Ampeg SJ-12R, quick question on biasing?

[Sarg_29]

I would like to be able to expieremnt with the power tubes but I don't know what type of biasing is used by the amp. I have looked inside and there is a small pot on the PCB and was wondering what it's function was for? Is it for biasing? Any advice on this matter would be greatly appreciated.

[Dave C]

The small pot you've found is likely the bias adjust. To confirm this take a voltage reading on pin 5 of the power tubes(if confident and comfortable doing so) while adjusting the pot up and down a little. If the negative voltage varies with the pot movement ......that's the bias adjust.
Dave C

[Blue Strat]

The small pot you've found is likely the bias adjust. To confirm this take a voltage reading on pin 5 of the power tubes(if confident and comfortable doing so) while adjusting the pot up and down a little. If the negative voltage varies with the pot movement ......that's the bias adjust.
Dave C

What Dave said. You'll probably want to have a bias probe since it's the easiest and safest way to do biasing. This will allow you to take measurements without probing the tube sockets and other dangerous areas of the amp.

www.tedweber.com (http://www.tedweber.com) sells bias probes.

[Sarg_29]

Thanks for the replies and the tip on the bias probe. I am fairly confident around amps, I just couldn't come up with a schematic for this amp and it varies a lot from the older ptp amps which I am use to and have the schematics for. Again thanks ....