How to bias a sunn beta lead???

[Bernardduur]

Hey all!

I got in this Sunn Beta Lead that works almost perfectly....... only downside is that underneath all notes played a weird 'rumble' is preset....... like a slight misbias sound, or that it almost gains........ when I listen to the preamp alone there is no of such sound so it has to be in the poweramp.......

Read in the service manual how to bias the unit...... but I can't get it to work.

It reads to trim R42 (trimpot) and measure across R18 for 1mV. I measure nothing there...........

[loudthud]

Bias should be adjusted with speaker disconnected. If you still can't get anything across R18, there is something else wrong possibly around Q13.

[Bernardduur]

Tnx!

That made a lot of difference! And it wasn't printed in the manual!

Tnx!

[Bernardduur]

But then the issue still remains as I can't get a reading across R18; still 0mV!

Tested all the other lines also and those don't respond either to the test voltages as are printed in the manual....... those are measured to ground, right?

[Jazz P Bass]

Are you sure your meter works?
I doubt that biasing the output stage hotter or colder is the rumble issue.
Bias too cold will have "crossover distortion".
It sounds raspy, hissy, real crappy.
Bias too hot you will never hear a difference.
The transistors will get toasty but you will not hear anything different.