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Critique and mod ideas for Mojo Princeton Reverb

Discussion in 'Shock Brother's DIY Amps' started by jrgtwo, Oct 13, 2020.

  1. jrgtwo

    jrgtwo TDPRI Member

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    Hey all,

    First post here, but this forum helped me get through a previous 5f2a build that turned out great.

    Just finished my Mojotone Princeton Reverb kit and wanted to get some feedback on it.

    It sounds great to me, the tremolo is a bit weak, I'm planning on adding a bias pot to it, hoping that helps a bit.

    Running it into a 12" Amperian VSOP that sounds good but needs to be broken in quite a bit.

    Let me know what you all think, second amp build but it's becoming addictive

     
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  2. 63 vibroverb

    63 vibroverb Tele-Afflicted

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    Looks great! I personally like to add a mid pot on blackface/brown face amps that don’t have one. Makes it much more versatile. A 25k pot should give you more range than you’ll ever need.
     
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  3. jrgtwo

    jrgtwo TDPRI Member

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    Great idea. Thanks!

    Will look into what it will take to add a mids control
     
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  4. King Fan

    King Fan Poster Extraordinaire

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    Welcome. Good looking build!!!

    What's your bias measure now? A bias pot is a super good idea. Doug Hoffman's method is widely used; it's easy and effective.

    I agree with Rob's take on PR mods. He recommends only a few, especially a bias pot. In fact, they're mostly in power handling. A 12" speaker is popular, but staying at 10" also has a lot of strong support.

    Always smart to agree with Rob, you say? Well, even before he put up his PR page, I listed and reviewed my PR mods here, after reading quite a few other folks' reviews, and we got other useful comments along the way. I ended up with a small mids pot and don't use it too much; same for my (later, further modded) 3-way NFB (off, adjustable, stock). Not bad, but not life-changing.

    Oh minor details, but did you follow the Mojo layout on AC wiring? They often switch the neutral; many experts prefer to switch the hot. I can't follow your hot, but the common wisdom here is hot to fuse center, then fuse side out to switch. Your amp will work the way Mojo drew it; experts just note safety trade-offs if fuses fail, switches fail, etc.
     
  5. tubegeek

    tubegeek Friend of Leo's

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    @jrgtwo

    Welcome! <- take this personally

    Don't take this personally -> I think one thing that often gets overestimated is how much can be determined from gut shot photos.

    For example - I'm often viewing the forum on my phone, and I hardly even try to trace wiring, ID resistors, etc., from my little phone screen.

    On the other hand, I'm sometimes surprised by what the other forum members can spot.

    Take this in -> even reputable websites contain errors or practices that are not universally approved. So you may step into mini-controversies from time to time and have to weigh whether the disagreements need to concern you. Some do, some don't.

    And: in my OPINION you don't want a switch in the neutral unless it is a two-pole switch.
     
  6. jrgtwo

    jrgtwo TDPRI Member

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    Thanks for the feedback.

    And thank you for pointing me towards the Rob Robinnete sure. Lots of useful info there to keep me busy!
     
  7. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Looks great! There is a mod to strengthen Tremelo, replace the 1 meg resistor to the left of the 3 tremelo disc caps with 470k or so. Are you sure you have 1 meg there now? I cant quite tell but doesn't appear to be....

    Bias pot yes!
    Mid control.. MEH.
    Stokes Mod etc? NO.
     
    Last edited: Oct 15, 2020
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  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Welcome to TDPRI.

    Critique...
    Good work! Your work looks very much like the originals.:)

    Nit pic...
    The heater wire could be wound tightly.
    The originals did not have the heaters wound tightly, so... you are in good company.

    Do you have any hiss or hum? Do you think it needs to be modded for something you don't like?
     
  9. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    I built one with the Stokes mod. It breaks up quite late. The next one I do will be stock with a 10" speaker.
    The trem is weak in mine too.
     
  10. tele_savales

    tele_savales Tele-Holic Platinum Supporter

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    I didn't scan your photos with a microscope, but it looks really good to me. I personally would do the heaters again, but I'm kind of anal. It's a hassle but worth it. I try to twist mine super tight and give myself about 3/4" of untwisted on each end, bent straight down towards the socket. Once the flailing ensues you might lose a bit of the twist but it's negligible.
     
  11. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Welcome!

    The way it sounds is most important. If you think it sounds good, it is good.

    Some minor point of constructive criticism, only because you asked for it:

    - The unused power transformer wires are a bit long. They will act as a small antenna.
    - Heater wire could be twisted a bit tighter. It's hard to see, but the heater wires should go up/down at the tubes and fly through the air inbetween them.
    - The yellow wires to the rectifier tube are a bit long and are running quite close to one of the power tubes.
    - You've twisted both grid wires of V1 together. One is low signal, the other depends on the volume. I would keep them apart, although it seems quite normal to do it as you currently have. In fact, I would use shielded wire for both.

    and then there are all the points where this and other forums are full of that are heavily debated :)
     
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  12. Mongo Park

    Mongo Park Tele-Holic

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  13. ElliotKnapp

    ElliotKnapp TDPRI Member

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    Congrats on this build! PR is one of my favorite builds so far (I've done 2!). Changing the 1M resistor for more intense tremolo is worth trying, but be aware--go too low on the resistor and you may run into issues with the tremolo distorting at the peak of the oscillation (I had this issue with a 6G3 I built and it took some experimentation with resistor values to find the best compromise). I'd suggest getting some other values between 470k and 1m and clipping them in place of the 1M to see what sounds best.

