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Thinking about a modified Princeton Reverb build

Discussion in 'Shock Brother's DIY Amps' started by Nickfl, Oct 11, 2017.

  1. Nickfl

    Nickfl Tele-Holic

    642
    May 24, 2016
    Florida
    The thought of building an aa1164 has been festering in the back of my mind for a while now and seems to have become more insistent since I finished my last project. I've had three possible builds in mind for my next project: a PR, an octal deluxe, or a single ended el34 thing more of my own design that I've been kicking around in my head for a while. The SE el34 idea still doesn't quite feel fully baked to me so I want to put that off and let it gestate a while longer and the idea of a Princeton build is eating at me a lot more than the 5c3, so that is where I think I will end up going.

    Anyway, I was planning on doing a straight aa1164 with a Hoffman layout, but looking Hoffman's site got me thinking about his non tremolo version. As much as I like on board trem in theory, I find that I rarely use the feature on either my silverface vibrolux reverb or my brown hybrid build. Both have great tremolo and I love the sound, but it isn't really something I use. It also seems that bias vary tremolo in particular has the inherent problem that it makes you either bias for the tremolo or for the amp, but they each have a different ideal setting. Since removing the tremolo from the aa1164 circuit has no impact on the function of the rest of the amp, it seems like the non trem version would be better for me.

    Removing the tremolo frees up a triode and two control pots, which really speaks to the part of me that can't leave well enough alone... So, I'm thinking of using one pot as a "fatness" control to fade in that spare triode as a parallel on one of the gain stages, probably the second one, but I need to think about where it would make the most sense. I also like the idea of putting a PPIMV in that other unused control spot. I also think I may replace the footswitch jacks with a passive fx loop. I don't feel the need to be able to use a footswitch for the reverb and part of the reason I want to build this amp is its reputation for being pedal friendly so this seems like a useful feature for me.

    TLDR summary: I'm thinking about building a blackface PR clone, but with no tremolo, an adjustable parallel triode on the second gain stage, a post PI master volume, and a passive effects loop instead of foot switch jacks. I think all of this will add more options to the amp but as far as I can see none of it should effect the sound of the classic circuit when not in use.

    Thoughts, opinions, criticisms and suggestions please!
     

  2. mgreene

    mgreene Tele-Meister

    200
    Jan 27, 2010
    south carolina
    I will reply good sir - essentially because they cant shut me up about my "discovery" of the Silver pull-boost PR. :lol:

    IMO this is the best "low volume" reverb amp I have ever heard - based on the fact that I want to play it every day. I have had many great sounding amps sitting in the place of honor in my LR - Ampeg Reverborocket, Gibson RVT 79, Holland Lil Jimi, BF Vibrochamp, etc. - but this is the one I have played the most and has got me thinking about selling amps I said I would never sell, etc.

    That said - I KNOW that it wont be tempting to a Fender purest, BUT if you make this amp, dummy up the pull-boost schematic with the mods I VOLUMINOUSLY mention in my other posts about this amp and listen.

    What I have done with the pull-boost PR does in some respects meet your desire to have additional functions in place of the tremolo knobs. On mine I have the "functions" on a toggle switch (Silverface or BF sound) and on the pull-boost switch (tweed sounding Texas Tea mod) a'cause I loves my tremolo.

    BTW: I have a early Silverface Princeton non-reverb and it does not do "it" for me like the modified pull-boost.
     

  3. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Nick, you might want to look at the Rivera era Super Champ, which is in essence a hot-rodded Princeton Reverb without the trem circuit.
     
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  4. King Fan

    King Fan Tele-Afflicted Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Smarter folks will have to weigh in on your mod ideas. ;-)

    What will you use for a PT? The stock PT famously works at or above its design limits -- lovely if you like vintage sag and bloom, not so great if your mods add any current draw (maybe they won't?).

    But as you'll know; upsizing the PR PT is very popular, so there's no reason to let that stop you. Good luck and enjoy the build!
     

