Set bias on Randall RB100 amp?

[Axtman]

How do I set the transistor bias on a Randall RB100 bass amp? And for that matter, how do you set the bias on a transistor amp without the schematic or bias information? Thanks.

[DrGonz78]

The Enzo method is good enough to keep your job. You dial in crossover distortion of a test signal 1k by making the bias pot set to coolest setting. You will need to measure current with an ammeter as you adjust the bias pot. Now turn up the bias pot while watching the scope wave form and the ammeter. As you increase the bias you will see the wave form become more linear on the scope. There will be a sweet spot where raising the bias pot causes the ammeter to register current flow. You will see the current meter start to rise and when it is in that sweet spot you just back it down a hair. So it will be idle and safe.

There is a lot of information about this on MEF but it's CURRENTLY down.

[Axtman]

Thanks DrGonz78. There is no crossover distortion when I adjust the trim pot. I broke down and bought the schematic from MusicParts.com. Luckily, it had some voltages, so I adjusted the pot for those.

I sure hope that Music Electronic Forum comes back up. It would be a big loss for the amp repair community if it was gone forever. There are lots of great tips and knowledge on that site.

[DrGonz78]

You don't need there to be crossover distortion to find the sweet spot with the ammeter. The scope is just there for detail. Honestly you just need to monitor current flow with a meter then find the sweet spot and back it of a touch. Try it on some amp one day and you'll see just how easy it can be.

MEF will be back up I am sure of it.