Next Adventure: Mojotone Princeton Reverb Kit Build

Ed Storer

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Hi Ed, I like the idea fo using the ground switch location for the bias pot, thanks. I love Rob Rob's site, it's an encyclopedia! How come you are selling your amp?

Assembling amp kits is one of my retirement hobbies. I have an Allen Sweet Spot that I assembled back in 2010 that is my "desert island" amp and I don't have need for another. I intend to sell at cost, no charge for my assembly.
 

Blue Bill

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Hi Ed, I like the idea fo using the ground switch location for the bias pot, thanks. I love Rob Rob's site, it's an encyclopedia! How come you are selling your amp?

Assembling amp kits is one of my retirement hobbies. I have an Allen Sweet Spot that I assembled back in 2010 that is my "desert island" amp and I don't have need for another. I intend to sell at cost, no charge for my assembly.
Uh-oh, I'm feeling the bite of the amp-building bug! I checked out the Allen Amp site, they look great. Let's finish this one first, though.
 

wabashslim

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Please feel free to speak up with any hints, criticisms, ideas, or jokes,

OK...
I also managed to strip out one of the little set-screws on one of the knobs, whoops. Here's a couple shots of progress so far:
It's Federal Law that every set of knobs has one pre-stripped or pre-cross-threaded setscrew. Happened to me twice, how else would you explain it? I blame a certain Presidential regime from decades ago...
 

schmee

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Hi Ed, I like the idea fo using the ground switch location for the bias pot, thanks. I love Rob Rob's site, it's an encyclopedia! How come you are selling your amp?

Assembling amp kits is one of my retirement hobbies. I have an Allen Sweet Spot that I assembled back in 2010 that is my "desert island" amp and I don't have need for another. I intend to sell at cost, no charge for my assembly.
I had mine there for years. I have to say though, the pot barely fits and the wires run right past the power tubes. For that reason I used shielded wires with one end of the shield only grounded.

I used a 30k pot I had laying around so it had a lot of adjustment range.

However, on a kit/non vintage amp, I would just drill a 3/8 hole in the normal bias pot location, clean and simple that way.
 

Blue Bill

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I had mine there for years. I have to say though, the pot barely fits and the wires run right past the power tubes. For that reason I used shielded wires with one end of the shield only grounded.

I used a 30k pot I had laying around so it had a lot of adjustment range.

However, on a kit/non vintage amp, I would just drill a 3/8 hole in the normal bias pot location, clean and simple that way.
Yeah, I was thinking also, having a pot on the back panel, it would probably get accidentally knocked around a bit.
 

King Fan

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My plan is to build the kit to "stock" specs, then go from there. Since this is a home build, a "partscaster", so to speak, I don't have to worry about losing value by altering a factory amp, I can go nuts modding.

Good plan -- but before going totally nuts, see Rob's finite list of suggested mods. I did a lot of reading (and I'm sure Rob did too) -- most builders find this isn't an amp that wants or needs a ton of *sonic* mods. Safety or function (adjustable bias): Slow the trem? Yes. And Rob's PI grid stopper? Yes. But after that...
I like the idea fo using the ground switch location for the bias pot, thanks.

Yeah, Ed's idea is super. Re your note about a knob on the back panel, it works perfectly, IME, with Doug's 10KL pot -- the internal slot adjust means there's no knob for your good buddy to give a big curious twist when you're not looking. Here's the 10KL bias pot on my 6G2...

1664293855330.png


FWIW here's the wiring topology on mine -- I found twisted hookup wire was sufficient; didn't need shielded cable even though this runs back to the AC-rich corner of the amp.

21C0D509-EBAE-4EC9-A56B-C8332620FAEF.JPG
Thanks for the diagram, I saved it for future ref.

This mod comes up most of all in PR threads, so for my own understanding I drew a pic of the 10KL pot mounted for external adjustment (I *think* -- smart folks please check -- pot function and on-face CW v. CCW warp my brain).