  5. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    281
    Apr 8, 2016
    Oakland, CA
    Sounds cool, Nickfl. Especially interested in your feedback on that adjustable parallel triode, which is in your recent tweed/D-style amp, right? Good luck!

    Princeton Reverb-esque stuff I have tried and liked or at least not disliked (no A/B testing or rigorous controlled experimentation implied):

    - Biasing for asymmetry in the preamp: center-cool-warm (stages 1, 2, 3, respectively)

    - Cathode-biasing the power tubes

    - Finding a way to preserve mids without resorting to a tone-stack lift, or at least having a way to remove excessive bass if lifted

    - Multi-tap OT with Switchcraft 14 jack to preserve primary impedance with different speaker combinations

    - Any mod Rob Robinette recommends for reduced noise

    - Replacing the rectifier tube with diodes and a sag resistor of choice

    - High-cut (tone) control on the reverb/wet signal

    - Main power switch on front panel
     
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  6. Nickfl

    Nickfl Tele-Holic

    642
    May 24, 2016
    Florida
    Thanks guys, interesting thoughts all around!

    I hadn't thought about the power transformer yet I have seen that mentioned before that a lot of people like to go with a bigger Transformer but I hadn't really considered it. I might consider trying to spec a lower voltage, higher current rating Transformer and go to solid state rectification entirely but I'll need to maybe do some listening and see if I can't seek out a stock Princeton to play with a little bit before I make that kind of decision. Even if I don't go full solid state I will probably try out a Weber copper cap instead of an actual tube which will mean I won't be using the 5 volt winding and will let the power transformer run a little cooler

    That Rivera era Super Champ does seem to be very much in line with what I'm thinking.

    Yeah that parallel Triad control was from my Tweed ODS build (not my idea though it was original to the Tweed ODS). I liked it a lot, I think it's a very elegant way to make use of a spare triode without doing something that changes the character of the amp since you can so easily dial it right out.

    I don't think I'd consider going to cathode bias I do want to try to stick to things that won't change the character of the amp or can be easily Switched Off and it sounds like too much trouble to make it switchable between cathode and fixed.

    I might consider throwing in a raw switch/ tone stack ground lift, if I did that do you think it would be wise to put some sort of bass filtering on a double throw switch with the ground lift? I take it these amps tend to get flubby with the tone stack removed? Maybe a fixed high pass filter with its ground being engaged at the same time that the tone stack ground is lifted?

    I'd really like to spend some time playing a Princeton reverb before I make some of these decisions I don't know that I'm going to get the chance though. It doesn't seem like any place in the area has one in stock definitely the two Guitar Centers in town don't...
     

  7. mgreene

    mgreene Tele-Meister

    200
    Jan 27, 2010
    south carolina
    Thats one of the interesting things about this mod, I did mine w/o the pot - its just a switch. So now the treble control works kind of like a quasi-preamp master volume. I did do the first stage bypass cap mod that halfs the value (IIRC). And may have done another bass reduction cap further down the line too - but IMHO the amp needed these anyway. I did the mid resistor mod on one PR but not on the other (I built two out of fascination with the awesome sounds I was getting. The first one I got already had all the signal caps changed to orange drops but the guy made some mistakes and it didnt sound good so I got it for song - I then wanted to hear what the stock circuit sounded like so I got another one).

    The interesting thing to do here would be to make the raw control footswitchable like Tone King. Since I am just a LR player these days, I dont mind changing the tone controls when I want to hear the raw mod.

    Note that all of my raving is about the suitability said mods for an amp run at LR to Cafe/small club volume. If I want to turn the amp up to hear some power tube distortion (which the PR isnt known for - AFAIK) I use a Tone King attenuator.

    I would also suggest that if you are thinking about getting a larger output XF, you are really building a single channel Deluxe Reverb? Which aint nothing wrong with if thats what you need.
     
    Last edited: Oct 12, 2017

  8. King Fan

    King Fan Tele-Afflicted Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Nick, an old Vermonter joke:

    Motorist to farmer: "How do I get to Barre from here?"
    Farmer: "If I were a-goin' to Barre, I wouldn't start from here."

