SF Princeton Reverb - what mods (if any?) for Jerry tones?

SF Princeton Reverb - what mods (if any?) for Jerry tones?

Postby redeyedjim » Fri Oct 18, 2013 2:32 pm

OK, so I seem to have become like the proverbial Crazy Cat Lady, only with amps instead of cats. This is me:

Image

Just substitute "amplifiers" for "cats".

The latest stray is a 1977 SF PR that was too good a deal to pass up. It's a bit tattered (a bit more than the following pic reveals), but not bad enough to warrant a new cab. It has stock electronics and a mix of new glass (EH 6V6s, some unknown 12AX7s, JJ rectifier), as well as a no name 10" speaker.

Image

I have a D110 that I intend to put in it, as well as some decent old glass. I also plan on replacing the electrolytic and coupling caps, but the rest of it seems to be OK, the pots aren't scratchy, the reverb works, etc. I may have a professional tech check it out just to be sure, as my knowledge and experience are limited and definitely "shade tree". But beyond those most obvious changes, I'm curious what else I should consider doing to make this amp more Jerry-esque? I am not planning on gigging with it, really just want a nice grab-and-go Jerry-flavored amp for jamming.

I know the PR is a great platforms for a lot of mods, most of which leave the amp as more hybrid than PR. I'm aware of the Stokes mod for more clean headroom and power, Paul C. mod for fixed bias, etc. I found a nice PDF of the most common PR mods here:
http://www.blueguitar.org/new/articles/blue_gtr/amps/john_paul_rick_PRmods.pdf

I know some of you have PRs. What do you like to do to them, apart from play the snot out of 'em? :lol:
Last edited by redeyedjim on Fri Oct 18, 2013 3:29 pm, edited 2 times in total.
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby caspersvapors » Fri Oct 18, 2013 3:02 pm

besides changing the speaker/tubes and getting routine maintenance I wouldnt mod it.
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby Smolder » Fri Oct 18, 2013 6:09 pm

I have several princetons... and they won't get you exactly there. The D110 is pretty great, but it's not gonna give you exactly what a jbl with a 4" voice coil will.

Do the Paul C mod, but don't do any others...

Here's a thread where I replaced the baffle to accommodate a 12" jbl and it kills... but I wouldn't do this to a virgin or all original princeton reverb.

http://www.thegearpage.net/board/showth ... ton+baffle
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby redeyedjim » Fri Oct 18, 2013 6:52 pm

Thanks, Smolder, that's exactly what I was looking for. If/when I do step up to a bigger JBL, it will be with an aftermarket combo cab. Any other words of wisdom?
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby Smolder » Fri Oct 18, 2013 7:45 pm

redeyedjim wrote:Thanks, Smolder, that's exactly what I was looking for. If/when I do step up to a bigger JBL, it will be with an aftermarket combo cab. Any other words of wisdom?



Allen amps makes an excellent princeton combo cab for both 12's and 15's. And... he's a cool cat, very helpful with great products.
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby redeyedjim » Fri Oct 18, 2013 10:33 pm

Smolder wrote:Allen amps makes an excellent princeton combo cab for both 12's and 15's. And... he's a cool cat, very helpful with great products.


Yeah, he is. Actually, I exchanged emails with him today about working on this Princeton, as he worked on another amp for me some months ago and I was very happy with what he did. I'd love to have him go through this one, too, and replace the caps and tune it up as necessary. His prices are pretty reasonable, at least by the crazy standards here in NorCal, lol. I'll also ask him to do the Paul C. mod while he's in there. I asked him about his oversized 12" Princeton cab, but he said he has an 8 week lead time now on cabs, so I may pass on his and have Peter Mather make me one that's similarly sized to his. Peter made me a cab for another amp (see "Crazy Cat Lady" description in my OP), and it turned out fantastic, and his turnaround time is more like 2-3 weeks. His quality is very, very high as well.

