Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #133596  by tatittle
 Tue Oct 29, 2013 12:01 am
Okay I got a mssg that files were too big to upload so the address below is for photobucket.

I am hoping that since it seems like the 1/2 12ax7 usually dedicated to trem is now involved in some kind of extra gain stage, that is what is causing the reading to be wacky.
If anyone knows an experienced Fender guy in the North NJ area that doesn't charge typical NJ bench fees I would be grateful too. ... t=3&page=1
 #133601  by Smolder
 Tue Oct 29, 2013 1:43 am
tatittle - your link, when clicked, tells me the library is private so I can't see anything.

Pasting the [IMG] link should work here better than the web link or an upload.

Lastly, the concertina phase inverter is a limiter to the princeton amp - it doesn't scale well. I's not unheard of to sacrifice the trem in order to put a (two stage) long tail phase inverter in... if you notice the princeton has two less preamp tubes (four fewer gain stages) than the deluxe. Two of those gain stages are used for the normal channel, one for the PI (for a total of two), and the last is used for the term circuit (utilizing a bug rather than use the bias circuit).
 #133603  by tatittle
 Tue Oct 29, 2013 5:29 am
okay I think I made the account public now. ... t=3&page=1

Pictures aren't the best, but you can see e.g. the grey wires are added on the circuit board, 2nd 12ax7. Looks like a preamp out to where the footswitch once was (1/4" jack).
Gotta say I am loving the juiced clean tones this thing has given me so far. Totally different league than my '78 Princeton, which I have always found sterile. Still stays clean all the way up if I set it right, like a Princeton should though.
 #133624  by redeyedjim
 Tue Oct 29, 2013 11:33 am
Sorry, pic links still aren't resolving to pictures, just to the private gallery page. If you go to one of the pics there should be a ready-made snippet or code to embed the picture in a forum post. Something like:
Code: Select all
[img]{**the address of your image, ending in .jpg**}[/img]
I'm curious to see what your chassis wiring looks like :)
 #133646  by tatittle
 Wed Oct 30, 2013 3:37 am
Sorry digital photos are not my thing obviously. I will try an fix it says "private" for me and wont let me login lol.

Remember this is a standard Princeton, not a Princeton reverb. I think I prefer a good Princeton over the reverb version actually...Mike Campbell does too.
Last edited by tatittle on Wed Oct 30, 2013 4:56 am, edited 1 time in total.
 #133657  by Smolder
 Wed Oct 30, 2013 6:53 am
ok... so the pictures are visible but a out of focus... so here are some observations...

It's a princeton NR so when stock, not as loud as a princteon reverb and it won't typically break up as much.

Good news... the original blue molded caps are there and they rarely fail - so leave them. Bad... so are the old cardboard electrolytic coupling caps (on the main board and on the bias board). Those are now 40 some years old and while purist would tell you to leave them until they show signs of leakage... I'd replace them. Old electrolytics often compromise clarity and volume and if they fail catastrophically can take out power tubes and/or transformers.

The concertina phase inverter looks to be in tact. The tremolo section has been gutted and it looks like there is an additional (cascading?) gain stage (first half of V2) put in (somewhat like a marshall). I can't see values of the caps and I can't really tell what the extra smaller cap and a couple of additional resistors are doing... but that's my best guess. It also looks like the additional gain stage might be turned of and on by the (green) pull pot on the volume knob.

You don't have a bias adjustment pot that I can see. That would be extremely helpful in managing the power tube bias... especially if you plan to swap out 6L6 and 6V6's at any point. AS I mentioned... if that truly is 115mA, that's way hot and I don't know many 6L6's or any 6V6's that would hold up to that in a push/pull amp like a princeton. A couple of great help pages on the bias topic:

It looks like you do have a three prong (grounded) power cord which is good. But the death cap (blue cap located just below the power switch) should be removed. If that fails, it will put hot current directly on the chassis (very bad). Lastly... with the three prong power chord, the polarity switch is virtually useless... and is a great place to but a bis pot.

Very nice little amps. I have a '74 that is stock other than the bias pot and the paulc phase inverter mod. They call then a little twin an they do stay clean up to about 8-9 on volume. Hope that is helpful.
 #133661  by tatittle
 Wed Oct 30, 2013 8:44 am
Thanks Smolder, you mentioned most of the info I have wandered upon. The original owner described the mod as an extra tube gain stage your guess is probably a good one. I do get bias pot's always when I have amps serviced too. I am pretty sure the volume pot is not a push pull unless it is frozen or something. As I said the trem controls now set gain...actually the Intensity seems like a Master volume now more than the volume control. I just cant believe how much better it sounds than my 78 Princeton which has had full service...maybe the cabinet is alot of that, but the difference is drastic. Maybe Ill throw the 78 chassis in the BF cabinet and see how much it improves it.
 #133663  by Smolder
 Wed Oct 30, 2013 9:38 am
I doubt that it is the cab, but it might be as your 78 would have the dato'd (not replaceable baffle system that changed in '74 isn't.

Are both Princeton's non reverb?

If they are, then you may just be a fan of the juiced tone of the extra gain stage and the super hot bias of your power tubes that you're getting.

Now that you mention it there is a pair of twisted wires from the power (PI) section to a pot over there - it's difficult to see exactly where they connect - but it may very well be an MV. The main volume pot has a green section on the back that is an on/off switch and it looks like something is soldered to those tabs... on looks like a ceramic cap (bright gap?) between it and the treble pot. Those switches are either pull pots or have an off switch past 1.
 #133665  by redeyedjim
 Wed Oct 30, 2013 10:13 am

First, thanks for sharing your PR/P-non-R knowledge. I think this is wandering off the make-it-Jerry-like question I initially asked, but you seem to know your shit with this amp and I appreciate the dicsussion we're having here, so I don't want to move this thread to TGP. I hope the mods are OK with that.