    I totally support your idea of trying to play a PR before you build or mod one. IME, neither of the modern production versions -- PRRI or '68 CPR -- sounds much like a real PR. No rap on those amps at all, but if nothing else the speakers are very different from the originals.

    IMHO, the PR is one of Leo's 'more unique' amps (grammar police, give me a bye). For sure some folks (MuchXS) successfully hot-rod them into bigger, louder, cleaner, tighter, or just more modern-sounding amps. A 12" speaker is another popular mod, and various versions of raw / mid / tone-stack defeat are also pretty common.

    But, all that said, the PR is not just a smaller DR -- it has a circuit and a sound that is all its own. (See Wally's and mgreene's posts about the Rivera Super Champ or the DR itself.)

    Please note I'm not saying don't mess with it -- you know where you want to go and you have the skills to get there. I'm just saying it'd be super-cool and a lot of fun to know exactly where you're starting from. :)
     

  9. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    281
    Apr 8, 2016
    Oakland, CA
    If I wanted a raw setting, then bringing in just the kind of filter you describe on the other pole(s) of a switch is what I would try to do. I think the Rivera-era 'mid boost', which brings in a larger cap in parallel with the treble cap accomplishes something like this in a fairly simple way. A complex dumble-like Rock/Jazz switching setup to change the whole tone stack from (scooped) Fender-style TMB to a flatter James-style stack might be cool too, but all the work of testing that out and adding the extra components never made sense for me, because I don't care for the classic Fender EQ sound--putting me very much in the EQ minority. Rivera's new collaboration with Fender on that DR with the factory 'mods' uses a big multipole rotary switch to shuffle through several cap combinations. I wish I could find a schematic for that.

    I guess this whole topic is highly conditional--not everyone wants to use their PR-sized amp for gain or dirt at all, and some folks that try it don't especially care for it. If you have another amp that shines for overdrive, and intend the PR to do sweet sounds or gritty sounds then I probably needn't have brought up the raw switch at all. Just that lots of people here mention them, and excess bass and missing mids is my hobby horse :rolleyes:
     
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  10. Phrygian77

    Phrygian77 Tele-Holic

    589
    Apr 30, 2016
    Crawfordville, FL
    I was planning to do a raw switch on my PR build with a 75k or 100k mid resistor on a SPDT mini-toggle. Now you guys have got me reconsidering it. I'm also going to have a 47pF-off-120pF bright switch. I would think a bright cap would make the raw switch more usable at lower volume settings. That works well on my mid heavy Express preamp, but there's a lot of bass cut there as well.
     
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  11. Nickfl

    Nickfl Tele-Holic

    642
    May 24, 2016
    Florida
    Can you link to the thread, I looked at your profile but it didn't jump out at me.
     

  12. LudwigvonBirk

    LudwigvonBirk Tele-Meister

    Age:
    115
    103
    Aug 26, 2017
    Madison
    Do it! And! Don't change anything at all, not one tiny thing, stick exactamundo to the AA1164 orginal circuit, which is a very proved-out Leo design. I guarantee you will love the experience and the end result will thrill you.

    After you get one of those done, and have debugged it and broken it in, and have learned how to play it (it is an instrument all to itself...) THEN start thinking about mods or bigger changes of your own design.

    *Don't attempt to "gut" a reissue, it is a waste of money. Get a real pine cab, and do some research on speakers (yep, tons of voodoo knowledge and nobody is right, can be exhausting. Fwiw I like a C12Q after the break in. YMMV)
     

  13. Nickfl

    Nickfl Tele-Holic

    642
    May 24, 2016
    Florida
    Yep, the proven design thing is a big factor and something I want to preserve. That is why all of the mods I am considering are things that can either be switched off or turned down so that they no longer interact with the circuit in any way, leaving it as effectively a stock aa1164, with the tremolo permanently turned off.

