Princeton Reverb II - lead vactrol and speaker - opinions please

Discussion in 'Amp Tech Center' started by Jowes_84, Mar 31, 2020.

  1. Jowes_84

    Jowes_84 TDPRI Member

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    Hi TDPRI amp experts,

    hope this finds you in good health!

    After finishing the restoration of my Rivera Super Champ, I could not help but buy a Princeton Reverb II off the bay recently. Playing it did sort of disappoint me.. Not at all like the Super Champ. Especially the lead channel is a disappointment, while I really like the Super Champ lead channel.

    After first inspection, I found weird looking modifications around the vactrol (I am pretty sure it is the lead channel vactrol), see in the picture. If you need a reference how it looks like originially, here is a good source: http://www.stratopastor.org.uk/strato/amps/prii/referencephotos/referencephotos.html

    Could you guys help me with opinions on my plan of action please?

    1. As I do not seem to understand the "mod" around the lead vactrol, I am not going to bother and remove it, wire all back to original circuit and check the amp again after that. ??Good idea??

    2. I have a spare EVM12L speaker, but in order to make it fit with the chassis (the OT), I would need to re-position the bolts for the speakers, OR move the OT, because the EVM12L frame would not fit as is. ??is it worth the pain upgrading to the EVM12L??

    While waiting on parts (electrolytics mainly), your input is greatly appreciated!
    Thanks a lot!

    Vactrol (1)
    image11.jpeg
    Vactrol (2)
    image9.jpeg
    No space for the EVM12L's frame - need to move speaker bolts or the OT
    image0.jpeg
     
  2. jhundt

    jhundt Doctor of Teleocity

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  3. Jowes_84

    Jowes_84 TDPRI Member

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    Thanks jhundt! While I know the website, it is true that have not taken enough time. Will re-read and study a bit more.

    But - If anyone has suggestions, I will listen.

    Also, next week I am off work and will have plenty of time for working on the amp - there is not much else to do these days. Will post my progress. Cheers.
     
  4. Wally

    Wally Telefied Ad Free Member

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    Congrats! I can’t help you on those mods. One would have to draw them out and compare to the schematic. fwiw, the preamp for the PRII is identical to the Super Champ, and both are single channel amps with a gain stage dropped in for the Lead mode. The PRII has a long tail pair PI compared to the SC’s cathodyne. In fact, the SC is a continuation of the vintage Princeton Reverb circuit while the PI of the PRII is like the bigger Fenders.
    If you get the amp back to stock, you might consider the changes to the foot switching circuit that removes the error in the design that renders using the footswitch negative in its results. I pointed out the problem with foot switching the in the Sc and PRII amps, and our brother Bendyha worked up a simple cure for the design error. That mod is somewhere in the Super Champ club thread. I have not done this mod to my SC amps, but it needs to be done if one wants to use the footswitch. in sock form, the footswitch
    neuters the amp’s production. The mod involves two resistors, iirc. I wonder if someone tried to accomplish that mod and got it wrong???
     
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  5. Dacious

    Dacious Poster Extraordinaire

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    Looks like that's what happened Wally. I did the mod and it works, but rather than piggyback extra resistors in the switching path I soldered in two resistors of the right value. I think it was something like 250k in place or 120 and 1.2m in place of 600k.

    In terms of speaker you won't get a 12"EV in the stock cabinet. I'd go for the 1028 or 1058 Legend or GA SC64 Eminences. Any of them will match very well
     
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  6. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Congratulations on the amp Jowes, now you have to keep your eye out for a Deluxe II:D

    I can't work out what that mod was meant to be, but it doesn't seem to be well executed. It looks like the IC is conected to the +leg of the Vactrol via a resistor, so it will be switching on and off with the footswitch.
    It also looks like like a version of my FS mod......but not being able to see exactly what is going on, that is a guess, but having the bypass cap there would not help solve the problem...the idea is to make a resistive path to block the highs, so I don't know.

    Anyway, I would recommend taking it all out, and reverting back to stock.

    As far as doing my version of the footswitch fix mod on a PR, it is both easy to do on these amps, as only one resistor needs attention, as well as also being more necessary on these amps, as the treble bleed-off with the footswitch inserted is quite a bit greater than with the SC. Princeton Reverb II - footswitch fix.
     
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  7. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    I see the original parts list states that an EV 12" was the upgrade option, how they fitted it in there I do not know.

    I guess it would have been the EVI 12 ( FORCE 12 ) rather than the EVM 12

    upload_2020-4-3_21-36-34.png
     
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  8. Jowes_84

    Jowes_84 TDPRI Member

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    Thanks Wally! Will probably follow my first instinct and remove all the mess, if I have time, I'll trace it out - the amp does work - but the result in the lead channel is not very pleasing. I do not care too much about the insides of an amp being very tidy, otherwise I would hate Rivera-era Fenders - however, this is just too much and even if it would work fine and the amp sounded well enough I would probably go in and remove the wiring as it is now. Will do the footswitch mod as Bendya says below - for sure - thanks for the reminder!