I've been spending some more time w/my recently acquired 1977 SFPR and I am totally digging this little amp -- it's so damn playable, lol. I had the chassis out last weekend and mine looks to be a 100% virgin board, untouched by anything other than time. Part of me wants to mod it "for the sake of modding it", because that's what I typically do, but I'm a bit worried that fixing some of the amps's known short comings will spoil some of its charm, so I'm hesitant. Still, I have a new set of NOS power tubes coming today or tomorrow, and then I am going to drop it off with a tech to have the amp biased, a bias pot installed, and the caps and coupling caps checked. And probably the Paul C mod, too.

So, I guess my question is, how much does the Paul C. mod alter the feel of the amp? I'm not really looking for louder, as I have that covered with other amps, but a less farty bass and (somewhat) chimier or more focused top end would be nice.

FWIW, I've tried it with a few 10" speakers I had around, and so far it seems to shine best with a Celestion Gold G10. I tried it with a D110 and it became more brittle, and lost some of its "feel." I think something similar to the Gold G10 but a little brighter/chimier would be even better, but the Gold G10 is a pretty good all around speaker for this amp. I also plugged it into a K120 I have in a 1x12 cab, and I was surprised at how much fuller that sounded. I may get a 12" cab made for it down the road, but for the moment, I think I'll be keeping it in the stock cab, heavy though it is.
 #133667  by Smolder
 Wed Oct 30, 2013 11:19 am
Hey redeye...

The princeton is it's own amp, and while the NR version can get close to the cleans of a twin... the tone will never (IMHO) have the range that the twin does... fewer lows and fewer highs... part of that may be the cab. A 12" JBL helps that even more that the 110. btw - I have a thread on TGP regarding how-to on changing the baffle to a BF design which makes the gaffes and thus speaker size interchangeable. I've done it to a couple of princetons and a twin.

The stokes mod is the one that increases volume... but put's a lot of stress on the stock power transformer. I've only done it with a PT upgrade. As I mentioned above, with the concertina PI design it will never be a deluxe (long tail PI) without a lot of sacrifices.

The Paulc mod simply balances the output section and the PI. The end results is a smoother overdrive with more even order harmonics... a tad less shrill when cranked. I've done it on all three of mine... no risk other than if you're holding a collectable.

One of my PR's needed a pretty complete restore (or full build) and I did set the tone stack to 120pf, .022uf, .022uf as in the waldo schematics. I also put in Dale 1% spec 100k resistors and the Sprague tantalum mill spec coupling caps and all of that turns out to make a difference.

As for speakers... you won't get it close to Jerry land unless you put the 12" and JBL (or equivalent). That said, I'm sure a gold or other alnico will sound amazing in it. They sweet little amps right off the assembly line. The tweaks are like adding a little salt and pepper to an alread gourmet meal.
 #133672  by redeyedjim
 Wed Oct 30, 2013 2:38 pm
Thanks, smolder, that really helps!

I did read your thread on modding the dado'd SF cab, and while that's within my skill set I think I'm more likely to leave this cab intact and have Mather make me a lighter, pine cab, maybe with both a 10" and 12" baffle, if the price for an extra baffle isn't too much. That would give me maximum flexibility. In the mean time I can keep the 10" and use the 1x12 cab I have if I need the PR to have a fuller sound.

A new cab is definitely a down-the-road thing, though; I feel like I've been single-handedly keeping the used amp and speaker economy alive the last couple of months, and I need to slow down a bit.

In the mean time, I'll pull the chassis and send this off for a checkup and tune up.

 #133675  by tatittle
 Thu Oct 31, 2013 5:16 am
I checked the volume pot again, I originally had assumed it was one after seeing the inside...the pot's post pulled clear out of the chassis when I pulled it this time lol. The post has a collar on it which could be for switching purpose...but it didnt caqtch on anything obviously. I am not looking for any serious gain/distortion from this amp...just some juiced cleans in a small package which it definitely has. Thanks for the help; I have learned it is wise to explain EXACTLY what one wants done when taking an amp for service, and I cant do that without some basic concept of what is going on.
 #133740  by tatittle
 Fri Nov 01, 2013 6:13 pm
An efficient high power speaker should help the farty bass tremendously I think, even if its still a 10". I dont have any farty bass with EV's or Altecs or JBL's in a PRev. I have a hard time using anything else since I have become accustomed to extremely flat response speakers for the clean tones now. As You are likely aware many folks are not crazy about distortion tones of JBL's though, myself included. A Celestion Gold is one of the highest regarded speakers for overdriven tones on the other hand, and standard PRev may be the dirtiest vintage Fenders. I have considered trying to rig up a switching box to change speakers/cabs for my clean and overdriven tones--similar to what guys do with 2 amps (Marshall/Fender e.g.), but I havent tried anything yet. I also have drilled mounting holes in my 10" baffle to install a 12" speaker and it seems to work great even though the edges of the speaker are blocked by the baffle (rear mounted) a little.
 #134692  by tatittle
 Thu Dec 05, 2013 11:03 am
I just scored a pair of vintage GE 6L6GC's which I am trying in the modded Princeton (or other 6L6 output amps).
Strong, spotlessly clean, and actually matched for mA; all for the price of 1 tube. I saw a funny quote from a stereo/music enthusiest: " the one who dies with the most tubes wins"...