This is from my thread on TDPRI, which can be found here if you’d like to see the topic in its original entirety: Click!

Most of this is copied and pasted from there because it was already so detailed and accurate to what is being used. My memory isn’t as good as the Internet’s is! As more time went on I kept cycling back to my pages and working with them slowly over time, editing more and more.

Mainly this was my first venture into modding audio circuits, besides build BYOC (electric guitar effect pedals) kits and selling them. Working with an amplifier is amazing! Hearing the differences you can make is very rewarding, especially when you are doing all the building yourself. I encourage all readers who own a Fender Blues Jr. to try the “basic kit” he sells for all versions of the modern Blues Jr models and the Fender Excelsior

Correction: BillM ISno longer providing his mods again, after he took a break due to health concerns. For a while his son was filling the orders, but on their site it sounds like Bill is back at it again. I may reach out to him and see if he wants to team up and let me be the guinea pig! Burson Audio and Sparkos Labs

New Article on “BillM”‘s Fender Modding site announcing he is back and working on a backlog of orders:

“We’re Back!”


This is great news for all current and future Fender Blues Jr owner’s! I believe the two or three modern versions of the Blues Junior (made in last 20 years) is the best trade off between tubed audio circuits and their (sometimes superior) solid state equivalent. BillM and his son create some of the best Fender Blues Jr (“BillM”) mod kits for your money of just about any other modern and vintage based guitar amplifier, not counting the all DIY from start.

Most mods for tube based electrical equipment will run at quite the premium if for nothing else, lethal currents & voltages present in multiple places… Without taking the proper precautions, doing a capacitor upgrade could end with your obituary. That isn’t a joke and BillM does an excellent job instructing the customer how to build and/or install their kits. For my entire life I have been picking the DIY kits for any circuits where performance mattered, if nothing else to keep soldering, at least 3 times a week 52 weeks a year if possible. You lose the touch you once had if you put down this instrument for too long and expected to not only jump back in, but that we had to suddenly be great at this. This is how I felt in my 2.5 years back at NCSU attempting to do the honorably thing.
Without anymore delay or l, here is the Fender Blues Jr tube amplifier (except for the rectifier and reverb)!

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I purchased the Blues Junior knowing it was pretty much the perfect practice/home amp in the sub $500 range for the classic American tone. After spending some time with it and stumbling through the internet onto BillM’s website ( I decided to start modding the amp. I have a long history of modding PC’s (even directly soldering on motherboards and video cards along with building custom guitar effects pedals. So I knew this would be a piece of cake with how simple the Blues Jr. is.

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I started with some basic BillM mods, in the end this was the list of mods I had purchased/installed from BillM:

  • Basic Cream Kit
  • Sparkle Mod
  • Presence Mod
  • Switchcraft Input Jack
  • Switchcraft Line Out Jack
  • Standby Switch
  • TO20B Upgraded Output Transformer

After installing all of those mods I was very satisfied with what I had in terms of overall tone and the amount of tonal possibilities with all the mods. The only thing still lacking was the bass response from the stock speaker.

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At that moment in time the “budget” wasn’t the main concern here and you are talking about a $200 speaker upgrade (that could be moved or sold at very little loss).

Here is a breakdown of everything you see in the completed pictures

Blues Jr. – $429
NOS Tubes – $105
BillM Mods – $91
Speaker – $199
Total – $824



I went with the G12H because I have heard them (lol ever listened to Hendrix? :lol:) versus some other 12″ speakers and nothing came close to it in my mind (from what I’ve heard in person). Looking back now I wish I had gotten a used Blues Jr. (since I was going to void the warranty anyway), but you live and learn.

Build Log Comments:

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I just got done installing the BillM Clean Boost board + Stereo Foot Switch mods and I am really impressed by that mod. You can really hit the output tubes hard and this is something no pedal can do going into the front end (no matter how expensive it is). Next to the basic kit, the clean boost is my favorite mod out of all of the BillM mods. The only possible thing left I can do now is change transformer and output tubes, but I like the EL84s. Now if we could put EL34s in it, that would be a different story.


Shot with all NOS tubes and the famous G12-H (Heritage) speaker by Celestion
My Schecter Tempest Custom w/ real Seymour Duncan pups


I definitely think NOS tubes do create a tonal difference vs. the stock Fender Groove Tubes. My only issue was I did so many changes so often that it was hard to pin down certain improvements. However I know for sure the NOS EL84s (GE USAs) created a much glassier sound than the Groove Tubes and were also quieter. Like I mentioned previously I went with the 12AY7 to create more clean headroom. Although with my BYOC ESV 2 Knob Bender, I can enter Jimmy Page land at the click of a switch. I went ahead and changed out tubes V2 and V3 because I figured I might as well go all NOS, however I did not spend near the money on those as I did V1 and the power tubes. Although V2 GE USAF 5751, but I didn’t pay very much for it compared to the V1 12AY7. So I’m running all GE US Tubes except for the Sylvania in V1, which from my understanding may have very well been made in the same plant as the GE tubes.

These are my EL84 GE Power Tubes that tested 7500 and 6800:…l?_sacat=0&_from=R40&_nkw=400562623054&_rdc=1

Then my Sylvania 12AY7 that test perfectly balanced:…l?_sacat=0&_from=R40&_nkw=190859227278&_rdc=1

Then my V2 and V3 12AX7s: (I got that one GE 12AX7 for $9.99 because for some reason there were no other bids, even though it was tested (U GETTER/ GREY PLATES- 52/54))…l?_sacat=0&_from=R40&_nkw=111190696077&_rdc=1…l?_sacat=0&_from=R40&_nkw=370917247520&_rdc=1

Overall from everything I did, my favorite mod is the clean boost + stereo foot switch. Being able to push output tubes harder from a completely transparent boost is something that no pedal is going to achieve, no matter how expensive when running it into the front end of the amp. I still need to get some more pictures now that I have the Clean Boost installed and I have the line out mod in there now also. I’ll see if I can’t get them in the next few days, if not today when I get off work. If anyone has any questions about anything or wants to tell me I am an idiot for spending so much on a Blues Jr. (again, lol), just leave a reply. By far the biggest tonal difference was when I installed the G12H (perfect balance all around and a true vintage sound), that was $200 well spent. I know I will never get rid of that speaker, if I upgrade later down the road that speaker will move also.

The David Allen-designed TO22 Heyboer output transformer

Low Profile TO20 Output Transformer

Premium TO20B Output Transformer

TO26 Output Transformer for 6L6 Conversion

TP24 Power Transformer





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