    The former owner did not know much, he said he had it in storage for the last 20 years or so and does not know enough to help. Well, it is all speculation from here.

    Thx Dacious, that sounds like a very clean execution of the footswitch mod. - probably this is what I will do.
    Also, I do not think I will put the EVM12L in there... first, it is already very loud, second, I want to bring back the amp to stock but include sensible, careful mods first (footswitch, maybe slightly changing a cap here and there) and then check the sound again. I might stick with the stock speaker. Thanks for the recommendations. I think those are also the ones Wally usually recommends in other threads and I will reconsider once I give it another sound check with the stock speaker and amp restored to (almost) stock circuit.

    Haha. Funny enough, I will check out a Concert II on Sunday ... it seems that I have serious GAS. But restoring/upgrading an amp and making it sound "better" (well, whatever that means), is just a very rewarding - especially if you sit in a (home) office all day and stare into your laptop for your job.
    Guess I will run out of space soon...

    It can be fit if the speaker is slightly turned in either direction, fitting the 8 bolts about 22.5degrees to the left or right. Then the EVM12L's frame would fit... or to put it differently, the OT would fit between the EVM12L's frame. I just dont think I will do this, even if I have a spare one.

    Ok guys, thanks a lot, I think I am more confident to proceed now. I shall keep you posted - starting Monday, I got lots of spare time.

    Cheers!
     
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  9. jhundt

    jhundt Doctor of Teleocity

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    If you start removing the unknown mod, just keep right on going and remove all the junk in that amp. Otherwise (in my opinion) it will never sound good. I know, because I bought one back in '83. I didn't try it out, I just bought it because I needed an amp, I had always played Fenders, it was a Fender and I thought I knew what a Fender amp sounded like. I was very quickly and completely disappointed. You can read about it on the PRII page. There's a fine little amp inside that box, but it's hidden beneath all of the Rivera nonsense.
     
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  10. Wally

    Wally Telefied Ad Free Member

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    I disagree, but that is why each of us has our own ears. I would never undo the design...ommv. If I did not like the amp, I would sell it. There is an AB763 DR circuit in the amp there...that is the clean mode. It is basically a BF DEluxe Reverb in the clean mode...the SC is a Princeton Reverb output. The overdrive works, ime....very well if one knows how to manipulate the controls. The footswitch as mentioned ruins the Super Champ and the PRII due to a design flaw. Unplug the footswitch and these amps come to life. The OEM Eminence and Pyle speakers leave much to be desired! Ime.....although the PRII’s Eminence is much better than the 10” Eminence in the SC.
    Ommv......this I realize. However, it is too good of an amp to gut, imho. Buy a DRRI. I would trade a DRRI straight across for a PRII and walk away grinning. I would be ecstatic if someone wanted to trade me their Super Champ for a PRRI...IF I found myself owning a PRRI.
     
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  11. jhundt

    jhundt Doctor of Teleocity

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    as they say - "your mileage may vary".

    By 1985 you couldn't sell a used PRII ($238 new) for a hundred bucks because they were so bad.

    Yes, there is an old Fender in there, as you say. But it is weighed down by so many extra and unnecessary little "improvements". I found that hidden amp, and brought it to life. It's easy - but it requires the removal of ever so many components. It does NOT require the addition of anything. It can be done with the existing rat's-nest fiberboard.

    Like so many things from Fender, this amp has gained in value because of age. But age doesn't make it any better. It is a great amp ruined by too much fiddly circuitry, and the worst lead dress you will ever see in any Fender amp. That's probably why they added all the little caps, to suppress the oscillation caused by the total lack of attention to the location of the wiring. Maybe the Fender company made a decision that it was cheaper to add a bunch of little caps than to have the circuit-board builders spend the time to route the wires correctly.

    If you think a stock-standard PR II is a score - good for you. I owned one, and it was the worst Fender amp I ever owned. I modded it, and it is one of the best Fender amps I ever owned. To each his own.
     
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  12. Jowes_84

    Jowes_84 TDPRI Member

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    Hi there!

    Good to see a discussion unfold. Wally, jhundt, thanks for the support! You are already a step further than me. First things first, remove the funky switching circuit.

    For now, I will not go and do a lot of mods. I am a little bit sick of working in this amp - removing the previous footswitch mods took a bit longer than I would have liked. At this point, I just want to get it over with - then enjoy the results while playing the amp.

    After that, I will consider other options and read your thread more carefully again. I already did prior to starting working inside the amp, besides other threads around Super Champs and PRIIs.