Simple PR style bias pot rev.1.png
 
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Blue Bill

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Good plan -- but before going totally nuts, see Rob's finite list of suggested mods. I did a lot of reading (and I'm sure Rob did too) -- most builders find this isn't an amp that wants or needs a ton of *sonic* mods. Safety or function (adjustable bias): Slow the trem? Yes. And Rob's PI grid stopper? Yes. But after that...


Yeah, Ed's idea is super. It works best, IME, with Doug's 10KL pot -- the internal slot adjust prevents your good buddy from giving the knob a big curious twist when you're not looking.

FWIW here's the basic topology on mine -- I found twisted hookup wire was sufficient; didn't need shielded cable even though this runs back to the AC-rich corner of the amp.

View attachment 1033643


This mod comes up most of all in PR threads, so for my own understanding I drew a pic of the 10KL pot mounted for external adjustment (I *think* -- smart folks please check -- pot function and on-face CW v. CCW warp my brain).

View attachment 1033641
LOL, that upside-down clockwise thing gets me too. Thanks for the info! Nice clean job on your wiring, I like the big buss bar, or is that white insulation? I'm thinking maybe the ground switch location could be used for a push-pull gain pot for an overdrive channel, future mod?? Just sayin'.
 

Blue Bill

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I built the Mojotone kit last year. They are notorious for not providing enough wire.

For the bias pot, I made a bracket using metal from an old license plate.

View attachment 1033656
Greg, that's brilliant, artistic license. I may copy that. It's fun to see everyone's guts; I have a whole new perspective on these amps now.
Yeah, big square pre-tinned bus bar. Sadly, I can't seem to find anyone who sells it any more. It makes bus building and ground-wire anchoring really easy...
KF, I just noticed in your photo, the bias pot is a recessed trim pot, which is not susceptible to inadvertent knob twisting, very smart. I also like the blonde/brown colors. I have some standard black Tolex and silver grille cloth, but I'm toying with the idea of other color schemes, maybe chartreuse snakeskin? :lol:
 
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King Fan

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KF, I just noticed in your photo, the bias pot is a recessed trim pot, which is not susceptible to inadvertent knob twisting, very smart. I also like the blonde/brown colors. I have some standard black Tolex and silver grille cloth, but I'm toying with the idea of other color schemes, maybe chartreuse snakeskin? :lol:

Heh, that's my blonde 6G2 -- just easier to see the bias pot than on the PR.

You're right, that's the trick I was talking about to use the ground hole without having a knob sticking out inviting mischief. Just use the Fender-style CTS 10KL "bias pot" with a slotted actuator... Doug Hoffman and others sell it. You can't see it here, but there's a slot fairly deep inside the bushing, easily adjusted with a small bladed screwdriver, but impervious to curious friends, toddlers, roadies, and passing furniture...

1664396306668.png
 

jordan86

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I built that same kit in 2020. My first build as well. It’s doable for a newbie. Even if others discourage you. Took me about 20 hours. I was stupid lucky. Worked on the first try.

Mojotone stiffed me also on wire. Not even close. Like short by several FEET. But their support was great and it’s a great kit.

As far as mods, for sure do the bias pot. I also did a lot of the mods that rob robinette mentions here. Some are simple and worth doing as you build. https://robrobinette.com/AA1164_Princeton_Reverb.htm

I used the slotted pot and drilled a hole in the chassis to Mount it where it would logically appear on the schematics. Just by the bias board. I went a little over the top though with lots of other stuff. Even used upgraded transformers. Got one from Dave Allen back when he was still doing that.

Robinette is a guy worth looking up as well as MuchXs here.

You’ll definitely catch the bug though. Consider yourself warned. And have fun!
 

SoK66

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I built a Mojotone 6G16 kit last year. I found they were pretty chintzy with the supplied wire and I also ran short on a few colours and had BARELY enough green.

