My new Mojotone Princeton Reverb build vs. a 2017 PRRI

Joulupukki

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86
At the beginning of January I placed an order for a complete Mojotone Princeton Reverb kit. At about the same time, I also found a used PRRI and bought it so I’d have something to compare with and tinker on in the meantime. During my wait for all the parts of the Mojotone to come in stock I’ve been having fun getting familiar with the PRRI, making the slower trem mod to it and studying up on the schematics.

My Mojotone kit finally came Mar 17 and I pretty much lost a lot of sleep staying up really late a couple nights working on it. It was better than Christmas morning. :) I made a few mods on the Mojotone while building it and they all turned out pretty well, mostly stuff I learned on http://robrobinette.com/

I didn’t realize that I needed to buy the ON-OFF-ON ground switch. The one that came in the kit is just an ON-ON switch so I wired it with normal NFB and half NFB. I may crack it open again and disconnect the one side just to check out what no NFB is like.

Because it was my first amp build (I’d done other electronics projects so fortunately it wasn’t my first time soldering/etc) I definitely took my time and measured all my resistors and wrote down most of the values while going along. I think that, for sure, took extra time.

I was kinda scared to fire it up for the first time. I used a light bulb current limiter but was still pretty careful with my safety glasses on. Fortunately, each step of the initial power-up and testing turned out great. But, when I went to plug in my guitar for the first time, zero guitar sound! That was the cause of a really late night past midnight. There was no way I would have slept without figuring it out. Fortunately, it turned out to be something with the input jacks. Those were about the first things I wired up and had accidentally not clipped one of the 68K resistor leads and so it was shorting out on the back of input #2. It was so fun hearing it for the first time. It sounds goooood!

The other thing that somehow I am not sure how I missed was that when I went to add in my lower-valued resistor for the trem depth mod, I ended up with a resistor that was about 360K instead of 330K. It was a noticeable difference between the depth of the trem on this new one vs. the PRRI which I had made the same mod to. I cracked it open again and replaced that resistor with the one that I had bought specifically for it … but had somehow forgot to do it. Crazy.

I will say that I think I prefer the Jensen C10R speaker that‘s in the PRRI vs. the Jensen that came with the Mojotone (C10Q). The Mojotone also seems to have a LOT more pronounced bass response. I can turn the Mojo‘s bass knob down to 1 and it still matches the PRRI’s set to about 3. I’m not sure that I like that part of it. I also have to turn up the treble higher than the PRRI to get it closer to match. The PRRI sounds maybe a bit more crisp? I’m not sure how to describe it – but maybe a tiny bit less muddy overall. I’ve plugged the PRRI’s speaker into the Mojo and vice versa just to get a good idea.

Both are definitely excellent amps and it was a ton of fun to build the Mojotone.

These are the mods I made:

- Slower and deeper tremolo mod
- Used higher wattage input resistors
- Ground switch switches between normal Negative Feedback and 1/2 normal negative feedback
- Higher wattage on the voltage dropping resistors
- Phase Inverter Grid Stop Resistor
- Power tube and screen resistors

There are a few things I’ll probably still change. One of them is to use thinner shielded cable. I ordered some from Mojotone but what it turned out to be was standard instrument cable like what I use to plug my guitar in to the amp. It‘s a bit bulky inside the chassis, but I guess it works. I used it for: input and volume to V1, speaker out to the ground switch, and from the resistors on the ground switch back to where the NFB returns onto the circuit board.

Here are some pictures…

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Here are a couple pics of what I did after using a solder sucker when swapping out the resistor for the trem depth mod. I bent the leads so it’d be easier to have some stronger contact with the eyelets since I didn’t want to take the entire thing apart to gain access to underneath the top board.
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Joulupukki

Member
Messages
86
The tremolo just didn't sound as good as the PRRI (which I have also done a slow mod to). It also seemed to be running physically hot. I should have done this before but I took some measurements of the plate current with a Eurotubes bias probe today. It read 34mA. Yikes. So, I cracked back open the chassis and took more measurements. Wow, it was running HOT.