    Today I started up the amp. No explosions :)

    Just stuck with a reliability issue, some crackling noises when the amp is turned all the way up, especially in the upper mid range. Could be bad solder, bad ground or tubes.. not sure yet and no time left to check today.

    This issue ignored because I am sure I can fix it later... below the results of

    - bringing the amp back to stock (mainly removing the IC-switching circuit)
    - identifying why this was done (original footswitch was not available to previous modder so he/she decided to make their own switching circuit)
    - exchanging electrolytic caps (sorry for cheaping out on the cap can...)

    ok, first a couple of words on the discussion

    -------

    Mh, different for everyone, I agree.

    I came to my Super Champ not knowing what I get, 4 years ago, knowing nothing or close to none about tube amp history and only owning some gear I got cheap - both tubes and transistors.

    The fact that I could improve it so much with rather simple restoration work(and a lot of research in the beginning) encouraged me. I love this amp (the Super Champ) now. That was the motivation for getting the PRII.

    For me, these are close-enough classics with a lot of room for improvement. For now, I will play them stock. I think a lot can be done with the Rivera amps to develop them in either direction... change some cap values, clean up the circuit, add a bias-wiggle tremolo --> there you have your silver/black face amp.

    Or go the other way, and improve the lead channel, make it a different, better amp, like Bendyha did with his Super Champ.

    I recently also got a Concert II - the clean channel is very nice. I dont own a true 60s/70s Fender - and I never will - too expensive for my taste - but I assume the Concert II clean channel comes very close.
    The Lead Channel is very flexible yet a little weird - I will have to get used to dialing it in.

    Any way you go, you have to have a clear goal of what you want to achieve, at least that is important for me. My goal is rather simple: I got hand-wired, sort-of vintage, decent Fender amps for an ok price (600-700 SC and PRII, the concert even for 500) that need a little refurbishment here or there.

    Now I have a Super Champ and a PRII in almost stock form, slightly upgraded, fully operational for the next 20 or thirty years. This was more or less my goal. The next goal might be more modificaitons .. or selling everything because I don't have space and neither time for this amp fun anyways ... did I mention I am a drummer not a guitarist ;)

    ----

    ok, now the pics with explanations - hope this can be helpful for others

    (pic1 -- footswitch -- non-original -- using single pole switches -- this makes modifications to the switching circuit necessary -- got to have respect for the tech doing this probably 25+ yrs ago w/o internet as we know it today)

    image6.jpeg

    (pi2 -- amp now looks like this)

    image7.jpeg

    (pic3 -- measurements)

    image9 (1).jpeg

    (pic4 -- russian output tubes)

    image0 (1).jpeg
     
    Last edited: Apr 11, 2020
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  13. Jowes_84

    Jowes_84 TDPRI Member

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    Ok - Update. I had half an hour so went back to the amp. Turns out that those non-original caps (MKP and other type) are not the best replacement - they are made for modern-style circuit boards, not eyelets. The .022u coupling belonging to the master volume circuit was slightly loose - at higher volumes in crackled and sometimes even cut out. I found this after a couple of minutes of chopsticking the amp with an old toothbrush (same one I used for cleaning the insides) at different settings.

    voila... so all non-eyelet-proof caps are now replaced with eyelet compatible ones == caps with long legs.

    Amp sounds good, still not 100% happy with the drive channel. Clean sounds great, especially with the mid boost engaged. Footswitch works fine without any noticeable differences in volume.

    I have to play it with the master on ten for a while to really know if it is a keeper or whether to carry out any further mods - lets see. Probably a different speaker or at least a different cab will be the next step.

    Thanks all for your input, highly appreciated. Whenever I know more, I'll try to update the thread the best possible.

    Cheers
    Happy Easter
    Joe
     
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  14. jhundt

    jhundt Doctor of Teleocity

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    If it works and you enjoy it - that's all that really matters!
     
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  15. Jowes_84

    Jowes_84 TDPRI Member

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    Thanks!
    Yes, I do ... for now.

    I already used this Princeton Reverb II with my Super Champ cab carrying an EV force 10 in it. Huge improvement. I'll make a list of what I dislike with the amp and come up with a plan. Not done yet. :)
     
  16. jhundt

    jhundt Doctor of Teleocity

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    to be fair and honest - I did all the work on mine back around 1993. I had bought the amp new in 1983, and never 'bonded' with it. I used it mostly at home, back then. When I took it to a live performance for the first time, I realized that it was NOT what I had hoped it would be.

    That's when I started to learn about tube amps. If I had the amp in its original state today, I might try a different approach (starting with speakers). But back then, I had a fairly costly amp that didn't sound right, and I was young and reckless!
     
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  17. Wally

    Wally Telefied Ad Free Member

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    ime, the speakers in the SC and PRII left much to be desired...unless one paid for the $100 upgrade for the EVI option. The EVI option is worth it, but there are other inexpensive speakers that greatly improve what comes out of a stock Super Champ especially.
     