Now, I just need to sell some gear to pay for all of this other gear I want to have worked on :D
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby redeyedjim » Sat Oct 19, 2013 11:57 am

Smolder, if you see this: what are you using for a rectifier tube in your SF Princeton? turns out the previous owner of mine put a 5Y3 in there, which seems very...limiting. The tube chart for the SF Princeton calls for a 5U4GB, but I see a lot of using something else (5V4, GZ34, etc.). What are you using in yours?
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby Smolder » Sat Oct 19, 2013 5:19 pm

redeyedjim wrote:Smolder, if you see this: what are you using for a rectifier tube in your SF Princeton? turns out the previous owner of mine put a 5Y3 in there, which seems very...limiting. The tube chart for the SF Princeton calls for a 5U4GB, but I see a lot of using something else (5V4, GZ34, etc.). What are you using in yours?



I have one PR with oversize transformers and 6L6's... super clean and I'm running a 5AR4/GZ34. It's 22 watts of super clean. The other PR and the P NR have 5V4's. They will handle 5u4's just fine (it pulls 3 amps vs the 5V4's 2, otherwise they are essentially the same. I use those two for lower volume and the slightly lower voltage is appropriate. If you want it to run a tad cleaner (and potentially louder) a 5AR4 is the way to go. Just don't do the stokes mod. Pushing it further to clean, you can get a solid state rectifier... but that sounds a tad harsh to me and out of character for a princeton reverb.
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby claytushaywood » Mon Oct 21, 2013 1:29 pm

Ive run into some problems replacing the rectifier on silverface amps that call for a 5u4 with a gz34. It can raise the plate voltage too high and cause it to sound terrible when breaking up. I had a pro reverb where the gz34 raise the plate voltage over 500v. It sounded great clean, but I couldnt get that slightly dirty jerry sound. And I am not under the illusion that jerry's tone was ever completely clean, besides 90's era though he did keep it pretty clean occassionally in 73 and some other random years. if you really wanted to jerrify this amp you could do the preamp out mod because this preamp is exactly the same. The power amp on the other hand is about as far as any blackface (style) fender amp there is away from the twin. the phase inverter section is different. I would just do the tone stack caps mod and the metal film resistors and preamp out if you want to, though the princeton has a different response than other fender amps (it's quite a bit different than the deluxe even) so the cap mods might not give you the same response change youd be expecting. You can do the paul c mod if you want, but I think the phase inverter is the key to the classic princeton charm if you have other jerry amps i would leave the princeton alone, just doing some of the standard blackface mods. check out jim campilongo for some of the coolest fender tele sounds ever done, he uses a jbl in some of his too, or a jensen.
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby redeyedjim » Mon Oct 21, 2013 1:44 pm

Thanks, claytushaywood and Smolder. My thinking on this amp keeps going around in circles. I had been making plans to go to work on it, but I'm now thinking I'll leave this one more or less as it is -- put in a better speaker (this amp has an unknown in it now), some old glass and a different/correct rectifier and make sure it's working properly, but otherwise keep it true to the Princeton form. I have other amps for BF and Jerry tones, but the PR is very cool on its own, and doesn't need to be made into something else. That's my plan of the moment; we'll see how it changes. :D
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby tatittle » Wed Oct 23, 2013 7:31 pm

I dont think a 5Y3 is wise...that is probably just what the guy had laying around and since it "worked" he left it. I dont have the specs in front of me but I recall it is FAR different than the 5U4GB or GZ34. The Champ and some tweeds use a 5Y3...maybe just the class A circuits. I imagine you are not only getting wacky voltages etc inside, but that rectifier is risking harming the components. Smolder etc. voiced what I have heard re: different acceptable choices for rectifiers in a PR. A 5V4 has been suggested to me (like Smolder referred to), I havent tried one yet. The 5U4GB will have more sag than the GZ34, and more still than a solid state plug-in. I have read solid state are not recommended for PR because of the level of voltage change.


If there is a code on the speaker you can decipher its manufacturer (usually 7 digits long). e.g. 1377422
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby redeyedjim » Wed Oct 23, 2013 9:00 pm

tatittle wrote:I dont think a 5Y3 is wise...that is probably just what the guy had laying around and since it "worked" he left it. I dont have the specs in front of me but I recall it is FAR different than the 5U4GB or GZ34. The Champ and some tweeds use a 5Y3...maybe just the class A circuits. I imagine you are not only getting wacky voltages etc inside, but that rectifier is risking harming the components. Smolder etc. voiced what I have heard re: different acceptable choices for rectifiers in a PR. A 5V4 has been suggested to me (like Smolder referred to), I havent tried one yet. The 5U4GB will have more sag than the GZ34, and more still than a solid state plug-in. I have read solid state are not recommended for PR because of the level of voltage change.


