, 09:21 PM   #38
DF96 is offline DF96  England
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Yes, you can minimise even-order distortion when things are not equal by slightly unbalancing the LTP to compensate for output stage imbalance. This won't affect odd-order distortion, which P-P cannot reduce.
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