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  18. Jowes_84

    Jowes_84 TDPRI Member

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    '93 I had other problems, sucking at soccer, trying to escape having to do homework, burying coins, records and other belongings of my parents in the forest - and never finding anything again. :)

    Congrats on the great amp work back in '93 - no internet references and already written guides on how to do it. It was a different situation at a different time, this makes a lot of sense.

    I also started with the Super Champ, knowing nothing but that it is possible to improve the neglected monster. And I did. The fact that most of these appear to be modded or at least upgraded by a speaker tells you that Fender could have done better back then. Still, compared to other "vintage" amps they go for ok-prices, still, and if you know your way around you can improve them. My SC stays as is for now, but the PRII had been messed with and I will mod away step by step as long as I feel I have to.

    Already did change the speaker in the PRII this afternoon :), I had a 12" Peavey Sheffield 1200 speaker lying around. It gets both good and bad opinions - but it is a vast improvement over the stock speaker. It's basically a poor-man's V30 - and for the PRII - a free upgrade.

    Already planning further improvements on the PRII and also browsing through Bendyha's Super Champ thread for ideas. Will keep you guys up to date.
     
  19. Jowes_84

    Jowes_84 TDPRI Member

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    Hi all,

    just finished playing the amp through all settings that came to mind with a Telecaster using all pickup selections available. All works fine. In my opinion, the amp is now a better amp (for me). I'll add other comments with more details but here first the changes I made, some need a brief comment. I mostly switched caps and resistors around - I almost did not have to add parts that were not already there.

    None of these mods are my idea, they all come from somewhere, mostly multiple sources.

    Here we go:

    pre amp
    - reduced both plate-cathode snubbers on v1 to 47pF (1200pF and 560pF)
    - v1b plate load resistors were swapped - 100k where the 47k was and vice versa
    - coupling cap off v1b (clean channel to mix 3.3meg/22pF) increased to 0.022u (0.001u)

    reverb/lead
    - 1200pF snubber cap ditched above went to replace 2200pF reverb/lead coupling cap leading to v2 grid inputs
    - v3b cathode 1.5k replaced v2 cathode 680R and vice versa
    - tried replacing 18k v2 plate load resistor with 33k (Super Champ has 39k) which yielded great results for lead channel but left reverb drowned out - so reverted back to stock value for now
    - replaced 330pF lead channel coupling cap with 2200pF from above (might need to undo this or go to lower value... tbd.)
    - 1.5meg lead channel attenuator replaced with 680k (because of footswitch mod)
    - 220k lead channel mixing resistor changed to 1meg (== footswitch mod)

    phase inverter
    - 22k plate load resistor was taken out of circuit and replaced with a bridge
    - input end from the .022 coupling cap of the master volume was connected directly to v3b's plate

    That's it for now, below separate comments for why I did what and whether I am happy with the results or not.
     
  20. Jowes_84

    Jowes_84 TDPRI Member

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    The references for the work described above are stratopastor.org, lots of threads here on tdpri especially Bendyha and Wally contributions, the Super Champ schematic, the PRII schematic, the ConcertII schematic and any AB763 schematic, preferably the Deluxe Reverb. Based on that, I decided on mods.

    Clean Channel
    I tend to think the PRII was designed to stay very clean in the clean channel, without any breakup. Whoever want crunch, can pull the mid boost. Whoever wants distortion can switch to the lead channel. That is my interpretation of the stock PRII.

    The Super Champ is a little different, a BFDR a lot.

    So the key to opening up the sound in my opinion is to get rid of all the attenuation going on in the circuit (not all the caps and all the extras). But if you do that, you run into problems balancing the different other features against the new clean channel ... at least I started struggling.

    I had achieved the best clean channel sound in this amp connecting v1b and v3b plates directly to the following coupling caps, not attenuating the output signal with a voltage divider as it is done stock. The Super Champ does not have these attenuating voltage dividers neither. So this was my first "mod".

    However, doing that made the clean channel almost louder than the lead channel, not quite, but fatter, fuller sounding and the reverb was again not as nice as it had been with the stock circuit.

    I can see now why Jhundt removed "all the extra" :) I am not giving up just yet.

    In essence, the challenges of further modifications for this amp seem to be a compromise between clean/reverb/lead sound levels as well as voicings. It was fine stock, but a bit sterile and not very similar to a classic Fender... nice, but...

    Best clean sound is achieved getting rid of all the attenuating resistors in the pre amp up to the master volume. But then, getting the boosts, reverb and lead functions to line up properly could be a nightmare. I kind of succeeded. Amp sounds good, but I cannot help and sence that further improvement is possible and even necessary. Will play as is for a while - I need to get back to practising guitar :)
     
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