Mojotone Princeton Reverb

[Minibucker]

Contacted Mojo and they are sending me three correct shorting jacks free of charge, which is nice of them. And I have the 22K that came with the kit, as well as a 4.7K...and with the tolerances together they measure 27.2K so I'll be using that on the pot. The chassis arrives tomorrow.

Currently, I have the 'Stokes' and 'Paul C' mod, but I'm a little concerned that it will make the amp sound too big so I have it set up so I can revert it back to the stock values if need be.

 

[Minibucker]

Finally got this thing done...took about two days to troubleshoot a bad buzz/ground loop but it was pretty simple. Still waiting for a 12"baffle to be delivered.

Ended up going with the reissue's chassis because of the faceplate dimensions were slightly different from the replacement chassis. Had to ream out all the holes for the new pots, as well as a new hole for the cap can. Might return the chassis or keep it for a future higher-powered project (but might not need that, noted later). Went with a Deluxe Reverb OT, and at first tried both the 'Paul C' and 'John Stokes' mods, with a higher value NFB resistor and a lower reverb mixer resistor (2.2M as opposed to the 3.3M). Wasn't crazy about this....a but too much of everything and sounded like you were literally trying to pack a really big amp into a small box, even through the 12" in a 5E3. Almost sounds like an early Boogie's hard cleans.

Went back to stock (except for the DR OT, of course). Sweeter and cleaner....very nice, but still felt like it could use a bit more push to hit the larger OT harder. So I went back to a 2.2M reverb resistor paralleled with a 15pF, and something like 3.5K on the NFB. And I dropped the bypass cap on V1 from 25uf to 10uf to tame some boominess. Now it seems like a good place to be...big and bold cleans, not spikey at all, and breakup starts at around 7 on volume.

Thing is...this amp is friggin' LOUD for its size now. Easily as loud as any Deluxe Reverb I've had. The OT probably has some to do with it...but I think more importantly, I am using a 5AR4 rectifier and JJ 6V6 tubes which are basically like using 6L6's. I pretty much have to because I'm using the stock reissue's power transformer which is a higher voltage than the various vintage-spec replacements out there. As a result, I'm getting a B+ of around 447 (as opposed to the stock 420), and a plate of 435 (as oppose to the stock 410). Pretty darn high for a PR, and no wonder they bias pretty cold from the factory to keep those stock tubes from exploding. With the JJ's I'm probably pushing at least 18+ watts with this thing now (biased at 23mA). Also, I'm using the JJ Cap Can which has a 40 for the first stage, rather than the 20-20-20-20.

If I want to use other more 'normal' 6V6's, I'll probably have to use the Weber copper cap 5U4 to get that voltage down. I kinda' miss the crunchiness of the original smaller OT, but I also have an Eminence Maverick 12" that I'm going to try when the 12" baffle arrives, and it has an attenuator, so I might just have it live at 7 on the volume. Also, the PR's cab is a bit smaller than the 5E3 one that I'm playing through. But again, I'm almost shocked at how powerful this new handwired project has made it, and almost feel like I overdid it a bit.

Come to think of it...I might just keep it 'big' like this and use the replacement chassis to build a nice quieter one with a lower-voltage PT.

 

[Don]

The voltages with the Mojo PT are much lower. I can only get the plates to 410 if the current is really low. I prefer to jack up the current and let the voltage be low.

I still need a little more "oomph" from my amp with the band I'm in. It's capable of being loud enough but there's not much headroom. It's pretty much the same as the '68 PR. I won't change the essence of the amp to get there- maybe just a more efficient speaker and call it done. Possibly an Emi Copperhead or a Weber 10F150.

I'm loving the tone!

 

[Minibucker]

Try a 12" baffle and an efficient speaker, maybe. I think what I have now is basically a Deluxe Reverb in PR's box...but with that 'rounder' timbre of the PR and a sweeter tremolo. I do really like the way it sounds, though.

 

[Minibucker]

Also thinking of putting diodes on the rectifier, like....

The amp came with BYD33V's, which I still have but a) they're a bit mangled from when I pulled them, and b) I can't remember which is the right 'direction' to have them....

 

[Don]

Try a 12" baffle and an efficient speaker, maybe. I think what I have now is basically a Deluxe Reverb in PR's box...but with that 'rounder' timbre of the PR and a sweeter tremolo. I do really like the way it sounds, though.

I'm avoiding a 12" speaker. If I have to I'll mic it with my e906. It's a great sounding mic and would solve the issue.

 

[Diablo1]

Also thinking of putting diodes on the rectifier, like....
The amp came with BYD33V's, which I still have but a) they're a bit mangled from when I pulled them, and b) I can't remember which is the right 'direction' to have them....

I would just use 1N4007 diodes, you can pick them up at radioshack. I do that on all my builds that have a tube rectifier for safety/reliability. You don't want a shorted tube to burn up your PT, or nuke your filter capacitors with AC voltage.

 

[Minibucker]

I would just use 1N4007 diodes, you can pick them up at radioshack. I do that on all my builds that have a tube rectifier for safety/reliability. You don't want a shorted tube to burn up your PT, or nuke your filter capacitors with AC voltage.

Cool, I have a bunch of 1N4007's already so it'll work out.

I'm avoiding a 12" speaker. If I have to I'll mic it with my e906. It's a great sounding mic and would solve the issue.

