Fender blues Jr

Jack Walker

Silver Supporting Member
Messages
857
Hi all!

I just picked up a blues jr. to practice with so i wouldn't have to set my other rig up. Can anyone tell me how to correctly bias this amp? Specifics will be greatly appreciated.

Thanks!
 

blair

Member
Messages
267
Unless they've made a major changes in the past few years other than the noticable improvements, they are cathode bias'd which require and/or don't have provision for external biasing. Just replace with a matched pair of good EL-84's and go! Sort of 'plug'n'play'.

You can individually measure the voltage each tube is drawing and change resister's but that's not necessary with this amp.

If the EL-84's are good, play with the preamp tubes to mold your desired flavor of tone.

Enjoy........

Hammer on....Blair
 

Jack Walker

Silver Supporting Member
Messages
857
Blair, Thanks! I think the guy I just bought this thing from paid a Tech to bias it for him. I wonder if he got taken to the cleaners.
I was wondering about that because I have a Maz 38 Sr and had a Mazerati. Both were EL84 amps and the circuit was kind of similar. Both of those amps were cathode bias. Makes sense. Thanks Alot for the info.
 

blair

Member
Messages
267
Hmmm, guess I stand corrected. I had one of the early ones that was factory preset, not requiring biasing.

My bad...........

Hammer on.........Blair
 

fullerplast

Senior Member
Messages
6,781
Originally posted by blair
Hmmm, guess I stand corrected. I had one of the early ones that was factory preset, not requiring biasing.

My bad...........

Hammer on.........Blair
Blair, I think Jon probably should have added that in order to change the bias, you have to modify the stock circuit, cutting some traces and inserting a bias pot (or two separate ones) onto the the circuit board. Probably not exactly what Jack had envisioned, but certainly an option.
 

A440

Supporting Member
Messages
4,640
correct, in their stock form you cannot bias the blues or pro jr amps. you would have to mod it to add a bias pot.

I've been happy with my pro jrs in their stock form :)

simple and inexpensive retubing does help the tone. if you're so inclined, you can also go for a spkr upgrade.
 

Jon Silberman

10Q Jerry & Dickey
Silver Supporting Member
Messages
40,784
Originally posted by fullerplast
Blair, I think Jon probably should have added that in order to change the bias, you have to modify the stock circuit, cutting some traces and inserting a bias pot (or two separate ones) onto the the circuit board. Probably not exactly what Jack had envisioned, but certainly an option.
Don't should on yourself and don't should on others. :mad:

Just kidding! :D

Sorry, I was in a rush when I posted + I've come to the point, in my old age, where I mod the circuits on every amp I own these days to get it perfect, so I guess don't even think twice anymore about doing or recommending it.

Jack, not sure what exactly you had in mind but for a tech who is comfortable with working on PCBs, modding your BJ per the article I posted is not a big deal.
 

blair

Member
Messages
267
I generally mod or alter my amps too! I had a first generation BJ, did the NOS tubes and spkr in various combinations and just wasn't satisfied with what I wasn't able to get out of it. The reverb was marginal too so I sold it. That was 5-6 years ago........

I understand that the BJ's are much improved in their stock config now and people are happier with them.

Anymore my "modded" Rivera Chub 40, Deluxe Reverb and the Marsh Champ Clone kit I built are doing me just fine and meet all my immmediate needs for elec playing. I'm still working on the UltraSound accoustic amp tho........... It's all a never ending game, same as strings, bridges, tuners etc, etc, on the axes.

Enjoy whatever you have and don't be afraid to play with it's inards a bit! Just be sure to bleed the caps so ya don't get zapped.

Hammer on.........Blair
 

Jack Walker

Silver Supporting Member
Messages
857
So obviously what we are saying hear is that the BJ has a cathode bias circuit. There just a better way to bias the amp with the bias mod. It doesn't look too difficult. I may just have to go for it.

Thanks Jon and Blair and everyone else.
 

tiltrite

Member
Messages
670
I'm looking for a Blues jr., and how do I tell the diff between the earlier ones and the later ones? Is it just Made in USA vs. MIM, or is there more to it?

thanks,
tilt
 

fullerplast

Senior Member
Messages
6,781
They have made revisions over the years. In March 2001 they made a significant change to the reverb circuit and changed the reverb drive and recovery IC. That revision level was from "C" to "D". It was intended to clean up the reverb hiss. I don't know how many revisions were made after that, but the rev level should be on the circuit board when you take the back off.
 




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