Fender Blues Junior Mod for Jazz Tone

[Maroonblazer]

I came across this video from Sandra Sherman where she mentions a mod for the Fender Blues Jr tailored for jazz.

https://youtu.be/4HkSXxnvoNc?si=Xsc7S53n5eM50nDl&t=913

Is anyone familiar with this mod and know where more details can be found? I would love to be able to take it to my local guitar shop and have them perform the mod.

I've searched "BillM mods" and it seems he's created a number of different mods, but I couldn't find any references to this specific one.

[stevo58]

Bill Machrone‘s site is down (he passed away in 2016) so we only have secondary evidence right now (at least that I could find).

As I recall, there are a bunch of “Bill M” mods, but in the video she mentions tone stack and bias circuit mods. I can’t find any exact description of what the mods are, but based on what I have found, it’s like this:

Adding a bias pot lets you set the bias voltage on the input grids of the power tubes. A tube can only handle so much current, and every individual tube is a little bit different. That’s why you buy “matched pairs” - two (or four…) with close behavior. The stock blues junior has a preset bias level (no adjustment knob). That means you have no control over the current, and it will be different every time you install new tubes, and over time with the same tubes as they age. This has a direct impact on the tone you get out of the amp (and tube life). Running high current will drive the tubes harder and is loved by the amp-distortion rock-and-roll guys. Low current will give you cleaner sound. Run it too low, though, and you get “crossover distortion,” which you do not want, it’s very ugly.

So the bias mod allows you to set the bias voltage where you want it. To do that, though, you have to measure the bias voltage on a live amp and adjust it on a live amp. That means the amp is open and you are sticking your fingers near potentially death-inducing parts of the amp. With the stock amp, you just pop in some tubes and hope for the best. So what this means is if you do the mod, you either have to get used to messing with high voltage or you have to pay someone else to do it every time you swap power tubes. So your choice. When I was building amps, I included externally-accessible test points and bias pot so I could set it quickly without opening the amp.

The tone stack mod changes the curves of the tone controls. When I have more time I can look at the changed values and see what it is. But I don’t know what values were used in her amp, I can only look at info I found on the net.

Before you do any of this try the easy thing. She also says she tried different V1 tubes. She mentioned a 12ay7 and a 7025. Back in the days when tube designations meant something, a 7025 was just a low-noise 12ax7, so that probably didn’t make much difference. A 12ay7, though, will reduce gain nicely and was used in many Fender tweed amps. Another possibility is a 5751, which is more or less midway between a 12ay7 and a 12ax7. I never liked a 12at7 in the first position, but then that tube was designed for a different purpose.

[JCat]

Claim: "I had my amp modded for more headroom and a less boxy sound. I also use a weaker tube in v1"

Let's break it down:

Get to know your amp

The amp in question is a Blues Jr III, Limited Edition. Factory speaker; Eminence Lightning Bolt, 50W.
There are four official circuit versions; ver I, II, III and IV, all fixed bias. There's also an unofficial cathode biased version IV, specific to certain geographies, e.g UK and Australia. (These markets used to report that the output tubes were running hot, indicating the need of a bias trim pot. If you got a cathode biased version, there's no need for a trim pot.)

They all got a traditional passive TMB tonestack (Treble, Bass, Middle) and Master Volume. The controls are all very interactive and sensitive in some part of the range (typical behavior of this tone stack, regardless of amp model).

Over the years these amps came stock with many different speakers (12", 8 ohm). "Limited Edition" typically means that the amp got a different speaker and different cosmetics, but that the circuit is the same like all the others of the same version.

The sound we hear is a closed miced recording on youtube. (We don't know anything about the actual sound in the room. For now, let's just assume it sounds good.)
Control panel settings as shown in the video:

Volume: 6.5
Master: 2.5
Treble: 2.5
Bass: 9
Middle: 6
Reverb: 2
Fat switch: Off

This setting produces a warm Jazz sound, apartment volume level.

Get to know your guitar

The Guitar in question is of ES-175 type (Heritage), played on a neck humbucker adjusted close to the strings, with Volume and Tone set to 10 as shown in the video.

A humbucker on full throttle will produce a bright, high gain in-signal that tend to push the amp into distortion, in worst case a stiff amp will produce a a spikey, harsh tone.

By turning down guitar tone about halfway you could increase amp Treble (amp tonestack is interactive, remember. Find your sweetspot)

By lowering the pickup and turning down guitar volume a couple of notches, you could increase amp volume (gain) before it breaks up. If the guitar got modern wiring, turning down guitar volume is also going to produce a warmer sound. If the guitar got 300k volume pots, Treble roll-off is significant and sound becomes muddy when guitar volume is set too low.

About the supposedly boxy sound of a Blues Jr

The Blues Jr wasn't voiced with the same deep mid-scoop like many other Fender blackface circuits. Scooped out mids is primarily a consequence of NFB that attenuates mids to mitigate distortion. The implementation of the tonestack also plays a role. (That's why it's hard to make a typical blackface amp distort in a pleasant way. And that's why some people are used to set their guitar controls on full throttle). The Blues Jr was made to distort, but you can still have it play clean, loud enough for most jazz applications.

