A Very Special Blues Junior
This is the 1000th Billm-modded Blues Junior.
Or as close as I can figure, given the number of components I’ve ordered over the years, shipping records, etc.
I decided to pull out all the stops on this one. I started with a basic black Blues Junior, the perfect platform for a stealth amp that can blow away some much more expensive hardware.
It’s got
- Basic mods — tone stack, power supply stiffening, adjustable bias
- Presence control
- TwinStack
- Heyboer output transformer with 4/8 ohm switching
- Audio-taper reverb control
- Clean Boost
- Clean Boost/Fat footswitch control
- Aux speaker jack
- Line out jack
- Cathode follower tone stack driver
- Standby switch
- Higher preamp voltages for more headroom
- and octal power tube sockets for 6V6s — or 5881s!
Here’s a shot of the innards. You can see where I removed the ribbon cables and wired in a pair of ceramic sockets. I used Amphenol military-grade ceramic sockets, which are probably indestructible, but were kind of a pain to install. As you can see, I’ve got Tung-Sol reissue 5881s installed for testing… and they are awesome!
The EL84 and the 6V6 were designed to be electrically equivalent to one another, but they go about it different ways. The 6V6 is actually a 14 watt tube, while the EL84 is rated at 12 watts. The 6V6 has more plate area, and like its brother 6L6, is actually a tetrode (four elements) with a special plate inside that focuses the electrons into a beam for maximum efficiency. The EL84 is a pentode, and it makes up for its smaller size by running hotter and using a suppressor grid instead of beam-forming plates. But output tube performance is all about plate area and current. Because of the socket and bias differences, it’s not easy to directly compare an EL84-based amp to a 6V6 equivalent, but I can easily compare this one to other Blues Juniors.
The 5881s have even more plate area, which really helps bass response as well as maximum power. The 5881 is also known as the 6L6GB, the version that preceded the current version, the 6L6GC. The 5881 is nominally a 23 watt tube and the 6L6GC is a 30 watt tube.
The 6V6 consumes less filament current than the EL84; the 5881 consumes more. The question is, will the 5881 overtax the Blues Junior’s power supply? And is the Blues Junior’s 330V plate voltage enough to extract power and tone from the 5881, or will it run out of steam? After all, 5881s are rated for 360V operation and it’s customary in guitar amps to run them at around 400V. Power (watts) is volts times amps, so if you increase the voltage, the current goes down. And you tend to get cooler-running power and output transformers with the high voltage/low current formula. So I monitored thermal rise closely when I installed the 5881s. I’m happy to report that even after hard running, the power transformer was just comfortably warm and the Heyboer OT hadn’t shown any signs of heating. I’ve got more about octal tubes, output power and power consumption here.
Power transformers are limited in the amount of volt-amps they can deliver. When they run out of steam, you get voltage sag. Going full out into a dummy load with 6V6s, there is no sag. With 5881s, the sag was just a few volts. I was frankly surprised that the BJr power supply performed that well. I’ve seen more sag in Princeton Reverbs, and the power transformers in many blackface and silverface Fenders get uncomfortably warm.
OK, so it’s not going to blow up. How does it sound? In a word, heavenly.
Here are some sound files, recorded while a local blues phenom whom I know only as “Matt” sampled the amp. He brought his own Tele, heavily customized to his preferences. These recordings started loud, went louder, with serious ear ringing after an hour or so.
1. Playing first through a modded Billm Blues Junior with EL84s and stock OT, then #1000.
2. Playing with heavily scooped tone controls, then turning on the Clean Boost.
3. Some dirty, bassy blues, tone rolled back on Matt’s Telecaster.
4. Some riffs on “Ghostriders,” really loud.
1000 loud ghostriders noodling
With 6V6s, the output power is about the same, maybe a watt or two higher at full bore. But the larger plate area makes the bass performance noticeably better than even an “improved” Blues Junior with power supply stiffening, tone stack, and Heyboer output transformer. With no other changes, plugging in the 5881s make the amp 2-3dB louder and gives you the kind of bass thump you’d expect from a much larger amp. Flat out, it puts out an honest 25 watts clean (5 percent THD). Despite the big bottles, it’s still not going to go toe to toe with a Hot Rod Deluxe, but it can edge out a Princeton and hang with a Deluxe Reverb effortlessly.