Fortunately I had an extra 5.6K resistor (which is reading actually 6.53K). So, I cracked it open and took a few voltage measurements first to confirm what I was seeing with the bias probe. Yup.

I connected the 5.6K resistor in series with the 22K one and now I'm getting about 28.5 - 29K which is making the bias probe read right around 21mA. The top of the amp doesn't feel quite as hot above the power tubes and the tremolo now has the depth I was hoping for in the first place!

Here's a shot of the mod. Hopefully it's ok to do it like that. Seems to work well enough. If I had a 10K linear potentiometer I would have gone that route. I might still do that in the future, but for now, this lets me fire it up without burning out the tubes. Whew!

IMG_0112.jpeg
 
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rumbletone

Silver Supporting Member
Messages
8,146
The tremolo just didn't sound as good as the PRRI (which I have also done a slow mod to). It also seemed to be running physically hot. I should have done this before but I took some measurements of the plate current with a Eurotubes bias probe today. It read 34mA. Yikes. So, I cracked back open the chassis and took more measurements. Wow, it was running HOT.

Fortunately I had an extra 5.6K resistor (which is reading actually 6.53K). So, I cracked it open and took a few voltage measurements first to confirm what I was seeing with the bias probe. Yup.

I connected the 5.6K resistor in series with the 22K one and now I'm getting about 28.5 - 29K which is making the bias probe read right around 21mA. The top of the amp doesn't feel quite as hot above the power tubes and the tremolo now has the depth I was hoping for in the first place!

Here's a shot of the mod. Hopefully it's ok to do it like that. Seems to work well enough. If I had a 10K linear potentiometer I would have gone that route. I might still do that in the future, but for now, this lets me fire it up without burning out the tubes. Whew!

View attachment 546928
When you say hot, was the chassis itself getting hot? I noticed mine is recently, so may try this mod.

I did the grid stopper, NFB, and mid pot mods to my mojotone kit:

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Joulupukki

Member
Messages
86
When you say hot, was the chassis itself getting hot? I noticed mine is recently, so may try this mod.

Nice looking build! I’ll probably pick up a pot for the mid mod as well. I’ve noticed it also has a huge amount of bass. Even at its lowest level it’s quite a bit more than the PRRI.

So the top of the amp right above where the power tubes are was definitely pretty warm. I noticed that when I’d turn on the tremolo the power tubes would pulse with the trem intensity. I have a Eurotubes bias probe and it was reading 34mA, which in my estimation was way too high.

I used this to learn more how it all is supposed to work and took measurements of my amp: https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

It confirmed the bias current was over 33mA.

My PRRI is running at about 23mA using the bias probe. With the 5.6K in series with the 22K resistor, my Mojotone is now running at about 21mA (bias probe reading). I believe I read somewhere that a bias probe will give values that are a little higher than actual.

Here are the measurements I took…

Output transformer voltage before the mod: 356.1 V
Plate voltate, plate-to-cathode resistance (warm, but powered off and caps drained), and voltage drop on V5 and V6 respectively were:
V5: 350.8 V, 160.5 ohms, 5.3 V
V6: 351.1 V, 149.5 ohms, 5 V
Bias current: Voltage Drop / Plate-Cathode Resistance
V5: 5.3 / 160.5 = 33.02mA
V6: 5 / 149.5 = 33.44mA


After the mod:
Output transformer voltage: 379.0 V
V5: 376.0 V, 157.2 ohms, 3 V
V6: 376.3 V, 141.8 ohms, 2.7 V
Bias current:
V5: 3 / 157.2 = 19.08mA
V6: 2.7 / 141.8 = 19.04mA

My power tubes are the JJ6V6S and I used a value of 376 V on this calculator (I think that’s what you’re supposed to do): https://robrobinette.com/Tube_Bias_Calculator.htm

According to that calculator…

Class AB Fixed Bias:
50% Cool: 18.6 mA
60% Average: 22.3 mA
70% Max Safe Dissipation: 26.1 mA

So it looks like it’s running near the cooler side. In any case, the tremolo depth finally sounds amazing and still plenty loud overall.
 

rumbletone

Silver Supporting Member
Messages
8,146
Thanks for all the great info!

