As Nigel said, the transistors have to be carefully matched - also at the same temperature and adjusted together at the same temperature in order to all measure precisely the same Re voltages and meet the original performance specs. However, it was probably never the case that all Re voltages were exactly the same or remained that way, since current sharing is never easy to maintain, even with precision components, in such basic control systems with large heatsink areas
This is a powerful amplifier with 4 pairs of output transistors - so it's important to take full sets of replacement power transistors from the same tube packages and in sequence, to get the closest possible matching. If you mess up, it will cost quite a lot to repair with the necessary full sets but at a pinch, carefully matched individual transistors from the same batch that you don't try to push to their spec. limits may work fine too.
This isn't typical consumer gear and for your safety and assurance, should be serviced by a pro. audio technician or someone with appropriate experience and the equipment for matching complementary power transistor sets and drivers if necessary. However, if the transistors are still in their original sets and all working, it should be possible to just re-adjust the bias current if necessary, if you take care and follow the instructions without compromises and let it warm up as stated before taking measurements.
The procedure is described in the service manual, and scans can be downloaded from a few sites but not hifiengine. Try this one: https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Nad-218-Service-Manual.pdf
This is a powerful amplifier with 4 pairs of output transistors - so it's important to take full sets of replacement power transistors from the same tube packages and in sequence, to get the closest possible matching. If you mess up, it will cost quite a lot to repair with the necessary full sets but at a pinch, carefully matched individual transistors from the same batch that you don't try to push to their spec. limits may work fine too.
This isn't typical consumer gear and for your safety and assurance, should be serviced by a pro. audio technician or someone with appropriate experience and the equipment for matching complementary power transistor sets and drivers if necessary. However, if the transistors are still in their original sets and all working, it should be possible to just re-adjust the bias current if necessary, if you take care and follow the instructions without compromises and let it warm up as stated before taking measurements.
The procedure is described in the service manual, and scans can be downloaded from a few sites but not hifiengine. Try this one: https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Nad-218-Service-Manual.pdf
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Hei and thx for your help.
The Nad 218 was just an example i used to own before and i actually replaced all trannies on both channels. I got them all from Digikey at the same time packed in long plastic tubes. Bought 20 of each, and on my simpel equipment they all were between 128-134 HFE and thats what i used and it worked/ works really fine actually. No i dont have a curvetracer. But here it was more easy to adjust because common bias point.
Other hand this Copland amp manufacturer recommends 8mV over one of the 4 emitter resistors (0.22R) and i was wondering how could i measure to get some middle point. For example now if i hook my probes up on the first emitter resistor i have 7.1mV and the last on i have 9.6mV for example.
I just have to live with it or? This is orginal trannies though, from the amp was manufactured in Sweden.
The Nad 218 was just an example i used to own before and i actually replaced all trannies on both channels. I got them all from Digikey at the same time packed in long plastic tubes. Bought 20 of each, and on my simpel equipment they all were between 128-134 HFE and thats what i used and it worked/ works really fine actually. No i dont have a curvetracer. But here it was more easy to adjust because common bias point.
Other hand this Copland amp manufacturer recommends 8mV over one of the 4 emitter resistors (0.22R) and i was wondering how could i measure to get some middle point. For example now if i hook my probes up on the first emitter resistor i have 7.1mV and the last on i have 9.6mV for example.
I just have to live with it or? This is orginal trannies though, from the amp was manufactured in Sweden.
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Your motives are your own, I guess, but it begs the question why you want to replace good working transistors anyway. You won't significantly increase their working lifetime or sound quality by changing for others of a similar type, if your amplifier otherwise remains the same.
Anyway, it looks like Copland or someone else set the mV reading to an average 8mV for both emitter resistors from your individual resistor mV measurements and according to your example. It would be sensible to adjust your mV readings for a similar 8mV average since there are no better options when you replace the transistors and/or resistors and enter new territory.
There are 4 different 5% resistors in series parallel to make the average current setting a bit more complex and all these variables limit how close you can get to good power sharing between the 2 pairs of output transistors which is actually much more important than bias settings when the amp is working above ~40% full power .
Building and repairing multi-transistor power amplifiers has all been done much like this for many years though - nothing hard to understand but maybe disappointing that a respected brand amp isn't really as wonderful, precise or special inside. That's the reality of most audio power amps, even so-called high end gear.
Anyway, it looks like Copland or someone else set the mV reading to an average 8mV for both emitter resistors from your individual resistor mV measurements and according to your example. It would be sensible to adjust your mV readings for a similar 8mV average since there are no better options when you replace the transistors and/or resistors and enter new territory.
There are 4 different 5% resistors in series parallel to make the average current setting a bit more complex and all these variables limit how close you can get to good power sharing between the 2 pairs of output transistors which is actually much more important than bias settings when the amp is working above ~40% full power .
Building and repairing multi-transistor power amplifiers has all been done much like this for many years though - nothing hard to understand but maybe disappointing that a respected brand amp isn't really as wonderful, precise or special inside. That's the reality of most audio power amps, even so-called high end gear.
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