Deep mid-scoop means deep bass and shimmering highs, a stereotypical Fender sound, not necessarily ideal for archtop jazz where we typically want more mids and less bass, nice and fat treble strings, not "boxy" at all. The Mid knob affects volume. Set it half way for starters.

About tube biasing and circuit modifications

(If this is what you love, then nothing else matters. Stop reading and heat up your soldering iron). If on the other hand, you just want to play your guitar, be aware that all amps are Volume dependent, meaning sound and EQ depends on volume and gain settings of the amp as well as the volume and tone setting of the guitar.

Also be aware that amps are different, also when it says "Fender" on the badge. We cannot use similar settings and expect similar results. (Like if you think you know how to operate a Fender amp using the "magic 6"-method, you'll probably be challenged.)

You don't have to circuit mod a Blues Jr III to get a nice warm Jazz tone, but you may try a setting similar to what's shown in the video. At the end of the day it depends on what speaker you got. Also the question about headroom (how loud you can play before the amp distorts) is closely tied to the efficiency of the speaker. A low gain tube in v1 won't provide more headroom, but it allows you to set the volume (gain) knob higher before the amp distorts.

Avoid the stereotypical guitar forum recommendations on settings and customizations as these are typically in favor of distorted blues rock (that may include common BillM recommendations as well. I totally disagree with him on speakers by the way.)

[Tal_175]

Does a weaker preamp tube give you more headroom, or is it an internet myth?

[JCat]

Does a weaker preamp tube give you more headroom, or is it an internet myth?

A low gain preamp tube is great sometimes, but for other reasons.

I use an 12AY7 (45% gain) in my Tweed Deluxe (the amp was designed for this tube).
I also sometimes use a 5751 (70% gain) as replacement for an 12AX7 (100% gain) in some of my other amps.

A low gain v1 tube reduces the gain of the first stage, meaning the amp can take a hotter in-signal and stay clean (if you think of this as "headroom", then yes). In practice, this typically means the sound will get brighter since a higher guitar volume setting typically means less treble roll-off.

Alternatively it allows you to turn up the gain (v1 volume) higher, which is beneficial for example when an amp got a bright cap (that makes the sound thin on low gain settings). But a low gain preamp tube won't provide more volume. On the contrary, the amp loses some overdrive power.

Clean headroom is about how much louder I can go before the amp breaks up. A low gain preamp tube cannot provide more clean volume, hence I won't get more clean headroom. It may appear like I have more clean headroom because now the gain knob is turned up higher, but that's just an optical illusion.

[LesB3]

I have a BJ (version II) that I bought in the late 90's as a practice amp, in lieu of the Peavey Classic 50 I would gig with. At the time, I played only distorted blues-based hard rock, and the BJ was a constant source of frustration. I tried speaker swaps, tube swaps, you name it. At some point, someone recommended that I make the "Bill M" mods too. I went in a different direction, put the stock speaker (a Fender "special" speaker) back in, and the amp was dumped in my basement for about 10 years.

These days, I play a lot more cleanly, and the BJ suits me far better. It is 100% stock, save for new JJ tubes that were installed around 5 years ago. The stock Chinese tubes are extremely "brittle" sounding, so switching to more elegant tubes is probably the only "mod" I would make at this point. It is currently in my office and is my main practice amp. I find it makes for a perfectly fine amp for Jazz. My settings are...



..which makes for a particularly nice clean tone, a bit on the "chimey" side (Fat switch is ON). I play Gretsch and Guild guitars usually, so like to have this as my default amp setting. I get "jazzier" sounds by using the tone control(s) on my guitar(s).

When changing to the amp settings in JCat's post above (Volume: 6.5, Master: 2.5, Treble: 2.5, Bass: 9, Middle: 6, Reverb: 2, Fat switch: Off) and playing through a Les Paul's neck P-90, I get a warmer, throatier, somewhat distorted, sound for sure (tone is 100% ON), but don't really have anywhere to go via the tone knob. The more I roll back the tone, the more virtual "blankets" end up being thrown on the amp. I much prefer the flexibility of my own settings, but to each their own.

Honestly, I don't think you need to mod anything on a Blues Junior to make it a good "Jazz" amp. Just fiddle with the knobs (on your amp and your guitar) a bit more and I think you'll get there. Good luck!

[JCat]

I have a BJ (version II) that I bought in the late 90's as a practice amp, in lieu of the Peavey Classic 50 I would gig with. At the time, I played only distorted blues-based hard rock, and the BJ was a constant source of frustration. I tried speaker swaps, tube swaps, you name it.

I know the Classic 50 and I also know the frustration you talk about, because these amps are very different. The normal channel of the C50 is like an inverted Blues Jr. The C50 normal channel got all the bass that the BJ seems to lack, and the BJ got all the fat mids that apparently exist only in the C50 lead channel. And if you think the Blues Jr is bright...wait 'til you plug into the C50...that's a handful!

Once you make peace with one of them, you're in for a fight with the other.

Solution: Play them both at the same time via an amp switcher

I think of the Blues Jr as a constrained lead channel. Mid focused and therefore suitable for many jazz applications.