    - the parallel triode is out of the signal path when the "fatness" control is turned all the way down
    - the PPIMV doesn't effect the signal when turned up all the way
    - a passive effects loop does nothing when not in use
    - the raw switch with low pass filter also does nothing when not engaged.

    Where is gets iffy on authenticity is using a bigger PT and potentially going to a SS rectifier...

    Thanks for the speaker recommendation, that is something I realize is a big factor in the sound of these amps, or any amp really. I haven't thought about it too much yet, but I have seen a few different recommendations. I'll definitely have to think about that at length before I start the build.
     

  14. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Nick, you can use a push-pull pot for the raw switch. If you leave the switch alone you have full tone stack bypass but you can simply solder a resistor across the switch poles to modify the amount of mid boost. The resistor is basically an additional mid resistor. Raw switch closed and the resistor on the switch is bypassed. Open the raw switch and the resistor is in the path for a controlled mid boost.
     
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  15. Nickfl

    Nickfl Tele-Holic

    642
    May 24, 2016
    Florida
    Ok, I put together a schematic showing some initial ideas. I implemented the spare triode as a parallel gain stage on a "fat" control. I also put in a master volume, but after some consideration I went with a pre PI master rather than post since I read in a few places that pre PI might work better with a concertina PI.

    The raw switch is an on/off/on DPDT, setup to give me stock tone stack/raw tone stack lift/15K resistor mid boost. I have also thrown a voltage divider on the other side of that same DPDT switch so that it is dropping some of the volume increase from the fully lifted tone stack position. I have it arranged so the voltage divider is bypassed in both the normal and mid boost positions and only in use for the "full raw" position. I would especially like feedback on this idea and the values I should be using (I just have two 470k resistors there for now).

    I have also gone with SS rectification on this one and as mentioned in my initial post it is a non tremolo version.

    Here is the schematic:

    Modded AA1164 schematic.png

    Ignore the OT specs, I haven't gotten around to updating that part yet, but everything else should be correct, though it probably could stand some proof reading.
     
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  16. ChopSauce

    ChopSauce TDPRI Member

    48
    Jun 1, 2013
    F-91
    That (really) is an interesting project. I'm going to have a more in depth look at your schematic...

    (thanks for sharing)

    ... but any special reason for not going with a Dumble-style tone stack, as you got the moddin'way?
     
    Last edited: Oct 14, 2017

  17. Nickfl

    Nickfl Tele-Holic

    642
    May 24, 2016
    Florida
    My intent is to preserve the stock aa1164 signal path as an option. Other than the solid state rectifier (and with that I am going to use a lower voltage PT so the B+ will still be about the same) everything I've done can be switched, turned down or up so that it doesn't effect the circuit and returns it to pure aa1164 (without the tremolo).
     

  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I like your raw voltage divider idea. You're probably going to have to adjust the divider by ear.
     
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  19. Teleterr

    Teleterr Friend of Leo's

    Feb 7, 2011
    Lewes De.
    I have a Silverface I bought in '77. Its the model where they finished fine tuning the circuit. Theres 1 cathode cap difference from the preceding model. Anyway , it came w a SS circuit plug-in rectifier. This was a primitive home brew, so maybe modern designs don t sound as bad, but once it was restored to original, by switching back to the rectifier tube, it sounded much , much better. I ve yet to hear a SS supply for tubes I like.
    Nick , You don t find them harsh/brittle? Whats the reason for SS rather than tube ?
     

  20. Nickfl

    Nickfl Tele-Holic

    642
    May 24, 2016
    Florida
    I'm kind of on the fence about the rectifier, I'm mostly considering it for simplicity's sake. If I do go that way, I am planning to go with a lower voltage power transformer to keep the B+ at the same level it was intended to be with a tube, so that should help minimize the difference in sound, other than giving it less sag. I could put it on a switch so I can have tube or SS, but then I have to decide to optimize the voltage for one or the other. I suppose I could go with a gz34 rectifier tube and a tube/ss switch, which would give me a smaller B+ difference between the two and maybe make it easier to make it usable. I have done that setup on one build before, but I'm not convinced it was worth the effort.
     

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