Can someone shed light on the bias adjustment in the Fender Blues Junior

[AmpMan1954]

Blues Junior (Green Board) Biasing

Can someone please help me understand the way a Blues Junior should be biased? There's a ton of different info on the web, some of it outright wrong info, such as stating that the amp is cathode biased etc.

From the service manual (http://schems.com/bmampscom/fender/Blues Junior Service Manual.pdf) I can read that:

"The output bias is derived from the -15Vdc supply. The voltage divider at R31 (R52 in later Rev. but still 22K) & R37 (R51 in later Rev. but still 33K)
determines the bias voltage (-10.7 Vdc). C14 filters the ripple to 1mV or less."

- or see: http://blueguitar.net/new/schem/fender/blues_jr.gif

Now, looking at the instruction video of the late Bill Machrone (YouTube) he states that one should "remove the output tubes or pull one of the red wires in order to lift B+"

He then places the ground terminal on B+ and measures voltage on the brown (transformer ?) wire, and sets a voltage. From the table on his page (Billm Audio >> Setting the Bias) I can see this should be 2,6volts with the stock tube and transformer.

Now, should the tubes not be in the circuit in order to set the bias? I mean, the bias should reflect the individual nature of the output tubes, and in this case also the transformer?

I also see that he has removed both reistors in the voltage divider and replaced them with a multiturn, but I assume that replacing only one of them - eg. R52 with a 50K variable would give the same option to set the bias the way described here?

I would be very thankful if someone could explain this?

Best regards
Hans

Hi, Appears you and I are searching for the same thing. I'd like to install my 50K Multi Turn Trim Pot as per the Late & Great BillM. My Green Board shows R31 and R37 on either side of C14. I think I should replace both Resistors . As to configuration, I think the Pot should rest just under C14, allowing the legs to connect in the corner, but not sure what to do with the 3rd leg. perhaps you might help me? Being new to this site, I'm having trouble uploading my BJ (Green Board) Pic. jf