I recall that I ended up changing one resistor in the trem to make it deeper and that worked great (with the stock value it was very weak).

On the heat issue, I’ll check the bias - even if that’s not what’s causing it on mine, it makes sense to keep it within an appropriate range, and as I’m learning (i) the stock Princeton Reverb circuit had some less than ideal values, and (ii) mojotone doesn’t seem to take any steps to ‘fix’ issues that almost always should be (like how they still show ‘death cap’ wiring on some of the kits, and don’t implement the fuse in the recommended manner …).
 

Joulupukki

Member
Messages
86
It’s definitely worthwhile to put a Bias pot on this amp. Work smarter, not harder.
Yeah, I will order one with the next bunch of stuff. This quick hack fortunately will hold me over ‘til then. I’d like to find a place I could mount it by drilling a hole through the chassis and access it with a screwdriver or even just the shaft of the pot from underneath without having to take the whole chassis out.
 

coltonius

Señor Member
Silver Supporting Member
Messages
14,160
Yeah, I will order one with the next bunch of stuff. This quick hack fortunately will hold me over ‘til then. I’d like to find a place I could mount it by drilling a hole through the chassis and access it with a screwdriver or even just the shaft of the pot from underneath without having to take the whole chassis out.
Not quite what you’re after, but I bought an L bracket at Home Depot and mounted it thusly for an Alpha pot inside the chassis:

D666A980-244B-46BD-BCF1-F19E567FEAF1.jpeg


Here’s my DR build on the left. I got my chassis and faceplates from Modulus UK and the head/speaker cabs from GuitarCabinetsDirect.com!

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Joulupukki

Member
Messages
86
Not quite what you’re after, but I bought an L bracket at Home Depot and mounted it thusly for an Alpha pot inside the chassis:
That is a really nice looking build! Good call on the L bracket. I'll probably do something like that. Looks easy enough.

Are your shielded cables just some of the extra cabling from the reverb wires? I noticed there is a LOT included in the kit. I coiled some of it up inside the reverb bag to take up the slack. I could probably easily use that instead of the thick instrument cable I used (for input/volume and NFB wires). Although, it doesn't have an insulated jacket on it. Maybe that's why you suspended it away from everything on your volume and input jack wires?
 

coltonius

Señor Member
Silver Supporting Member
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14,160
That is a really nice looking build! Good call on the L bracket. I'll probably do something like that. Looks easy enough.

Are your shielded cables just some of the extra cabling from the reverb wires? I noticed there is a LOT included in the kit. I coiled some of it up inside the reverb bag to take up the slack. I could probably easily use that instead of the thick instrument cable I used (for input/volume and NFB wires). Although, it doesn't have an insulated jacket on it. Maybe that's why you suspended it away from everything on your volume and input jack wires?
Thank you! It’s one of my favorite builds for sure.

Yeah, that’s extra cabling from the reverb wires! It’s not super flexible in a way I would’ve considered neat to run under the board, so I just went over the top with it. I think it really makes a difference in keeping out noise.. I used smaller insulated wire for my 3-way NFB toggle since it’s a longer run and I had more of it. Gets the job done!
 

Joulupukki

Member
Messages
86
I'm thinking of installing an SPDT switch for a bright cap switch. In the middle position it'd be stock (no bright cap). Bottom would be maybe a 47pF cap and top position would be maybe 120pF. I'll experiment with the caps to see what kind of effect they'll have on it. If I end up liking the extra options, would it be weird to replace Input Jack #2 with that switch? I know I could drill into the faceplate and chassis and put it in between Input #2 and the Volume knob but I have never used Input #2 anyway. Thoughts?
 