If there is a code on the speaker you can decipher its manufacturer (usually 7 digits long). e.g. 1377422


Thanks for the info. No, I'm not going to keep the 5Y3 in the PR.

Actually, according to all the references I've been able to find the recommended rectifier tubes for SF Princeton Reverbs are either GZ34 (early silver face PRs with the BF circuit) or 5U4GB (70s+ PRs) (one source: [url]http://www.thevintagesound.com/ffg/]The Fender Amp Field Guide[/url]). I happen to have both an ElectroHarmonix 5U4GB tube and a 5V4 Weber copper cap in my parts bin, so I'll give one of those a try while I look for a vintage 5V4 to plug in there.

Just to be clear -- the amp is still on its way to me, but I should have it by Friday. I have a couple of decent 10" speakers to try, as well as some nice vintage tubes, so I hope to spend some time this weekend trying out different combos and at least narrowing the field a bit. I'll report back after I make some progress, or if I get stuck.
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby tatittle » Mon Oct 28, 2013 4:00 am

This 64/65 Princeton I just got is really fabulous. It was modified (by VA reportedly) in several ways which I unforunately am not experienced enough to begin to decipher. It gets phenomenal sustain now even on clean tones...it really does seem to approach the Jerry sound in that regard. It doesnt have the super tight low end, but even that is significantly improved over std. Fender's Ive played.

I need to take it to a real pro for a check-up though. Primarily because the 6L6's measure a whopping 115ma bias on my BiasKing. That may be part of the reason it sounds so good...but 3 times the normal bias doesn't seem like something that can be smart. Yet it sounds so good, and it has not presented any problems so far; since it is a 1 off I really am hesitant to let anyone start working on it...once its been changed I am clueless on how to get it back. The original owner said nothing had been changed since the mod (including tubes), is it possible for drifting values to be responsible for such an enormous bias?

I took some pics of the guts which I will try to post later.
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby Smolder » Mon Oct 28, 2013 6:54 am

@tatittle... yikes...

At 115mA 6L6's should be red plating and 6V6's would basically be gone... I'd guess that something is wrong in your readings...

A lot of guys like to set the bias at 70 percent of plate dissipation, but I tend towards a bit lower. That would be in the 20's for 6V6's and the upper 30's for 6L6's. On a princeton the tremolo is directly connected to the bias circuit so with 6L6's biased in a reasonable range it will kind of suck. In fact if you want a good trem with 6V6's keep the bias point in the low 20's or even 18/19 depending on the tubes and the plate voltage.

If you take pics of the guts any mods should be easy to detect. This doc is a pretty good primer on the common princeton mods. As I mentioned before, the paulc mod is damn good... the stokes mod is tough on the stock power transformer and I wouldn't have it on a blackface (they are too valuable):

http://www.blueguitar.org/new/articles/ ... PRmods.pdf
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Re: SF Princeton Reverb - what mods (if any?) for Jerry tone

Postby tatittle » Mon Oct 28, 2013 11:39 pm

yeah I know...crazy high right...I am afraid Im gonna burn the house down lol! I havent measured bias with a multimeter, but the BiasKing gizmo I have seems accurate on all the other amps (both 6V6 and 6L6).

They arent red plating though. The trem has been disconnected entirely. The volume pot has some kind of green gizmo on the back of it now that makes it as deep as a dual-ganged (but its not). The speed and intensity pots now interact with the volume. The Intensity seems like a Master volume, the Speed gain. Its got a giant capacitor on the board now too. This is all greek without photos though. I dont think it is any of the typical mods, but I am green as Spring on circuits. The techs at VA in that day were Paul Rivera, Steve Fryette and Ken Conrad.

Lemme try and post the pics, gotta switch computers; Im also sick now so my energy is sapped.
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