I have the same mic, and it's great live. Used to use an MD421, but this one sounds about as good and is much easier to tote around.

 

[sonar]

Stock with a 10" or tweaked with a 12" really shows the versatility of this circuit.

 

[Minibucker]

Looking over the Reissue's schematics...


...looks like they have 445 B+ and 440 on the plates, so yeah this stock power transformer is higher voltage than what's traditionally called for. Probably the same unit they use for other amps in their lineup. Surprised they didn't try to drop the voltage down with zeners or the like, and again, no wonder they come from the factory biased really cold.

 

[Minibucker]

So I got tired of trying to knock voltage down with the reissue's power tranny (had to use a 5Y3 rectifier just to get it under 440 B+ and use Tung Sol 6V6's) and got the correct spec power tranny from Mojo. 330-75-330, 4A at the filaments so I could use a 5U4GB (just like the original schematics of the era). With that, I get 412 B+ and plates biased at 22MA...which is much nicer. Sweeter, a little less loud and not as 'stiff' as it was before with the reissue's PT. And I can switch to a GZ34 and JJ's if I want a bit more.

Got a 12" baffle from Larry Rogers with aged grillecloth. For some reason I seem to come back to the Jensen Neo 12 a lot in Fender amps....it's a very rich, clean speaker with no harsh highs, great sensitivity, and nice and light.

 

[Don]

Looks good! I like the aged grillcloth. The sparkly grillcloth on mine looks a little funny next to my Vibrolux Reverb/

 

[Minibucker]

I do get a little bit of rattle at higher volumes...killed some of it with some felt padding behind the rear panel. Lot;s of people get that on PR's...I think what it is mostly is that the PR's cabinet dimensions are especially sympathetic to a low F (which is where a lot of people get it) and thereabouts, whereas amps bigger don't have it, or have it lower enough that it's not as much of an issue with guitar. It's almost all gone now, though. The amp is still pretty clean up to around 7 because of the bigger Deluxe Reverb OT in it.

 

[Don]

I don't get any rattles with my clone. I'm not putting out the kind of power that you are. The thing's built like a tank, though!

 

[Minibucker]

I got rid of most of it, you'd have to really try to make it rattle now.

 

[Minibucker]

It's a mids pot except that it's a much higher value (I used 250k because I had a few on hand, I've read about using 100k). Turn it up beyond a certain extent and it lifts the ground from the entire tone stack. I moved the original mids resistor to the back of the new pot so it's at the stock setting when the pot is set all the way down. At about 9:00-10:00 o'clock it starts to fatten up a Strat really nice. Higher than that increases the volume of the amp tremendously.

It's a cheap and reversible mod. I'm thinking of trying it on my Vibrolux Reverb as well.

I just put one in, but using a 25K. Very interesting...definitely gives the amp more gain/balls and moves it towards Tweed Deluxe territory, but at the cost of some 'chime'. It's staying in since it's stock value when turned all the way down...will have to see how I like it at higher volume settings.

 

[Don]

I like it with the band, especially with a Strat or Tele. I turn it down at home.

 

[Minibucker]

I could see it being especially cool with a Tele, yeah. I'm actually debating going back to the original smaller OT (as opposed to the DR one I have in now) since vol 7, where it starts to hit its sweet spot, is too loud for apartment playing.

 

[Minibucker]

A little update....

When I rebuilt this thing, I used a Deluxe Reverb output transformer because I thought I could use the extra headroom/low end. But then......I've done this same thing, putting a larger OT from the 'next step up' amp in a Deluxe Reverb, a Pro Reverb, a Bandmaster...and never did I like the results and always went back to the original. It always felt like the amp was a bit anemic and couldn't really get that nice 'bouncy' compression unless you literally dimed it.

But I thought heck, with an amp this small this will be the exception. Well...it really wasn't. So I put the original spec output tranny back in (which took all of about ten minutes), and holy shit does it sound SOOOOO much better...exactly what I felt the amp should sound like when I decided to rebuild it with hand wiring. I should have stuck with my past experiences that when it comes to Blackface amps, the output tranny that it designed with is what will work best. That balance of power in and natural compression/saturation just gives it that life at every volume setting.

As far as whatever volume or low end I thought I would be gaining with a larger OT....hogwash. Again, that natural compression and drive as you hit the original OT makes everything just sound meatier and ballsier...whereas each time I tried larger OT's in other blackfaces, it almost made them feel like the later Silverface ultralinears. Plus, it now really shines with the 12" speaker and I've got more than enough low end to do the job without being complete mush. It's warmer, ballsier, snappier, sweeter...it's a friggin' Princeton now. That's why I tried the 'midrange' knob before, so it could maybe fill in what felt like it was missing. No need for it whatsoever now.

Just stick with the transformers that they were designed with....it's why we like the sound of the old ones in the first place. This amp will still do great in small gigs. If someone needs more volume/headroom from a Princeton...they need a bigger amp than a Princeton, plain and simple.

Night and day......the amp is perfect now.

 

[thin sissy]

Glad to hear you got it sorted! Have you done the Paul C mod on it, and if so do you like it?

I have a 12" alnico silver bell in my clone. I rarely get to crank it these days when at home, but I don't think any other amp has the same chunky sweetness as this one when volume is at around 5.