The thing that surprised me most about the tone is the touch sensitivity. I expected more clean headroom and I got it. Yes, the amp still gets crunchy between 5 and 7 on the volume control, as a BJr should, but it’s louder than a stock amp at that point. And the power tube crunch comes on in a whole different way, more gradually, right in line with the energy you’re putting into the strings. The Clean Boost helps deliver the extra kick to the output tubes to make this even more evident, but even with the Clean Boost off it feels like a $1500-class amp. I did all the bench testing with an Eminence Swamp Thang, but I was pleasantly surprised when I put the amp back together with the stock Fender Special Design speaker: solid bass, sweet highs, nice transitional midrange, and smooth progress from clean to dirt.
The cathode follower mod is supposed to increase touch sensitivity by taking the load off the previous preamp stage, so that it performs the same regardless of tone stack settings. That appears to be the case, but it’s not an easy thing to verify other than by playing the amp and saying “Wow!”
Beyond dirt and breakup, the highs are higher and sweeter, the lows are more powerful, and there isn’t a hint of boxy.
I also decided to leave the stock Accutronics reverb tank in place. I tested it with the Ruby tank and I could go either way, but the bright, percussive stock reverb just felt more in keeping with the amp’s personality and the audio-taper control lets me bring it in much more gradually than the stock control.
Here are some of the other mods–you can see the Heyboer output transformer, impedance switch, the additional jacks, and the Fat/Clean Boost jack. I’ve converted it to a stereo jack, so you can use a 2-button footswitch to control Fat and Clean Boost independently.
Here’s how it looks with a couple of EH 6V6s–bound to surprise the casual Blues Junior owner who peeks back there!
I’ll probably play some more with screen resistor values because they affect the feel of the output stage, but I’ve gotta say that it sounds pretty darn good.
Now I just have to figure out what do do with it. I need another Blues Junior like John Mayer needs girlfriends. It doesn’t make sense to keep it with my demo amps, because I suspect that most people aren’t going to want to go this far.
What a good problem to have.
Bill,
I love your site, and I am impressed with the amount of work you have done in mod’ing the BJ. I just purchased a ‘cream-circuit’ one and am considering various mods to sweeten and “unbox” my Junior. I’ll be ordering some of your kits soon.
In the meantime, I’ve got a couple questions regarding the TS 5881 mod. I assume that the Heyboer output transformer is a required mod to better support the 5881s, correct? Do you recommend the ceramic sockets over phenolic, and does it matter whether I use top mount versus inside mount (e.g. http://www.mojotone.com/item.asp?pid=617779&pg=569613&id=4050004)? Finally, in consideration of a mod like this, do I need to budget for NOS tubes or will the $50 TS5881 tubes from Mojo sound fine? Finally, would you be willing to provide wiring instructions for this if I pursue this in the future? Thanks
The cost difference between ceramic and phenolic is minimal–why bother with phenolic? It doesn’t matter whether the sockets are top or inside mount.
It definitely needs the heavy-duty OT, and the Tung-Sol 5881s are terrific. No need for NOS. I’m still working on heat issues for this mod, and may have a better power transformer to offer in the not-too-distant future.
Can this mod be done to the Pro Jr. & how difficult as I’m a novice. Thanks
It could be done, but it’s definitely not for a novice because it would also require revisions to the bias circuit and a new output transformer at the very least.
I’d like to do this on my Pro Jr., I’m planing on turning it into a 212 combo & already have a Blues Jr.(stock) just wondering can I use my Hammond 272fx Transformer for this? I know it will supply the voltage for El34′s but I prefer the larger bottle tubes. also I have 4 of NOS Sylvania 5881′s around.
cheers
The Hammond PT provides way too much voltage for the PJ’s full wave bridge. It’s intended to be used with a pair of diodes in a center-tapped configuration. The PJ probably only supplies 140-150V to generate 330V. So you’d have to rework the rectifier setup.
I’m working with David Allen on designing an aftermarket PT for the Blues Junior that would probably work for the Pro Junior as well. It would supply more current for the filaments in octal tubes, correct voltage for the bridge rectifier, and more available bias voltage for high-power tubes.
ok, thanks I see now.
Hi Bill,
Is the tube socket going to become a kit? Are there any instructions anywhere? This looks like a fun mod.
Thanks,
Reid
I’m not ready for this to be a kit–there are some heat issues under gig conditions. Also, the bias supply is marginal for 5881s and requires some additional modifications for more voltage.The octal socket upgrade is now available as a kit. I recommend JJ 6V6-S output tubes, but you can run 5881s at your own risk.