Regarding wire, although cloth pushback looks vintage correct and is fairly easy to use, after building a Metroamps JTM45 kit some years ago with Topcoat wire I would never use the old stuff again. Topcoat is a plastic coated, pre-tinned stranded wire that is as easy to form as the single strand cloth covered wire, but far better insulated. Plus the pre-tinned feature makes for far easier soldering. You can get the stuff from Valvestorm or AES.
 
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Ed Storer

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I haven't seen a post from Much XS here in a couple of years. Before I got banned from TGP, I did see his posts occasionally there.
I agree with you that Rob Robinette is a terrific resource.
 

Blue Bill

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I got back home last night from a trip. I went straight to my workspace. I lifted the leads on the reverb tranny and re-tested them. Well, it tested fine, so now I have a spare. Re-checked the wiring, hooked up the light-bulb rig, tried it with the rect tube, check. Tried it with the preamp tubes, check. Tried it with the power tubes...ummm...uh-oh. I did remember to plug in the speaker before I powered it up. It's quiet, really quiet, like, not a peep of any kind.

So now I'm stuck.

I have appropriate voltages everywhere I check. On turn-on, the bulb limiter glows for a second, then fades. The pilot light lights up normally. Maybe the speaker is bad? Tried another speaker, still not a sound. :cry:

As usual, I sure would appreciate any troubleshooting tips, thanks.
 

King Fan

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I’ve been AWOL — mighta missed stuff. But the symptom now is good voltages / no sound? The signal path is separate from the voltages, of course; might be anything from input jack to output plug. Someone here can steer you to a build a simple DIY audio tracer to see where the signal disappears.

I lost my notes on the subject, but I recall, tho it was some years ago, it was easy and saved my bacon.
 

King Fan

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OK, assuming the symptom is just totally missing sound, I found the audio trace notes I stole from FenderLover some years ago: "Remove the plug from a guitar cable and solder a 0.1uF cap on the center lead. The other cap lead is your probe. Connect the cable shield to the PR chassis with an alligator clip.

Plug the other end of the cable into another amp with the volume as high as necessary (start low) and then follow the input signal from stage to stage. You will listen for when things start going south. With respectable volume settings on the test amp, it should be clear as a bell as you follow it through.”

You can start by just probing the grid of each triode / tube. Once you find out which sector is eating the signal, you can dig into the signal path Rob shows near the top of his PR page.

I hope that makes sense, and I hope I'm interpreting your symptoms right.
 

GPoint

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Lookin good! It will be interesting to see if you like the E110. They are super bright speakers, but the PR can use a little help in that department, so ... report back how you like it in there!
JBL ceramic magnet E110 (or alnico magnet K110) brightness is coming from an aluminum dust cap, which works like a presence driver in high mids. Can be replaced with a paper dust cap.
 

Mr Mojo 54

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I got a chance to pick up a JBL E-110 for a good price, so I did. I've heard the talk about the PR being the best studio and small room amp ever. Threads like @Ed Storer and others wrote egged me on, and I've never built an amp from the ground up, so here we go. I shopped around and decided the Mojotone kit seemed the best value. I have some Tolex and grille cloth left over from another project, so I got the kit without speaker nor cabinet.

Here's the kit:

View attachment 1033157
Good choice. Mojotone is a good way to go. Have fun.
 

schmee

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JBL ceramic magnet E110 (or alnico magnet K110) brightness is coming from an aluminum dust cap, which works like a presence driver in high mids. Can be replaced with a paper dust cap.
I know this is the 'byword' on the internet, and an aluminum dust cap does add a 'mini touch' of highs in some instances. But actually I have found it to have a more minimal effect than one might think.

I have had D120, D130 JBL's reconed and went with paper dustcaps, minimal difference.
I have put my own aluminum dustcap on some Jensen speakers, Still warm speaker.
I have a PA speaker that came with aluminum dustcap but is still too warm on the highs to use for guitar.
My EV Force 10 quad were probably the brightest speakers I've ever had, and they had paper caps.

So .... just saying.... there are definitely other things going on also regarding brightness in speakers. 👍
 




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