Joulupukki

Member
Messages
86
I'm thinking of installing an SPDT switch for a bright cap switch. In the middle position it'd be stock (no bright cap). Bottom would be maybe a 47pF cap and top position would be maybe 120pF. I'll experiment with the caps to see what kind of effect they'll have on it. If I end up liking the extra options, would it be weird to replace Input Jack #2 with that switch? I know I could drill into the faceplate and chassis and put it in between Input #2 and the Volume knob but I have never used Input #2 anyway. Thoughts?
Changed my mind and just drilled a small hole between Input #2 and the Volume knob for an ON-OFF-ON mini switch. I’ve got it wired BRIGHT (50pF) bottom position, normal in the middle and BRIGHTER (100pF) on the top. I definitely like having both options and with it on the bottom position it seems like the clarity is better. I’ll probably play it in that position most of the time. I’ll post a pic later.
 

Joulupukki

Member
Messages
86
I built a little bracket out of some scrap I had to mount a 10K potentiometer so I could dial in the biasing. I had to add a little extra wire since I had cut the 22K resistor lead when connecting it earlier to a 5.6K resistor so it’s not as clean as I’d like, but it all works and now I’ve got it dialed in. :)

I’m not so happy about how close the shaft is though to the cap can. Should I admit that the tip of my thumb got a couple shocks when I wasn’t being really careful? Hmm. Maybe I could spin it around the other way eventually so the knob is on the left.

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Joulupukki

Member
Messages
86
Another mod I tried last night and this morning was a MID control potentiometer mounted in place of the external speaker jack. I didn’t really like much of what I got out of it so I removed it and I think for me it’ll be just great.
 

Joulupukki

Member
Messages
86
Here’s a picture of how the BRIGHT - OFF - BRIGHTER switch turned out. I think this is one of the best mods I did in terms of having the option to bring it out of the mud when wanted.
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Joulupukki

Member
Messages
86
Regarding the NFB switch I was going to change it to an ON OFF ON SPDT switch but I accidentally ordered the wrong ones twice (doh). So, I’m just going to keep the one Mojotone sent with it and switched to the left it’s not connected, so zero NFB. To the right, the normal NFB. When I tried half NFB with a 5.6K resistor I didn’t really hear too much diff between that and none so this should work great.
 

Joulupukki

Member
Messages
86
A little bit ghetto, but last night I used a Boss Katana as a power amp to connect the speaker leads instead to the speakers in my Mojotone PR and the Fender PRRI. Then, I used some software (Impulse Response Utility on the Mac … bundled as part of Logic Pro) to record and create IRs for each cab.

I recorded on and off axis samples of both cabinets using these different microphones:

- Shure SM57
- Shure KSM32
- sE Electronics sE5

This is my first dabble in creating IRs and I’m shocked how well it actually works. I loaded them up into the Torpedo Captor X and they sound pretty legit. I had a few little snippets of playing saved in my looper so I could compare.

I powered on each amp separately and recorded a few different tracks into Logic Pro for some comparison and here’s what I found…

1. The Jensen C10R inside of the PRRI is a brighter speaker than the Jensen C10Q that came with the Mojotone PR.
2. When recording the samples I had to adjust the mic sensitivity up higher when recording the Jensen C10Q. Pretty sure this is because it’s a little higher wattage speaker (35w vs 25w) than the C10R.
3. The PRRI amp is a little brighter than the Mojotone. The bright cap switch I added to the Mojotone helps get them closer to each other but there’s still definitely a difference. In isolation and without playing the other amp for a while, they both sound great. But if you’re switching back and forth between the two and switch to the PRRI it feels like a light blanket got lifted off of your sound and it’s just more clear and present.

As far as my personal preferences in which IR turned out best (which mic sounds best), in order from favorite to least favorite…

1. Shure SM57 gives the best guitar amp tone I feel and this was true for both cabs
2. Shure KSM32 is a very close 2nd and it feels a bit more full-range and more articulate
3. sE5 I probably wouldn’t use because it sounds too bright and harsh. I could probably re-record it with a better mic placement, but probably won’t bother since the other two work well already

My next experiment will be to record the amps directly with the microphones straight into Logic and no IR just to compare those. I’ll do that a little bit later today using the same pre-recorded tracks from my looper and then include some files.

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