Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fresh_Start on September 01, 2010, 01:06:44 pm
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Well, the guy I built the UberVerb for liked the result so much he's asked me to do some work on his Blackface Princeton Reverb. The only absolute thing is that he doesn't want to get shocked by the 2-prong power cord anymore :shocked:
Anyway, I'd like some guidance from you all about how to approach this project, things to look out for, etcetera. Frank was really clear that this is a musical instrument for him - not a collector's item.
Don't worry, I'll take plenty of pics before touching anything. I also intend to measure & note voltages throughout the amp and test the values of every cap & resistor I can. I'll also check for DC voltage wherever it shouldn't be.
A 3-prong cord is a MUST.
Frank wants the Ground switch to have a Standby function.
Unless it's been replaced already, I'm assuming that replacing the cap can is in order.
We're also in agreement that adjustable bias would be nice to have, but I'm not about to add external test jacks. Right now my thought is to remove the existing bias board including the wires to the Intensity pot & rectifier socket, put all that in a well-labelled baggie, and make a new board with bias adjustment to fit in the same place. 1-ohm bias sensing resistors could go on the power tube sockets, right? Either that or I could make a bias checker (at long last).
What about screen resistors?
Power tube grid stoppers?
My thought about these two is that the amp sounds good without 'em, but what do you all think?
Having this original PR right next to my Prankster Reverb has been "ear opening" if you know what I mean. The original somehow sounds warmer, with more upper mids and less clarity in the bass... maybe? It's not all the old speaker because I already tried connecting each amp to the other's speaker. The tubes must have something to do with it, but I'm guessing that some serious component drift will turn up while measuring various components. Who knows how this thing is biased after all these years too. I can't do anything about my OT, but I'm going to try to get my PR to sound closer to the original.
OK guys - tell me what to do!
Cheers,
Chip
P.S. This guy has a beautiful electric Gibson L-5, an early 1970s re-issue of a 1961 Strat, and pre-WWII Martin tenor guitar (4-string that is), to give you all some idea of the other goodies in his basement. After playing that Strat again, all I can say is that "I have lust in my heart!"
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It's all in Hoffman's 'library of information'. It even has pictures to show you what to do.It covers the three prong cord installation and the bias pot as well.
Here's a pic of the three prong cord installation.
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Here's Doug's advice under "Servicing Fender Amps" in case any one else wants it:
http://www.el34world.com/schematics.htm
Thanks Doug! :grin:
That 3-prong power cord pic doesn't show the Ground switch serving the Standby function.
Sounds like I should just plan on replacing ALL of the electrolytic caps, not just the cap can.
I'll post an action plan after I go inside and do measuring/testing.
Thanks,
Chip
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Just bypass the ground switch when doing the power cord.Then to wire the standby you simply lift the wire off the rectifier tube(pin 8) that goes to the board and splice in that wire to one side of the ground switch and run another wire from the other side of the ground switch back to the board.Done.
Your ohmmeter is your friend here.Check the ground switch to see which terminals will work.
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Unless you want to lift the ground in case of studio ground loop problems, the ground switch is worse than usless after the 3 prong cord is installed.
Dave
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Sounds like I should just plan on replacing ALL of the electrolytic caps, not just the cap can.
I'll post an action plan after I go inside and do measuring/testing.
Hey Chip,
You know my limited skills so take this with the grain of salt.
I would be hesitant to replace anything BUT the power supply caps.
My reasoning: it's a working amp. You've stated the unique quality of tone it has right now. unless some thing tells you that a cap maybe going south, like a bulge or leak I would leave it. even if it has drifted way out of spec. that may be that amps mojo.
Just my .02 thats all. I trust your skills and judgement with these matters more than my own.
Ray
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I'm with Ray. Don't fix it if it ain't broke. The customer wants a 3 prong power cord and a STBY switch, so do it. Use your judgment about replacing filter caps. If you don't see stuff dripping out of the can and can't hear any hum, leave 'em be. Don't add stuff to the original circuit just to hot rod it unless the customer specifically asks for it. Remember, the PR was Fender's bottom of the line reverb amp. It doesn't need all the bells and whistles of the SR or TR. The PR is what it is and that's it's magic.