Blues JR Versions vs schematics

[Nathan]

Hey all, at the risk of sounding like a newb:


Do the schematic versions that can be found online correspond to the different releases?


Ex. Rev. B = BJr II, Rev. C = BJr III and Rev. D = IV?


If that is the case, I am guessing that component values will vary from one version to the other.


BillM mods generally refer only to green board vs  cream board.


Thanks for your help!

[vampwizzard]

The revisions don't really correlate to the release editions. This is a handy guide: https://en.wikipedia.org/wiki/Fender_Blues_Junior#Blues_Junior_revision_history

[pdf64]

The latest Blues Juniors look to be cathode biased 
https://www.thegearpage.net/board/index.php?threads/blues-jr-2019-black-board-fried-resistor-and-no-schematic.2319222/#post-33871460

[TitaniumValhalla]

The latest Blues Juniors look to be cathode biased 
https://www.thegearpage.net/board/index.php?threads/blues-jr-2019-black-board-fried-resistor-and-no-schematic.2319222/#post-33871460

Whaaaaa??? How has nobody else noticed this since the OP in that thread says his BJr was made in 2019? It sounds confirmed though with the second poster mentioning the same configuration in his 2021 model.

[acheld]

Wow is all I can say.  I'd love to see some hi-rez photos of that board.

That's a huge change to make in that amp after so many years . . .

[mxrshiver]

Wow is all I can say.  I'd love to see some hi-rez photos of that board.

That's a huge change to make in that amp after so many years . . .

I think there was a single version... pretty sure it was a blackface but it could have been a blonde, that had a weird hybrid grid & cathode bias, that apparently they stopped almost immediately because it was so universally hated 😂 kind of a "worst of both worlds" situation, i believe.

[pdf64]

Various Fender models had revisions for mixed bias in the late 60s. eg the AB568 SR https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_ab568_schem.gif
I’ve never tried any, but I strongly suspect they sound fine and a whole lot of nonsense gets endlessly repeated about them, mostly by people who also have never actually heard them. Or if they have heard them, it’s with worn out valves and ecaps.

Except for the handful of the very early valve rectified versions, Vox AC50 use mixed bias. Conversely, they are pretty well thought of, and I can confirm that they sound absolutely massive.

[ListerHayes]

Wow is all I can say.  I'd love to see some hi-rez photos of that board.

That's a huge change to make in that amp after so many years . . .

A little late, but I just picked one of these up today.  I pulled the back off and saw the "black" board for the first time...  I have more detailed pics if anyone cares to see them.

[acheld]

Interesting!  Thanks for the pics.

Also interesting is that R51 and R52 appear to be 300R.   Earlier in the model run, folks have reported 270R in that position. 

I still don't really understand why they made the change from fixed (well and truly FIXED) bias to cathode bias.

[shooter]

don't really understand why they made the change from fixed (well and truly FIXED) bias to cathode bias.

based on the # of burnt up PA's I seen on the bench, maybe it was better to "customer relations"? 

[pdf64]

It beats me why they ever thought EL84 in hot fixed bias was  a good idea 
Cathode bias here seems all round beneficial, no drawbacks.
Independent cathode resistors even avoids the need for the valves to be reasonably well matched.