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FDP Forum / Fender Amps: 1985 - Current / How the Blues Jnr operates?

Modal Magic

MBJ, Highway Hound.

You Can't Bend It Aussie!
Oct 26th, 2004 02:39 AM   Edit   Profile   Print Topic   Search Topic

I have a circut diagram with my new (2004) Blues Jnr, and would appreciate if anyone could give me a rundown on how it operates. Things like why the Fat switch seems to come into the circut after the first pr-amp tube and it's effect and why there appears to be 5 pre-amp tubes (does the last one have two tubes in one?). Anyway I'm just intrigued by it's operation. Any coments would be appreciated.

Billm
Contributing Member

New Jersey, USA

Hey! What exit?
Oct 26th, 2004 07:00 AM   Edit   Profile   Print Topic   Search Topic

Yes, there are two separate tubes in each glass envelope in the preamp and phase inverter. It looks like five tubes on the schematic because one half of V2 is unused. The first three tubes are identical.

The Fat switch adds a 22uF capacitor to the cathode of the second preamp stage by shorting out the 100K resistor. This increases the gain of the stage and increases the bass frequency response.

The volume control between the first two sections controls the overall gain of the preamp and causes the second stage to distort (heavily, as you turn it up). The signal then goes to the tone stack. Much volume is lost in the tone stack, so V2 re-amplifies it. Reverb is added in at this point, and the combined signal goes through the master volume control to the phase inverter. The PI splits the signal into mirror images, so they can drive the output tubes in push-pull.

There's more to it, of course, but that's the gist of it.

Have you seen the distortion wavevorms on my site? They give you some idea of the effect of preamp vs. power tube distortion.

Billm's Blues Junior Mods

Modal Magic

MBJ, Highway Hound.

You Can't Bend It Aussie!
Oct 28th, 2004 03:48 AM   Edit   Profile   Print Topic   Search Topic

Thanks, Billm. I got onto your site last night and had a read. That in itself gives some great insight into blues jnr operation. One question though, what is meant by a "boxy" sound. The amp sounds great to me, but I havn't had it long enough to push it's full potential.

Billm
Contributing Member

New Jersey, USA

Hey! What exit?
Oct 28th, 2004 06:09 AM   Edit   Profile   Print Topic   Search Topic

Cup your hands around your mouth and speak or sing. That's "boxy."

It comes from the cab being too small for the speaker and as a result, causing the cancellation of some bass frequencies. Fender solved the problem on the classic Princeton Reverb with tone stack values that compensated for the loss. I took the same approach.

FDP Forum / Fender Amps: 1985 - Current / How the Blues Jnr operates?




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