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Post subject: Princeton Reverb test procedures
Posted: Tue Sep 03, 2013 7:55 pm
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I recently acquired a 68 drip-edge Princeton Reverb in near mint shape cosmetically. I also own 3 other silverface PR's of various later years. This 1968 just doesn't sound right. I was expecting the blackface tone, as I have heard the 68 has the old circuit. But so far, I am disappointed. It has about half the volume of my others. I also notice a slight tremolo sound, even when the tremolo intensity is turned all the way down. I have checked all the tubes, but haven't really gotten into it any further yet. I would like to do 2 things: 1) Fix the low volume/tremolo problem. 2) Do a complete tune-up to put this thing back to the intended specs. Is there any kind of procedure available for checking this thing out from stem to stern? I have a background in electronics. I just need some test points and things to look for. I would also like to find a schematic that is actually legible. Most I have found online are blurry copies that can't be read. Any advice would be greatly appreciated.


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Post subject: Re: Princeton Reverb test procedures
Posted: Tue Sep 03, 2013 8:11 pm
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With the symptoms you describe, I'd look at the idle-bias supply first as it has a direct impact on both the amp's volume and the tremolo circuit. From there, the remainder of the HV supply should be inspected -- power tranny, rectifier, cap can, and the resistors of the power rail. I'm betting you'll find your answer(s) there.

Arjay

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Post subject: Re: Princeton Reverb test procedures
Posted: Tue Sep 03, 2013 11:46 pm
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Retroverbial wrote:
With the symptoms you describe, I'd look at the idle-bias supply first as it has a direct impact on both the amp's volume and the tremolo circuit. From there, the remainder of the HV supply should be inspected -- power tranny, rectifier, cap can, and the resistors of the power rail. I'm betting you'll find your answer(s) there.

Arjay


This protocol should be followed with all vintage amps, of unknown maintenance history. :mrgreen:

Also, the bypass caps on the 12AX7 cathodes maybe dried out (just replace them -- they are cheap enough). Dried out caps = very poor bass and transient response.

Good luck! Worth amp to add to your collection. You have any other amps besides PRs?


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Post subject: Re: Princeton Reverb test procedures
Posted: Wed Sep 04, 2013 5:07 am
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Greatly appreciate the responses on this. Not having a schematic or other documentation, can you guys point me in the right direction on how to check the things you describe (i.e. what voltages to look for and where)? In answer to the other question, I don't currently have any other amps. I ran across my first PR (a 1973) when I was in college and bought it for $90. I later realized what a jewel I had and have used it ever since. I have since developed a greater interest in these and have collected the others. Thanks again.


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Post subject: Re: Princeton Reverb test procedures
Posted: Wed Sep 04, 2013 5:40 am
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Here you go. Layout is at bottom of link. Voltages are within +/-20% tolerance. Note, your voltages maybe consistently higher, as the outlet voltage is 5-10 volts higher than when this schematic was drawn.

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: Princeton Reverb test procedures
Posted: Wed Sep 04, 2013 8:27 am
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BMW2002Ti wrote:
Note, your voltages maybe consistently higher, as the outlet voltage is 5-10 volts higher than when this schematic was drawn.


+1

In my geographical region, wall-socket voltages are consistently higher than when Leo designed his amps back in the day -- commonly swinging between 118 and 127 VAC. Thankfully, he over-engineered his circuits to accommodate these vagaries.

Arjay

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Post subject: Re: Princeton Reverb test procedures
Posted: Thu Sep 05, 2013 11:35 am
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Thanks for all the great tips and schematics. I will definitely check these out. Best regards!


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Post subject: Re: Princeton Reverb test procedures
Posted: Sat Nov 16, 2013 6:59 pm
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Ok...I'm finally getting back to this project. I checked some voltages against a schematic marked AA1164. My amp tube chart says it's an AA764, so hoping there isn't much difference other than a different rectifier tube. Voltages at the cap can are as follows: The line marked 240v on the schematic reads 251. 320 reads 324. 400 reads 401. 420 reads 415. But the most interesting thing is when I flip my meter over to AC. I have 6.6 volts of AC on the 420v line. All others are well under a volt. I'm thinking the cap can may be my problem. This would also explain the hum. Do you think I'm on the right track? If so, where's the best place to get a correct cap can for this thing? I pulled the chassis on a 73 PR for comparison and don't see AC component anywhere near that. Thanks


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Post subject: Re: Princeton Reverb test procedures
Posted: Sat Nov 16, 2013 7:58 pm
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ec, the main filter cap can is shot. At least the first filter section is bad. You should be getting mV AC readings off the B+ rail. Pretty sure replacing that cap can will solve a lot of issues. I recommend the 30/20/20/20 @ 525VDC CE can, or some equivalent. 525VDC rating over 475VDC, if possible.


The AA764 has a GZ34 rectifier stock. The AB1164 has a 5U4GB stock. Both circuits use the same power tranny. The recifiers are interchangeable. I like the tone of the GZ34 bettter.

http://www.tubesandmore.com/products/C-EC30-20X3-525


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