Fender Blues Deluxe Adjustable Bias Mod

[pompeiisneaks]

Hey all, I'm recapping my old '95 Fender Blues Deluxe (One owner, me :).  I thought, while I was at it, I'd do a couple mods.  One is the reverb mod where you remove the cap at the recovery stage that's s upposed to make the reverb sound nicer, but the bigger question I have is how I would mod the circuit to allow adjustable bias.  I've got the schematic section here, I think if I just replace R87, the 27k Resistor with a 50k pot for bias instead, then I have the 27k minimum as well as higher and lower to adjust the bias.  Does that sound right?  I've got the section attached as an image, or do I want to keep the 27k and just add the pot?

~Phil

[sluckey]

That's a bit dangerous because you could turn the 50K pot down to zero ohms which would send zero volts negative bias to the output tubes and burn them up. A better way would be to replace R87 with a 25K pot wired in series with a 15K resistor. Then you can safely dial in 15K up to 40K, eliminating the danger of zero volt bias.

[pompeiisneaks]

ahh excellent, I was kinda thinking I should keep that resistor in, but then I thought I couldn't go below the voltage at the 27k so yeah, that sounds great.  Thanks!

~Phil

[pompeiisneaks]

Oh Doug's only got 10k or  50k.  Should I go for 50k and a 15k resistor?  It would go a lot higher than would ever be needed I'm sure, but g ets the job done?

~Phil

[HotBluePlates]

Yes, that will work.

[pompeiisneaks]

Excellent, thanks!

[pompeiisneaks]

I just wanted to update this thread, I had someone recently ask me about what the final schematic for the bias mod looked like so I drew it up.  If anyone needs to do a bias mod, you can see the attached schematic change from that first one I showed.

~Phil