Blues junior bias mods?

[Dingleberry]

Hi.

My friend gave me his Blues junior for little maintenance and I noticed that it didn't have adjustable bias.
I stumbled across BillM:s site and there was article about modding Blues junior to adjustable bias. It was kind of fancy with trimmer for each tube.
BUT. It's a pain in the ass to take the circuitboard out and cut the traces and solder jumpers etc...
I was wondering could it be done simpler?
What if I just wire a pot (for example 10K or 20K) as a variable resistor and put it in series with R31? Doesn't that just make the bias voltage adjustable?
Is there anything special why this could not be done?
Why the bias voltage is always (I haven't seen anything else) taken from the wiper?

There are the tutorials:
http://home.comcast.net/~machrone/bjr/bjbias.htm

Hope you Guru's can help me.

[martin manning]

I'd do it like the attached. The current through the branch will be about the same as before, and the trimmer wiper will be nearly centered at the nominal bias setting.

You'll have to lift the board to work on it, but there is no need to cut traces or run jumpers. Start by pulling the existing 22k and 33k resistors. Twist one lead of the new resistors around the 1 and 3 legs of the trimmer and solder them. Pick one of the empty holes that leads to the pad that formed the junction of the original resistors and the negative end of the 100uF cap and solder the wiper leg of the trimmer there. Solder the free ends of the new resistors into the holes formerly occupied by the other end of the original resistors. Try to arrange the new resistors so that they brace the trimmer against rotating while you adjust it, and maybe use a dab of silicone to help fix the trimmer in place. Watch out for the hot end of the large filter cap that is nearby; best to keep the trimmer away from there.

[Dingleberry]

That's a great idea. Should have figured that out myself.
Thanks anyway. That will be the easiest bias mod to perform.
Beautiful, simple, classic.
Probably a 10k or 20k trimmer will both be fine. I'll check what I have on hand.

-T

[Andy Le Blanc]

That one wire mode to the tone stack does well.

I see a lot of bad resistors in fenders, or hot resistors that create their own
bad solder join, You might want to simply replace the OPT, that really helps.
Then bias to taste, better coupling caps really helps too.

[PCollen]

Hi.

My friend gave me his Blues junior for little maintenance and I noticed that it didn't have adjustable bias.
I stumbled across BillM:s site and there was article about modding Blues junior to adjustable bias. It was kind of fancy with trimmer for each tube.
BUT. It's a pain in the ass to take the circuitboard out and cut the traces and solder jumpers etc...
I was wondering could it be done simpler?
What if I just wire a pot (for example 10K or 20K) as a variable resistor and put it in series with R31? Doesn't that just make the bias voltage adjustable?
Is there anything special why this could not be done?
Why the bias voltage is always (I haven't seen anything else) taken from the wiper?

There are the tutorials:
http://home.comcast.net/~machrone/bjr/bjbias.htm

Hope you Guru's can help me.

I removed one of the two resistors in my '97 green-board BJ bias divider circuit, and wired in a multi-turn mini 50K trim pot as a variable resistor. I just got some clear silicon adhesive and stuck the pot to the board; that and the wires hold it in place. Works great, and has for over 10 years.

[Dingleberry]

I found 10K multiturn trimmer that will fit there perfectly.
I think I use that because I didn't find any 22K or 25K trimmers.
Workshop is a mess right now 'cause I just got the space few days ago.

If I use 27K feed resistor + 10K trimmer + 18K to ground, the total resistance from bias voltage rail to ground would be the same as in the original schematic. 55K.

That arrangement will probably have enough range to adjust the bias to where ever I want it to be settled.

-T

[AmpMan1954]

Hello: I am new to Amp Garage. Could someone please help me with my 1995 Blues Junior (Green Board), Bias Trim Pot install? I've found my vertical R-37 and my Horizontal R-31. Also below are my R-21 and R-22. Perhaps some one could help me as to placement of the Trim Pot? Thanks,

[maxkracht]

Old thread, but thought I would share a bias pot perfboard layout for the blues jr III and save the next person a few minutes of thought. This replaces r51 and r52 and doesn't require board modification. Leave the top resistor leads long and add a piece of solid core wire to the bottom corners going to c-. Normally I would just drill holes in the PCB and glue in the new trim pot, but there's a b+ line going right underneath the bias resistors. This seemed the easiest way to do it.