Blues Junior Replacement

[Fresh_Start]

Every time I fix one (or more) things inside my Blues Jr. something else starts making noise sooner rather than later.  I've decided to just gut the amp and start over. 

The amp has five 9-pin tube sockets.  The controls are Gain, Treble, Mids, Bass, Volume, Reverb and a "Fat Switch".  There's not a lot of room on the faceplate to add more.  I'd like to keep reverb in the design, but it's a short tank and I don't know what's available in the right size and compatible with tube-driven reverb.  (the solid state reverb sounds crappy to my ears)

I sent tubenit a summary of the circuit I had in mind and asked what SCH file might be lying around that would come close.  Going above & beyond the call of duy, he assembled the bits & pieces from several schematics.  A big THANKS! to tubenit. 

Here's a thread in the Schematics section that I'll keep working on.

http://www.el34world.com/Forum/index.php?topic=11719.0

It'll take me a while to get to this project, but I wanted to get it fleshed out now - especially after tubenit's generosity.  As I explained there, it seems like it's best to have volume controls both before & after the pentode stage.  However, I'm not sure about the best places for the cathode follower and TMB tone stack.  My thought for this initial cut was to smack the pentode even though there's only one gain stage before it.  tubenit offered up an alternative approach that looks good too.  Also, the cathodyne PI offers a chance for another (sort of) gain stage, but a LTPI might work better.  Finally, I went cathode biased because virtually all of the 6BM8 poweramps I've found are cathode biased. (Blues Jr. does have a fixed bias tap)

I've never designed a circuit before so please accept this for what it is - a very rough first attempt.  It isn't really a scratch design, just an amalgam of the various 5879 circuits here lately, Geezer's Little Wing, and Gibson's GA-8T.

Thanks for any thoughts, suggestions, corrections, etc,

Chip

[jjasilli]

There are old threads on this topic.  IMHO the Blues Jr. is a great amp, poorly executed quality-wise.  I would consider rebuilding a point to point Blues Jr.  Also, I've found that the Traynor reverb circuit (see, YGM-3  Guitar Mate) will drive a SS reverb tank.  If you want Fender reverb, maybe an outboard reverb chassis will be needed for layout space.

[stingray_65]

Hey Chip!

Good to finally hear you're puttin that beast to it's final rest. I think you've been fixing that JR on and off since I've joined the site!

It's also great to see you stepping outside the comfort zone and giving your own designs a try.

I have nothing but confidence in you that this build will be outstanding!

Ray

[jojokeo]

Hi Chip, wondering if you got started on this, went another direction or aborted it?

[Fresh_Start]

Unfortunately, other things have got in the way of amp building.  Also, I'm really unsure about starting off on my own like this.  I would need to build it point-to-point in another chassis first, then design a layout & turret board to fit in the Blues Jr. chassis.

One additional thought has been bouncing around though.  It might be cool to have a triode/pentode switch for the 5879 pentode coupled with some variation of Merlin's "morph" control as a variable boost.  Still haven't put pen to paper on that idea...

Cheers,

Chip

[jojokeo]

I know what you mean about being distracted and/or busy.

Don't let 'being on your own' bother you. It's like reading the paper - the more you do it the better you get at it. Plus if you get stuck or frustrated you have all the help you need here to right the ship and to fall back on. I always find it a fun challenge and exercise to fit what I want into whatever I want. It really brings out the creativity from the building side of things. Sort of like doing crossword puzzles or other games.
I have never done the morph control but I've done the pentode/triode option on almost every pentode amp I've built. No big deal at all but the option one way or another is worth it if you want more headroom at times. More important actually and more useful for me I've found is a variable cap bypass control. No preamp pentode should leave home without it. T has done a great job already & I wouldn't impose but if ya ever need help w/ something just give a shout.

btw, I have a B-Jr w/ a bunch of mods also. I'm recalling work from several years ago but I did the re-rout of the reverb which helped, as well as added the extra unused triode gain stage, added a pot for one of the other popular mods for something I can't even remember now. I haven't even played the amp for well over a year and there's a few other tweaks here and there from several of those sites by BillM, MarkH, SteveA, etc...most of them kick butt really or atleast make a cool difference from stock. I liked that amp but I like mine more. :)

[catnine]

 I was going to turn a Pro Jr into a point to point when twice removing a 12ax7 caused the PC mounted tube sockets to break a solder joint . I once saw on-line a fellow who took a Pro jr and made a plate where the tube socket PC board is and drilled that to mount the new chassis type sockets then he went further and turned the amp into a vox style screamer .

