Anyone do the Fender Blues Junior bias mod?

FatJeff · #1 01-05-2011, 09:38 AM

OK, so I bit the bullet and decided to order a new "relic" version of the FBJ. It's on its way to me as we speak. But, given that my primary genre of playing is jazz, I'm seriously considering doing the following mod after having read this article: http://home.comcast.net./~machrone/bjr/bjbiastone.htm.

Anyone else try this? Was it an improvement?

spiral · #2 01-05-2011, 11:38 AM

Quote:

Originally Posted by FatJeff

Anyone else try this? Was it an improvement?

Sorry this is slightly tangential, but hopefully useful: I did Bill's Princeton (RRI) mod with the output transformer and it made a huge difference in tone. I'm typically very skeptical and think that mods are too minor to bother with, but in this case i was pleased. FWIW.

Other Bill M. mods:
http://www.billmaudio.com/

FatJeff · #3 01-05-2011, 11:45 AM

He's got some interesting stuff on his pages. Is this the article you're talking about? Billm Audio » Upgrade Output Transformer for Blues Junior Seems pretty compelling...I'm afraid I might be on my way to becoming an amp hacker!! (I do have a graduate degree in electrical engineering, after all)

spiral · #4 01-05-2011, 11:53 AM

Quote:

Originally Posted by FatJeff

He's got some interesting stuff on his pages. Is this the article you're talking about? Billm Audio » Upgrade Output Transformer for Blues Junior Seems pretty compelling...I'm afraid I might be on my way to becoming an amp hacker!! (I do have a graduate degree in electrical engineering, after all)

That's the one. I can't speak to what it does for the Blues Jr. but it helped the Princeton Reissue. I did that one and made a new baffle out of hardwood plywood and you would never know it was a reissue (or i wouldn't know). Sounds awesome. The bass was really flubby and boomy but the transformer mod seemed to make a difference (i'm sure the baffle helped too).

For the mod you were asking about, the parts didn't seem that expensive. If you are interested, why not try it?

DArmstrong · #5 01-09-2011, 08:08 AM

Hi. First post to jazzguitar.be (and probably the last that won't be a newbie question)!

I like a big, strong clean tone. Probably a Twin Reverb would be my ultimate amp. When I became aware of BillM's great Blues Junior site I bought a cheap used one and modded like crazy, trying to achieve a mini-twin. I made the bias adjustable, put in a huge Hammond 1650E output transformer, added a presence control, changed the tone stack capacitors, changed the speaker, doubled the power supply filter caps... All the BillM stuff.

Here's what I'd do, roughly in order of tone-effect-per-modding-hour.

- Changing the tone stack caps to BillM's suggested values. More bass available. In a band setting you might not need that, but you can always turn the bass down then. Very easy mod.
- Lowering the bias. My BJr never really quite sounded clean until I did this. Very easy mod. Once you've done this, changing tube V1 to an AY7 keeps you clean even with stronger humbuckers. (Changing the power tubes? I tried some JJs and they were a little different from the stock Sovteks but not as big a deal as I'd been expecting.)
- Double up the filter caps. Very easy. I did this at the same time as changing the output transformer, but doubling the caps alone might give enough extra oomph to the bass to suit you.
- Output transformer. Made for a somewhat harder-hitting bass, but a speaker with more bass response might help too, if it let's you keep the bass control lower so the OT doesn't have to deal with as much bass in the signal. (If you change the OT, DON'T use Hammond 1650E. It's Too Big, and you'll have to move the speaker to fit it. Use BillM's suggested OT instead.)

Hope this helps. It's fun stuff to do.

And thanks to everyone here for a great forum.

kenbennett · #6 01-09-2011, 09:15 AM

If you install a mod to allow for adjusting the bias, then you might want to check out some of the technical articles at Aiken Amps website. He describes a very reliable way of setting the bias to a calculated point based on your own preference of cooler or hotter. It doesn't require a scope or any subjective evaluation of the crossover distortion waveform. A little math and a milliamp meter is all you need.

Some people don't like Groove Tubes. One cool thing they do though is rate their output tubes according to how early in the gain curve they clip. So you get a 10 rated tube if you want the cleanest possible sound with the most headroom.

medblues · #7 01-09-2011, 08:42 PM

I have a Blues Jr with BillM's mods (most of them). I increased the headroom by installing an Emi Wizard speaker which which is 4 dB more efficient than the C12N it had. Plus I put in a Ruby reverb tank which is darker than the stock one. It is now a very acceptable tube jazz amp. I read negative reviews about the speaker (C12R ?) in the relic version.

DArmstrong · #8 01-09-2011, 08:49 PM

I also got the Ruby reverb and liked it more than the stock. I also changed the bright cap on the reverb for a lower value to darken it a bit more at the low reverb settings I like.

FatJeff · #9 01-09-2011, 09:40 PM

I've been playing through it now for a few days. It sounds absolutely great! Of course, I'm used to playing through my little Lunchbox, which I've found to be very "boxy" and mid-rangy. The bass on this thing (12" vs 6") is incredible and lends a very rich fullness to my guitar's sound.

I'll wait to mod this thing until after I've played out with it a few times. I've got a jam tomorrow night and a few gigs this semester, so that should give me a better feel for where I'm at with it then.

Thanks everyone for the tips.

DArmstrong · #10 01-09-2011, 09:52 PM

Glad you're enjoying your new amp, Jeff! After all the messing around I did with mine, I heard a guy playing jazz (big hollow archtop) in a combo setting with a BJr and he sounded great. His amp was...

... completely stock!

Made me think I should practice guitar more and amp modding less!