Model Two BIAS


#1

I just acquired a Model Two shoe box amp which I used to own in the early 80s. I checked the bias current on both channels and they were not the same. I also updated all the electrolytic caps and transistors while I had it apart. Now it’s time to set the offset and bias. Does anyone here still know what the idle current is for its output stage?

Thanks!
Stanley


#2

This was a bit of long shot but I did some digging though our Archive and I was able to find something.

Here’s what I found, apologies for the poor quality:

I’ll attempt to translate as best I can…

V1 + 38 VDC
V1 - 38 VDC
V2 + 30 VDC
V2 - 30 VDC

Offset
Both Channel: 0

Bias Adjust -
Meter in series with V2- at REC 1 on MMA scale. Both pots turned toward center of board all the way. Nominal idle current should be 19-20 MMA adjust right channel pot to REC (I’m not sure on this bit) 45 MMA

After this it’s illegible to me. It seems like it’s something about the left channel though I can’t make out any of the details…

These notes are attached to a schematic. If helpful, I can email you the whole schematic - just send me an email at scott.schroeder@psaudio.com letting me know you want it and I’ll send it right over.

Hopefully this helps some! Maybe others have details that can help out as well.


#3

Hi Scott,

Thanks for the quick response!

V2 goes to both channels so I guess the instruction was to read the combined current. From reading what I can make out. When I turn both pots toward the center, the bias for both channels will be 20mA, or about 10mA per, and it should be the minimum. Then adjust the right channel to read 45mA that means I am adding 25mA to the right. I guess then the right channel bias would then set to 35mA? I now can add another 20 to the left. I think 35mA is good for Class AB.

I am sending you my email in a separate email.

Best regards,
Stanley