 Once I found out that the pro jr and even my 94 fender tweed blues deluxe were all built using PC boards I was sort of bummed out , they advertised them as true tube amps which they were except the blues deluxe had a effects loop so that part and the verb were not like the old fenders . That's when I began building my own tube amps. I had a pignose GV40 whcih was PC boards but at least the sockets were chassis mounted. Yet all had those cheap pc mounted pots and ribbon wires just looked like trouble to me.

[Rev D]

 I agree the ribbon cables are problematic. You could wire sockets that are hard mounted to the chassis like Bill Machrone (?sp ) did on his 1000th modified Blues Jr. anniversary amp (it has all his mods plus 5881 octal output tubes) like in this picture:



 If I still had mine (sold it due to finances when I got ill and stopped gigging, it was a well used little guy though) I would consider doing all the sockets including the 9 pin preamp sockets onto the chassis and wire with short jumpers like Bill did in the above picture on the octals. Of course I'm sure most have seen the other boards that one of the guys out there (it escapes me at the moment his name I'm sorry) has put into the blues jr's. If someone did build a PTP blues Jr. board I'm sure it'd be like field of dreams... They would come... haha, that said, its a can of worms dealing with people who have no business being on the cool side of a soldering iron so I don't think I'd envy the person coming up with the idea of selling the boards to anyone...   (Back in lurk mode, but hoping you get to this someday Fresh Start!).

Kindest regards,

Don

[jojokeo]

"his 1000th modified Blues Jr. anniversary amp" - jumpin' jahosephat! I would've been bored to tears doing and/or dealing w/ that many of the same darn thing!

"If someone did build a PTP blues Jr. board I'm sure it'd be like field of dreams... They would come... haha"
Funny, I was thinking that too. Also, if it would even be worth it or not even to do my own?! (I don't think so?)

"it has all his mods plus 5881 octal output tubes"
Then I thought of PPR saying something like, "why try to put a V-8 engine into a Ford Pinto? Then the transmission & rear end couldn't handel the tork, the suspension couldn't handle the weight even if the motor fit and the brakes wouldn't even be able to stop it!" 

[tubenit]

Hey Fresh Start,

IF you will let me know what schematic design you settled on ............... I'll draw you up a layout.

With respect, Tubenit

[catnine]

 Just looking at the PC board based amps makes me tense . It's one thing to use a PC board to mount the componants but not tube sockets and pots and the INPUT jack!
 I know when I saw the inside of my 2001 Pro Jr changing the speaker and then because of some screw at the top that might have been the cause of a hum . I looked at the pots and input jack which float in a hole and are secured soley by the PC solder joints and really wondered how long will this last.

 I had a 1994 tweed Pro jr but never had a reason to look inside. Then like a complete fool and not having the funds I needed a larger amp . I wanted the 1994 tweed blues deluxe yet they were out of them so I took the Peavy classic 30 . smaller sounded good and they offered me time which was a few weeks that the new supply of blues deluxes would arrive and a I couls bring back the Peavy and pay the $80 more for the deluxe .

 What stood out to me the most at that time was as a fool like I said I traded two SF champs , the 94 tweed pro jr and some horrid ss park amp in. The fellow there took all four amps plugged in each and cranked them full up in a med sized room and to be honest the pro jr really was not any louder than the SF champs and did not OD any better . It was sort of a surprise to me. The SF champs were stock .

 Well I finally took home the deluxe . had issues and even with that amp I am not sure if the pots were bolted to the chassis or not I do know the tube sockets were not. I even had what fender put out at the same time a new bronco , all ss but claimed to put out true tube tone , this was just a year or so before the pro jr came out.

 To be honest the chassis on the pro jr or blues jr are really not all that sturdy. They may serve well as a build like some do using ss amp chassis .I was going to do that once using a working vox ss small combo but decided against it and sold it.

[tubeswell]

... I'm really unsure about starting off on my own like this.  I would need to build it point-to-point in another chassis first, then design a layout & turret board to fit in the Blues Jr. chassis.

Hi Chip, its not as daunting as it first seems. The BJ chassis is a cinch to work in if you are gutting everything and doing your own board and handwiring in new sockets. If it were me, I'd make the output sockets bigger for octals, and make another noval socket hole for a 4th pre-amp tube.

But if you wanted to keep them the same, you could use 6BM8s and use the triode part of each for 1/2 an LTP. Then you would still have 3 pre-amp sockets, which is enough for a one channel BF style pre-amp with reverb (with something like the Allen Old Flame pre-amp topology)