I am getting a blues junior tommorrow. had one before for a short period and remember it being warm. I have only one primary requirement from it. a warm ,articulate, focused jazz tone. My main guitar is an gibson ES-175. I have a “71″ fender super reverb as well… and a real nice re-issue fender “57″ strat. for blues… I really just want too get a wonderfull jazz tone …. I belive the BJ with some of your mods and a real articulate speaker will be, both portable and tonefull …. can you recommend the way to make this happen… I do not want to worry about “getting that sound” I just want to have it and focus on playing!
thanx. Steve. M.
The basic mods, presence control, TwinStack and Clean Boost greatly expand the range of available tones. You could give that a listen and then decide on a speaker to push the amp in the final right direction. The output transformer elevates the quality of the tone, but doesn’t change it.
how much would you sell this bad boy for? you have me drooling.
Not for sale, at least not yet. I still need to do a little more with the bias circuit and think some more about the heat load, although it’s better with JJ 6V6s than the 5881s.
I noticed that you did the high-voltage preamp mod on this amp, and this is a cream board. How many, and which, resistors did you replace?
I replaced two power supply resistors, but not the preamp resistors, so this one has stock high gain. I’ve done other versions of this amp with lower gain in the preamp, and it sounds very nice, more Twin-ish or blackface.
I notice that under the description of the new Octal Mod Kit that you say “cream board only”. Will there be a version for the green boards?
Thanks
I don’t plan to do a kit for green boards. There are too many components on the tube board that have to be moved elsewhere when you cut away the sockets. Each “invented” connection is another potential point of failure. Even people who are over their heads have a reasonable chance of making the cream board conversion work, but there are many more ways to screw up the green board conversion.
Can I install a Headphone Jack so I can play thru headphones and get the same output settings as those selected for he speaker?
If so, how would you suggest.
Tube amps always need a speaker-like load. So a product like the Weber Headphone Tap or one of their other attenuators with a headphone out would be a good choice.
Hi Bill,
Just about finishing the mods but one thing is not clear and no layout came with it. On a Green board, adding the Line-Out and Aux. Jack, the hook up to the Main Speaker jack is not very clear. Do I hook to both Green mounting tabs with the green Aux wire and the Black wire to the inner two upper conections as shown for the Cream board, it looks as if the connections are soldered to two connections each. Thanks……John
One lead goes to green and one lead goes to black. You have your choice of places to attach them, but I just stick the aux jack wires into the gap between the pins and the circuit board on the stock output jack.
Hi, the sound file link doesn’t go to sound files. I was wondering if you know that?
thanks
do you do mods for bj’rs for harp players ?
I just checked the sound files and they are on the server and they played in my browser. They’re just 128K MP3s, nothing special.
I do mods for harp, but I haven’t packaged them as a kit because every harp player seems to need a different tone. Some want more distortion, some want clean, etc. Email for more info.
Hey Bill -
That’s a really cool little “labeller” you have for the jacks and switch. Who makes that?… 3M maybe. I had something like that a good long while ago. But it would not print two lines of print on the same width of tape. IOW, one would have to make two lengths of the label the way you’ve got it. I’d appreciate a nudge in the right direction.
TIA
Geos
I use an old Dymo Labelmaker PC. With the PC program, you can force it to do two lines and some special stuff. Another alternative, if you have a laser printer, is clear label stock. Print anything you want, but put some clear tape over it to protect it from getting scraped off.
On your 1000th Modified Blues Jr. there is an impedance switch for 8ohms/4ohms. You also offer a mod. kit for an extension speaker jack which, if I understand correctly, is simply a parallel connection to the existing speaker jack and requires no other mods. (other than drilling the mounting hole). What then is the purpose and function of the 8ohms/4ohms switch on this 1000th mod. unit?
Thanks,
Bob
If you run the amp hard into a 4 ohm load, an exact match (4 ohm winding on the output transformer) gives you greater efficiency and less wasted heat, less wear and tear on the tubes. If you only play quietly, it doesn’t matter whether you run 4 or 8 ohm loads–the tubes don’t work hard enough for it to matter. So you would use the TO22 for the most demanding applications where you run a 4 ohm load. For occasional 4 ohm operation, you can use the TO20, which is simpler to install.
