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Old 17th November 2010, 11:29 AM   #11
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Updated schematic:

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Old 17th November 2010, 08:50 PM   #12
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OK Artosalo...I think I have enough know-how to build that amp...except for a couple schematic symbols I don't quite understand. What is the line (and perpendicular line) through the 2nd 33K resistor coming off the plate of the 6N7S? Similarly, what are the symbols in the bias supply?

Thanks!
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Old 17th November 2010, 09:53 PM   #13
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Quote:
Originally Posted by redplater View Post
OK Artosalo...I think I have enough know-how to build that amp...except for a couple schematic symbols I don't quite understand. What is the line (and perpendicular line) through the 2nd 33K resistor coming off the plate of the 6N7S? Similarly, what are the symbols in the bias supply?
Thanks!
Those are the symbols for preset potentiometers (pots). The line with the perpendicular line at the end of it represents the slider. In the case of the 33k pot in the amp, this is not a very good idea, IMHO, because of the high voltage involved. Basically, it enables the plate load of the right-hand triode to be adjusted to overcome the imbalance of the LTP splitter, caused by using a finite resistor as the tail resistor instead of a constant current sink. Normally, this requires the plate load resistor of the right-hand triode to be about 10% higher, so instead of using a 33k pot here, you could probably get an acceptable result by using a 3.3k resistor in series with the existing 33k resistor.
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Old 18th November 2010, 05:30 AM   #14
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Quote:
Basically, it enables the plate load of the right-hand triode to be adjusted to overcome the imbalance of the LTP splitter, caused by using a finite resistor as the tail resistor instead of a constant current sink.
This is right, but only partly. I have had also other things in my mind when designing this way.
As can be seen from the amplifier schematics, both output tubes have own bias adjustment. By these trimmer pots the quiescent current of both output tubes can be adjusted to precisely same level.

If the output tubes are not well matched - which is not even necessary - the gain of the output tubes are obviously slightly unequal too. This problem can be solved by adjusting the output voltage of the other side of the phase splitter.

In summary, the separate bias and driving voltage adjustments makes possible to build a good performing and -sounding HiFi-amplifier from not-so-good matched output tubes.
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