    My other favorite mods are 3-way NFB (I use the standard 2.7, none in the middle, and AB763 820Ω values), a 3-way bright cap switch across the volume pot, and especially the reverb Dwell control, which makes the reverb way more tweakable. All are easy to implement with a few parts, though most require chassis drilling, which can be challenging without a drill press. Also, you definitely want a bias pot--I'd recommend doing that first and adjusting to ~50%+ plate dissipation prior to making any trem resistor swaps. Make sure you're discharging those filter caps between tweaks!!
     
  14. jchabalk

    jchabalk Tele-Meister

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    @ElliotKnapp Do you remember what value resistor you settled on for the tremolo intensity? Last night i was trying this with mine, I had 3 1M (total, 2 in addition to the one that is already in the circuit) clipped in and it was really nice and deep. I want to replace the 1M i have in there, i'm thinking maybe a 220k or there abouts. i was going to order a few different values between 100k and 470k to see what they sound like.

    I also added a .022uF cap to the tremolo circuit (.01, .01, .022 +.022) and it slowed it down pretty good too. It went from ~230bpm stock to ~175bpm at its slowest with the additional capacitance. As you turn up the speed pot it still gets way faster than i use.
     
  15. ElliotKnapp

    ElliotKnapp TDPRI Member

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    @jchabalk For the 6g3 I believe I upped it to 470k from the stock 220k, which was giving me distortion (very noticeable on clean amp settings). I tried values above this and the intensity dropped more than I preferred. Are you saying you clipped in 3 1M resistors in parallel? That'd put you at 333k, which is a reasonable value. I've actually had differing results in the two Princeton Reverbs I've built--the first was getting a touch of distortion even with the 1M, and the second sounds a little weak at 1M and with cool bias--I might have to bust it open and start experimenting with it, too! As far as slowing the trem, I don't even bother with the stock cap values--.022 for all 3 is my go-to. If the max tremolo speed is way faster than you need, you could also experiment with different resistor values across the Speed pot--higher than the stock 100k will reduce the top speed.
     
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  16. jchabalk

    jchabalk Tele-Meister

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    I was just out in the garage digging through my parts stash and i might have found what i need to sort this out this evening.

    I was at 3x 1M (333k total) and i'm pretty sure i have a 270k sitting here, i'll probably pull the 1M and clip in a couple of options this evening and solder my choice in place.

    I was also thinking of trying the raw mod to dial out the tone stack but i think i'm going to skip that one. That and the change to negative feedback are things i've done in other amps but i tend to not use and it just adds another thing to mess around with. I really like the tone of this Princeton.
     
  17. jchabalk

    jchabalk Tele-Meister

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    I just went ahead and did this. @jrgtwo this might be useful info for your situation.

    I pulled the 1M (carbon comp) resistor in my trem circuit. It measured 1.25M sitting on my desk. and i replaced it with a 250k (measured 253k on the desk before installation). The tremolo is intensity is pretty amazing now, almost makes me dizzy how deep it is.

    Then i added a .022uF capacitor to the trem circuit and it slowed down from ~230bpm (stock) to ~140bpm at its slowest. I wouldn't tend to use it that slow but it sounds great and it still gets real fast at the top of the pot - faster than i can measure with a tap tempo, and faster than i'll use.
     
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  18. jrgtwo

    jrgtwo TDPRI Member

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    Thanks all for the input it is greatly appreciated,

    Sorry for not getting back earlier, been busy with work.

    Definitely the heater wires could have been better, they started super tightly wound, but my fat fingers slowly messed all that up when I got to soldering :lol:

    After about a week heavily running the amp, I can say that I couldn't be happier with it. I had been chasing fender sounds but never actually knew what I was chasing (took a while to realize I wanted that fender sound). /Actually had a Hot rod deluxe for my first amp but it just wasn't for me.

    The amp has very little noise and just sounds great. I have the parts for nfb switching and bias pot upgrade, but I've been so happy with the sound that I've actually started to gut my 5f2a chassis and parts to make a ga-5. Just want to try something a little different without screwing up the Princeton :p. Will post updates when I get deeper into that.

    I've been playing almost only soul and motown and it is just great, I had a supro Statesman which is a great amp but just too much power for me, this Princeton is the perfect amount of power for my house, and it has just enough crunch when my wife isn't home:D

    Oddly the more I use the Princeton the better the trem sounds, maybe I'm just getting more used to how it sounds though

    Thanks again for all the input, once I get some more time with the amp I'll be updating it with some of these updates, just want to get used to the sound first
     
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  19. Wally

    Wally Telefied Ad Free Member

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    The AC input needs to be re-wired. The neutral should go directly to one PT primary winding end. The hot black should run through the fuse, to the switch, and then to the other PT primary end. That is ‘best and safest’.
    Do you have voltage and bias measurements?
    If you do a midrange pot, I would suggest going with a 50K. Ommv.
     
  20. Nickfl

    Nickfl Friend of Leo's

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    If you decide to add a bias pot, it would also be a good idea to add one ohm 1% resistors on the power tube cathodes to make bias measurement much easier. If you do change the bias supply, I would suggest measuring your current bias first since you are so happy with the amp and you want to have a baseline of its current operating point so that you can recreate that and use it as a starting point for adjusting the bias in the future.
     
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