On your BJ tube mod you use 5881′s or 6V6′s. What would be the problem using the larger 6L6′s? Internal differences, clearance issues or heat build up? I know I can order a Hot Rod Deluxe sized cabinet set up for a BJ chassis which would take care of clearnce issues and further enhance the overall tone. Is this something that would work, much like it does in the Deluxe Reissues with the 12AX7 and 6L6 configurations? Thanks for all the tips you give us and work you put into the site.
6L6s run hotter, require more plate voltage than the BJr can supply. It also can’t supply enough negative bias voltage without rebuilding the bias supply from scratch. The larger cab wouldn’t do that much for tone. If you’re going to go with a big cab, just go with a Blues Deluxe or Hot Rod Deluxe.
Bill, what mic did you use on the 1000 dirty bassy Blues sound bite?
thanks
Joe
I used the stereo T mic on my M-Audio Microtrack recorder. It wasn’t particularly close to the amp or anything, maybe 4-5 feet away.
hi bill..running 5881′s in the bj..realisticly is it possibly to run them 100% of the time.. practice/live at all different levels and have no problems? what is the longest test you have done on them..longest time you have had them in one amp?
thanks.
One of my customers runs with them all the time and reports no problems. A couple of other customers use them all the time, too. Some have had concerns about them running hot and have switched back to 6V6s. The risk is up to you!
hi bill..concerns or was there an actual problem..and what would be the first thing to be damaged?
thank you
No actual problems that I’m aware of, just hot-running tubes and hot-running power transformer.
Howdy Bill, I just finished performing several of your mods on a Blues Junior for a customer of mine. Thanks for the concise instructions and complete parts set…everything went well and the amp sounds very good. 3 of the mods were the installation of 6v6 tubes, an adjustable bias control, and the Heybouer Output trannie. I set the bias according to your instructions (to 4.2 volts difference between the red center tap and the brown plate connection.) My question is: when I checked the current draw on the tube by the transformer shunt method (I have been using this method for a very long time) I read 42 milliamps current draw at the plate, but the tubes appear to be operating well. My normal setting for a 6v6 running at 355VDC would be around 23 or 25 milliamps. When I tried to use the adjustable bias control I got little reduction in the current reading. Is there something about the Blues Junior circuit which renders the transformer shunt method of biasing inaccurate? Thanks for your time, I know you are busy. Bird Dog, Bird Dog’s Music House
But the BJr only has around 320V on the plate, so the current has to be higher. If you were reading 4.2 volts across the brown-red leads, that’s actually 31 milliamps across 133 ohms, or about 10W idle. A little warm, but not bad.
If you jumper R51, the 5K resistor, you’ll have a few more volts and can turn the bias up a little higher.
Hi Bill, getting ready to order some upgrade kits for my cream board BJr. But was curious about the octal power tube socket kit with the 6v6s’. What exactly is involved workwise installing this kit ? Couldn’t find any info on the site. Just want to make sure that I don’t get in over my head on this one.
hey bill ,did the octal conversion wow!…my question is about bias for 5881s .. is it the same as the jj6v6 w/ the t020 ot across red / brown?
For 5881s, turn the bias up all the way and hope for the best. The BJr’s bias supply voltage is marginal for 5881s. They won’t redplate, but they’ll run on the hot side. You may want to consider eliminating R51 (jumpering it) to make a few more volts available.
hi bill
im in the process of installing an extention jack socket to my pro junior
what do i exactly need & have to do to do this ?
do you recomend taking off the negatve feedback loop on this amp ?
many thanks
steve ( stevierayb fender forum )
Just extend the green and black wires to the new jack. I like NFB. I think the amp sounds harsh and unrealistic without it.
what kind of socket do i need ? same as fitted ?
The Switchcraft 11A is the right jack.
thanks bill – & your right about the NFB !
hey bill… im running jj 6v6s in my junior one of the many billm mods ive done theyre biased pretty warm as you suggest 32-33 ma(across 1ohm resistor).. i was going to try the tunsol .before i put them in i wanted to cool off the jjs …the lowest i could get was 31.9 per tube before bias pot was maxxed..i was thinking 26-30ma will get me closer to the 60-70% range @ 325v…do i need to change out the r51 resistor? if so what value ?what do you recomend? thanks so much for your help …joe
Jumper the 5K resistor and you’ll get a few more volts of bias.