Fender Pro Junior IV

[Tauruspain]

Hello,
The fender Pro junior is one of my all time favourite amp.
I would like to know what changes fender put into the IV version. Anybody have a schematic or components values with the changes?

Best Regards!

[pdf64]

Well, the Blues Junior IV seems to have moved to cathode bias, so maybe the Pro Junior has too?
No schematics for either seem to have found their way into the public domain.

[ChopSauce]

Don't know, sorry but I didn't connect to the forum to write that. I think you rather should list the flaws you found in the amp and then read/ask/search about how to tune yours to your taste.

At least that's how I would do. There are many examples of a product being degraded to spare a few (fractions of, possibly) cents from one version to another.

[Tauruspain]

Well, I need a softer transition from clean to overdrive and maybe a little less congestioned sound with maxed volume pot.

Best Regards.

[Tauruspain]

In my clones I use 250k log Alpha pots, but maybe the taper is a little abrupt and the amps goes from clean to ovedrive very fast...

[Stevem]

If you currently have more then enough maxed out distortion/ overdrive then a to get a greater dynamic range between clean and maxed out clipping a nice simple change Is to replace the first preamp tube with a 5751 or a 12AY7/ 6072.

You could play around with the feedback resistor, but you need to do so in small steps ( like a 25% change at a time ) and really play the amp a lot in the transition range between clean and dirty and then again at flat out.

If you increase the value of this resistor you will have less feedback voltage.
This will provide a slower change over from clean to clipping , but it will also provide more gain and looser / less tight flat out bass response .

So this needs to be judged with what tube you have in the amp for that first one.

Another side effect is that the amp will have a bit less top to it.

If you go the other way ( increase the resistor value) the result is less gain, tighter low end and more top end and a faster change over from clean to clipping once a certain volume level is reached.

Output tubes can have a big effect on this transition range also.

The amp ships with Groovetubes in a middle of the road number 5 rating stamped on them.
If you install a set with a number 10 rating your clipping will take place some 5% later in regards to your volume level , and this will also help to slow that change over point .

Also in regards with what you posted is taking place, is this happening mostly on the neck pickup, or when both neck and bridge are on?
If so then it might help to move the bass side of the neck pickup away from the strings a big more if you still then
have enough low bottom end for you clean tones.

[rooster]

I have to disagree with Stevem here. Given the same circuit, as the feedback ratio increases, say from 5:1 to 20:1 (‘59 Tweed Bassman - 5:1, to JCM800 - 20:1) the amp has more gain and is looser (as Stevem states) but the higher ratio will also have MORE treble (along with more bass), not less. Remember the typical NFB circuit, as it directs reverse polarity info from the OT to the PI, affects the entire frequency spectrum at the speaker.

FWIW, I have changed the NFB resistance in an earlier BJr and what I heard I didn’t like. There is a Presence ‘roll off’ cap in the PI circuit, a basic shunt to ground pf cap, that you can alter along with the changed NFB ratio if you want, but I’m not sure changing the NFB is a great path. My opinion is that Fender has by now sorted the single biggest selling guitar amplifier of all time.

My opinion? Find an OD pedal that does what you need. You have more than 100 to choose from these days.