Suggestions/help on biasing Marantz 500 amplifier

I finished rebuilding a Marantz 500 and I’m wanting to make sure I get the bias done correctly. The instructions are a bit difficult.



After feeding a 20 kHz signal until I got 4.5v output I got no change to the waveform as far as “large peaks indistinguishable from the distortion”. I read that someone was setting each channel for 14.5w of power consumption at idle, but that’s somewhat difficult with it having a 10w drift. I’m measuring across one of the emitter resistors and would like to calculate for bias current.

Also, the instructions don’t say, but should bias be set before the amp has a chance to warm up? I did a Kyrocera that insisted on having the bias set within the first 2 minutes of power up.

What’s a good number to shoot for? At what I felt was close to 15w consumption I was seeing about 3 mV across the .15 ohm emitter resistor which is about 20 mA. It did fluctuate, liming to 4.4 mV (30mA) and then back down to 3 mV after about an hour of being powered up.

I’d like an idea of how much current once warmed? 15-20 mA? A little higher, 20–25?

I guess anything is better than where it was at, they had 4.7 kohm resistors in which was giving me a reading of about 0.25 mA, so it was way under biased.

Right on power up I see right around 75-80 watts of power consumption with both channels having no bias. Right on power up, no warm up, I adjust both channels to be right at about 3.3 mV, so about 22 mA of current and the power meter is fluctuating around 104-108 watts.

But I’m guessing because if the unstable power of the US power grid it for example just jumped up to 120w consumption and bias is sitting at about 4.4 mV or 30mA. I know it’ll go back down, but does this seem like a good place to be? I’m going to let it run for a little bit and see where is settles after an hour or so. They thing that worries me is that it says that if it jumps more than 15w per channel at idle to turn the bias down, kind of difficult when it just all of a sudden jumps 15w.