Blues Jr. -- Tone Stack -4

Posted by Jim Black on 2/27/2015 to Mods
A Word of Caution About High Voltages in Power Amps
Discharge high voltages before you start work on any amplifier. This is how you do it.

I use a 1Kohm, 1 Watt resistor to short across the power supply filter cap. We want the voltage to get down to less than one volt. It might take a moment to discharge the cap. Check the voltage with your DVM before you try any of the mods. You can now safely partially remove the PCB from the chassis.
The chassis is on the bench and the PCB is partially removed so we are ready to actually start the mods.
The tone stack mod replaces two caps in the tone stack itself and two coupling caps.
  • C6 0.1uF Bass
  • C7 0.015uF Mids
  • C2 0.022uF Input Coupling
  • C8 0.022uF Output Coupling
The goal of the mods is to increase the bass and the mids. The choice of the components reflects this goal.
For me, the easiest way to change out the components was to locate the part then highlight the hole on the solder side of the PCB.

Past experience taught me that the best way to get a component out without damaging the PCB, traces or holes is to use solder wick with flux applied. Just add a quick touch of the solder iron and it all comes out nicely.
I do recommend to cut the capacitors off on the component side first if you don't want to preserve them. For me, I wanted to be able to go back to the original if I didn't like the results of the mods so I took the extra step and preserved the caps.
The caps are all in place and it is time to move on to the next step. Next time we'll apply the power supply stiffener and the bias trimmer.



Date: 9/1/2016
Hello, For your tone stack mod, what are the V ratings of your coupling caps? C2 0.022uF Input Coupling C8 0.022uF Output Coupling Thanks!
Date: 11/1/2016
Hi Jim, So, I've had all the tone stack mods done now: C5 (Treble) – 250pF, Silver mica C6 (Bass) – 0.1μF, Orange Drop C7 (Mid) – 0.015μF, Orange Drop C2 (input coupling) - 0.022μF, Orange Drop C8 (output coupling) - 0.022μF, Orange Drop The Emi Texas Heat speaker cut out too much presence, so now I have an Emi GA-SC64 in there. The issue I'm facing at the moment is that the Middle tone control brings in too much low-mid. I'll turn it up to bring in the high-mids so my sound has more clarity in the mix, but those increased low-mids muddy it up with too much "chug" sound. I know you used different values for C6 and C7 than the above. Which would I change, and to what value, to allow the Middle tone control to increase more high-mids and less low-mids? Thanks again!
Paul Bennett
Date: 10/6/2017
Help... I did the Billm cream board basic mods, power stiffening, filter caps, bias and twin stack to my BJ. Now I have barely 1/3 the original output volume. Mids and treble barley work and are only very low frequency (and muffled sounding)... almost no treble or mids. Then there is the odd distorted tone (oscillation) beside each regular note played (almost sounds like a detuned effects pedal). I've check and re-checked all connections... everything is good and solid. Any ideas? Thanks
Jim Black
Date: 10/7/2017
Hmmm… Sounds like the capacitors got mixed up. Did you adjust the bias after installing the potentiometer?? That was the trickiest mod in that package. (sorry got to ask the dumb questions first) Where did you get the parts? Do they match the values in the blog? I ordered my parts straight from the Billm website. If you give me a list of the components with their designators (C10, Cxx) I can cross check them for you. Maybe take a close up photo of the bias mod both sides of the board. As I mention in the blog, I did not like the twin stack mod. I have since taken it out.
Paul Bennet
Date: 10/7/2017
Thanks for your response. Purchased all parts from Billm except C6 & C5 (I think they were left out of the kit). Sourced a 0.1uf Orange Drop type 225 (600V) for C6 & 250pf Mica for C5. C2 & C8 were changed to .022uf (in the kit). I found that most of the replacement caps were very wiggly after installing so I put a little blob of hot melt glue on each to keep them in place. Not pretty, but gets the job done. Yep, set the bias to 2.6 so she's not running too hot. Been reading about problems with the tube ribbons causing problems somewhat like these... oscillation, reduced output volume. What do you think? Going to open her up again tomorrow and poke around. Think I will pull the Twin Stack jumper and see what happens.
Jim Black
Date: 10/7/2017
I double checked the capacitor type and values. It all sounds good to me. When you say the caps were wiggly, even after you soldered them? They shouldn’t wiggle at all. Double check the solder joints. Make sure there are no voids. A photo of the bias mod, both sides, would be most helpful.

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