Grateful Dead Music Forum

Hey Now,
I have a 1964-ish Princeton that has been modded and I believe that includes a pre-amp out. At least that's what it looked like to my green eyes when compared to schematics detailing the mod. I am about to test it out for the 1st time but since I haven't had a tech confirm that is what has been done, I am a bit scared I have misdiagnosed and I will fry the power amp somehow. Is this a possibility if it is not the preamp that is tapped? I have some pics of the board somewhere so Ill try and add them soon.

OKAY: NOTICE BLUE WIRE FROM CIRCUIT BOARD TO [VIBRATO PEDAL] JACK ON LEFT OF PICTURE (which is now a 1/4 jack)...the black wire from the jack goes to the chassis/ground.

Here is photobucket link to some pics (I hope lol...I keep forgetting how to do this since it is infrequent):

http://s1358.photobucket.com/user/Tittl ... 5.jpg.html

http://s1358.photobucket.com/user/Tittl ... 5.jpg.html

http://i1358.photobucket.com/albums/q77 ... 841ee5.jpg

Just pull the power tubes, then there is really nothing that could go wrong. The only thing you want to avoid is removing the speaker while the output transformer is still in the circuit, but pulling the power tubes alleviates this.

Just to clarify, when I preamped my princeton, I simply tapped in to the circuit right after the .022 cap preceding the phase inverter (immediately following the second half of V3). Now, I did all of this while trying to understand more about amps, so it's possible that you could grab the signal right before the .022 (I guess then you would have DC voltage on the output, I don't know what this would do) but I didn't.


Have fun, I really liked the sound it running through my Mac.

Thanks S. No Mc yet...just a Carvin DCM1000, but Ive heard good things about the Carvins here. Itll be a few days before I receive it, but I have high hopes as the Fender power section is sometimes not quite what Im looking for. The Tap is in the middle of the circuit board I recall...and I cannot imagine what else would be attached to the Trem pedal output (1/4" jack now); the Trem has been disconnected and the 12ax7 used for extra gain stage. Pics must be on my other computer...though I believe I posted them here maybe 10 months ago or so.

Photos added above...

That jack was always a 1/4" jack. Right? Pretty sure. That may just be the wire for the vibrato?

Chris

No, it was not originally a 1/4 jack. It has been since I bought it, but the original Vibrato pedal connection was an RCA style jack like the ones stereo components link together with. I think some modern Fenders use a 1/4" jack for the footswitches, but the original vintage ones had the RCA jacks for pedals and reverb tanks.

The Vibrato feature has been disconnected on this amp and the tube used for vibrato is supposedly now acting as an extra gain stage. The pots originally controlling speed/intensity now work like Gain and Master Volume pots. The original volume looks like it should be a push/pull pot, but it does not pull out properly hence fails to change the tone when pulled out. Its a mystery amp and I don't have the knowledge offhand to know whats going on with it. Eventually I will consult someone, but it sounds really nice as it is so I in no rush.

Is the blue wire coming off that 470 ohm resistor?

The blue wire connects to the smaller resistor which is mostly obscured by the larger resistor standing on its end.
It also looks like the [.022?] chocolate colored cap (not the redder maroon one) is attached to that joint, and runs to the joint underneath the giant light blue capacitor that is not on the edge of the circuit board, but about 1 inch towards the center of it (and has one of the original yellow wires meeting it there).
This is the part of the circuit that is modified (early 1980's by VA in California I believe), but I imagine one could make an educated guess as to whether it is a preamp out without totally deconstructing the modifications to the preamp.
I should have a picture of tapping an AB763 circuit originating thru Wald, but I haven't found it yet, and that is a different circuit (if similar) anyway.

I just cant imagine what else it would be though, besides a preamp out.

It may have something to do with that extra gain stage?

I took a look at the layout diagram and was disappointed to see that the standard vibrato pedal hookup is a blue wire to the same area, so maybe the RCA just got switched to a 1/4" for convenience.

careful, if you pull out the power tubes the voltages in the amp will then be sustantially higher and you might blow some of the power supply caps. Be certain that, with the power (output) tubes pulled, non of the capacitor voltage ratings are exceeded.

Thanks for the warning. I have just begun considering the dummy load question. I'll check the specs on those. Looks like it may be a bit further off than I had hoped though.

Well I spent some time looking at layouts and Dozin's preamp material (I had thought that went with Wald) and am more hopeful that it is indeed a preamp out. I wish I knew for sure which capacitor is the phase inverter input one, but between the mods and my lack of basics I can only guess lol.

Okay after some cramming I am coming to the conclusion that my Princeton is "tapped" a bit further down the line than usual. After the .022 cap Smiddy mentioned (I think) but after a couple resistors. The .022 cap isn't original so I could be being tricked even easier than usual. It is where pin 7 of the 12ax7 (V2) meets. (I can only guess Smiddy's reference to V3 is a typo as V3 is an output tube?). Im still too ignorant to rule out it as a pedal to switch out the modified gain stage , but I my intuition says it couldn't work for that. So I guess I will go ahead and plug her in and see what happens!!!

Time well spent anyhow. I know just where to tap my Tremolux if I want to do her (she's pretty hot).

V3 as you are staring at the front of the amp, left to right, or staring at the back and reading right to left, like Hebrew

V5 if you are counting the rectifier as V1, etc.

EDIT:
I gots my rights and lefts all mixed up!

Thanks Smidtz. I figured you must be talking about a Princeton Reverb in hindsight...I am working on a Princeton standard. Thererin lies the discrepancy. Should be very similar though, especially there. I thought V1 was generally known as the 1st preamp tube. Anyhow...I get ya, Ill have another look Thanks!

Okay. That looks closer to the spot in the circuit on a diagram that has a "power shunt" (per Wald I believe)...as Garcia's actual amp apparently had. It allows use of the Trem from what Ive read, whereas the Dozin' mod disconnects the Trem (so I read I think).

Do you have another wire besides that one running to the preamp out jack from memory? Or is it just the one wire (with perhaps a ground at jack sleeve)?

Its your other left.