correction....I didnt mean 'Threshold Restoration' I have melos on my mind as Im looking at a melos pre. to buy.
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The bias procedure on a Threshold amp is a little special.
As I remember it involved removing a screw near a heat sink and inserting an accurate thermometer, letting the amp cook until it is stable and setting the bias for a given temperature. It then has to cook again to recheck.
You should check with the Threshold restoration group to be sure. If the amp has never been updated it is probably time to replace the power electrolytic caps anyway and this may make a bigger difference than readjusting the bias.
Here's the response I got from GRollins at DIY Audio Forums before when I was working on mine:
"First, check the temperature of your heatsinks. You may not need to adjust the bias.
If you still think you want to mess with the bias, start with 90mV across one of the emitter resistors with the amp cold. There's not really any point in reading all the emitter resistors. They will read somewhat differently depending on the characteristics of the individual output device. All you're looking for is a starting point.
Put the top back on and monitor the temperature by inserting a thermometer into the screw hole at the back of the amp. You'll need to pull the top to get to the bias pot, obviously, but be sure to put it back on every time, as it is part of the thermal environment. Plus or minus a degree or two between the channels is okay. Let the amp idle for an hour or more in order to get everything stable.
As I said above, the bias pot is fairly twitchy. Patience is the key. Adjust in small increments. Given the possibility of thermal runaway, it's better to run the amp too cool than too hot, although it will sound better if you run it warmer."
And here is what Mr. Nelson Pass posted to my inquiry:
"The best measurement comes around 40 deg C. on the
base of the heat sinks, best sound at 50 degrees."