In plain English how to adjust the bias
The following applies to the current range of Fender amps with bias adjustment points on the rear panel of the amplifier. There is no need to remove the chassis for this procedure.
Fender obviously consider this job to be well within the scope of the average owner, because the adjustments and test points are mounted on the outside rear of the chassis. And detailed instructions of how to do it are in the owners manual. Fantastic, clear as mud, if only the instructions made sense!
Now, I am from the U.K. West Country, Devon, just over the Somerset boarder. And contrary to popular belief, we don’t all drive tractors, combine harvesters, and dig swedes. Only the minority do that, the rest of us are not that bright! So even I had trouble getting my head round Fender's instructions of how to set the bias on this type of amp. Here is my version of how you do it.
First I will say, though very simple, the adjustments are very precise, the tiniest amount of adjustment to the trimmers will make a considerable difference to the readings you will see on your meter.
You can not do this job by what you hear, or by pot-luck. Even I can’t do that.
You will need at least to have access to a volt meter, or a multimeter with a DC voltage range.
You can buy one like I am using here for about ten or twelve pounds. No need to pay a fortune, I buy them two at a time as I manage to destroy one at least every three months!
So make sure the leads are in the right place when dealing with high voltages.
Secondly, the voltages you will be dealing with while making this adjustment are less than 1 volt. There is nothing you can do here that will cause you any harm, except burn your fingers on the hot valves, or break your back lifting the damn thing.
Now for the Devonshire procedure to set your bias.
2. Turn on the amp and let it warm up for a while, 4 or 5 minutes, both mains and standby switches on.
Set power switch to high, nothing plugged in to the input. Turn all gain and volume controls down to 1. If your amp has vibrato set both speed and intensity to 1. Foot switch unplugged. Reverb down to 1.
5. Let the amp settle, check again, and repeat the whole adjustment procedure if required.
That’s all there is to it, your amp bias is now correctly set. If you are unable to obtain those readings you may have faulty or out of spec valves, or a problem with the amp that needs attention.
It is not important which lead you stick in the middle hole, Red or Black, the readings will be the same. But if you stick the black one in the middle you will see a Negative sign -- on the left side of the meter's display.
I have come across a few issues with both versions of this amp and will add a feature dealing with them at a later date. There are a few separate problems individual to each version.
I hope you found this article useful, but please, don't blame me if you blow your amp up, I accept no responsibility, cause I'm vrum Devon!!!
Now back to the muck spreading.
Ump er dedle dedle ump er dedle dedle ump er dedle dedle de. Oh ar oh ar ay, Oh ar oh ar ay, drink up thee zider Gearge.