A question concerning my current repair project of an audio amp that uses big (600 W) but otherwise rather classic complementary output stages:

I assume when testing an audio power amp with 6 pairs of paralleled npn/pnp transistors, it is o.k. to assemble just 1 pair while testing as long as you have no significant output load connected. The advantage would be that, while testing, I would not blow as many of the precius MJ15022/23 transistors I have available as spare parts.

In what ways (if any) will the bias current circuit be affected?

Any problems I may not have considered but may be important for this repair technique?

Any other really good tricks for fixing big amps?

Once I am closer to finishing the repair: Does 10...15 mA of bias current per transitor branch sound like a good number to you?

  • \$\begingroup\$ How are the transistors balanced? On an IC, you can get away with having pairs of transistors that need to have precisely matched parameters, but it sounds like you've got discrete parts, which would require some kind of load balancing circuitry which could damage something. \$\endgroup\$ Jan 6, 2011 at 0:22
  • \$\begingroup\$ There are the usual emitter resistors: 0R33, 4W for each transistor. \$\endgroup\$
    – zebonaut
    Jan 6, 2011 at 12:49

1 Answer 1


1 bank of 6 at a time ok. Bias not affected significantly. 10-15 mA plausible.

  • 3
    \$\begingroup\$ Do you think you could elaborate? \$\endgroup\$
    – tyblu
    Jan 6, 2011 at 11:28
  • \$\begingroup\$ Well, I guess I get it: 1 npn/pnp pair is o.k., the bias circuit won't care if there is one path or if there are six parallel paths of big transistors, and 10...15 mA in each branch of power transistors is good. I actually like the answer: +1. \$\endgroup\$
    – zebonaut
    Jan 6, 2011 at 12:47
  • 1
    \$\begingroup\$ Thanks again. The repair work on this monster (dynacord.com/downloads/pm2600_datenblatt.pdf) with its four massive banks of TO-3 transistors is almost done and it already looks good enough that I can confirm your advice was really valuable and is much appreciated. \$\endgroup\$
    – zebonaut
    Feb 8, 2011 at 21:26

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