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Princeton Reverb Head Amp, 3nd guitar amp build

Discussion in 'Shock Brother's DIY Amps' started by pfarrell, Mar 1, 2021.

  1. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I'm not sure one or the other is 'better.' Someone else may know.
     
  2. pfarrell

    pfarrell TDPRI Member

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    Might be able to give this a listen over the weekend...

    PF-AA1164-Build_11.jpg
    PF-AA1164-Build_12.jpg
     
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  3. pfarrell

    pfarrell TDPRI Member

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    Amps alive! Some pics. The chassis images are before I changed the wiring and some dressing. Was pretty loud/hum at first fireup—I don't have ANY experience with the PR—so not at all sure what it's supposed to "sound" like at rest—and there's a lot going on—reverb tank sitting on the bench (proximity seems to have a huge effect on noise), not sure how far to take the wire dressing—mid mod pot, etc.. On previous builds—The Champ has some low level hum, seems normal—bad buzz it had went away as soon as I added a separate power switch... The 5E3 is a ghost. You can't tell it's on sitting right next to it. The PR seems to have some low level "noise". Things got A LOT better when I coaxed the input, vol control and mid-mod pot wire. The mid-mod pot was a definite source of noise dialing it out would dial out the noise. After coax it got a lot quieter, but not silent—i clipped in the fixed 6K8 just to see—and it got LOUD—mid mod pot was quieter and you could balance things out with the bass pot. I AM judging things here with the volume maxed—no input—so perhaps unfair? I may swap the input resistors on the jacks for 1W versions as I see @robrob has a note about them reducing hiss. Not sure that's what I'm hearing.. but... quiet(er) is always good? Tremelo is silent. Maybe some twisting of the other pot wires is in order? Coax on input/vol/mid-mod took most of the noise out plus figuring out the reverb tank—really loud one direction—virtually silent the other. Oh, and I'll try some other preamp tubes—that made huge differences in 5E3. One other observation is that the amp gets a little louder with an input... lot's of factors though.

    More later! And... B+ voltages are about +/-20VDC above the noted values on robrob's wiring layout....My transformer has slightly higher voltage though...Sweet amp. Reverb is awesome.

    PF-AA1164-Build_15.jpg PF-AA1164-Build_13.jpg PF-AA1164-Build_14.jpg
     
  4. pfarrell

    pfarrell TDPRI Member

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    Well...
    Massive differences between these two setups—based on some chatting in this thread here. Go there for details....Related to trying for a quieter amp and heater CT config—

    Changes: 1W resistors on inputs (I did have metal film in to answer a question in the other thread), deleting of artificial CT in favor of the ready to go Mercury heater CT lead—added a 10R resistor there for safety (not really noise related if I'm taking in the merlin article correctly..?), and lastly, I covered the inner top of the case with copper foil—I was able to hear the effects of my hand touching the metal of the handle! Vastly diminished with some shielding—perhaps eliminated. I can tell the amp is on if I stand right near it. In a normal room moving about it's not audible. I would call it "normal" having nothing to compare it to other than our Champ and 5E3. If I work the guitar volume and the PR volume and dial in the bass, a little treble (noise here when pegged—normal, I'm thinking), and tweak the hidden mid—I can get everything really tidy and quieter—then it's what pickup(s) that are in play. This is a GREAT amp. LOVE IT. Totally different tool set than the 5E3.

    I suppose the one question I might have—is the reverb seems very subtle to me—turning it on adds a shred of noise floor—and glorious-ness—but it's not a huge difference between off and on when I work the foot switch. Volume seems to be a factor here as well—as is perhaps distance from the amp and letting notes ring...

    The only other change that's occurring to me—based on my other audio projects—is changing out the volume pot entirely for something higher quality. I bought a kit of pots—the volume pot in the kit is one of the main brands I see used frequently—might be CTS—it's the style without a "greased" action—or whatever it's called (Treble is the same style btw—but the others are all really nice feeling). Volume is not a nice feeling pot to turn... My other experiences tell me that pot quality—especially volume—matters. Not sure if that's so true for guitar amps...

    Oh, and checked the heater voltages while I was in there— ~6.5V, yea!

    PF-AA1164-Build_16.jpg
     
    Last edited: Mar 16, 2021
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  5. pfarrell

    pfarrell TDPRI Member

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    Reverb issues!!

    I had the leads reversed on the tank—hence, "subtle reverb", er...none...... I can read it all both ways in terms of ins and outs on schematic... does "IN" match "IN" on the tank and amp or does the "Out" from the tank go to "In" on the amp?
    Anyway—I have reverb now—"Out" from tank going to "In" on amp.... Really bad hum and a little buzz that gets louder with the reverb knob—not the main volume knob incidentally—I would say on reverb knob around 2 it might be usable... any higher and things get loud. Assuming a grounding issue.... all the jacks are in contact with the chassis on the amp. I've read a place or 2 that they should be insulated with a dedicated ground from tank to amp...I can affect some buzz by touching the pedal switches with a finger....so, something is off...

    Ideas?
     
  6. pfarrell

    pfarrell TDPRI Member

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    Update—for anyone out there that's working on getting reverb quiet—
    Tank rotation—took care of the majority of the hum "Out" opposite PT. What I still have could be described as BUZZ—kicks in around 3—grows louder to 10 (really not usable). If I unplug the reverb side of the pedal—all is quiet—is that effectively turning it off though? Because playing with the pedal switches I can change the buzz (reduce it by half) when becoming a giant ground—I alligator clipped the foot switch to the chassis and buzz was killed by half—maybe more.... The foot switch is the typical purchased re-issue style. hmmmm....

    Even though I followed @robrob layout—I've also seen a couple of other builds where the black wire from the reverb transformer goes to a ground lug on the input jack—as opposed to the ground lug on the output jack tied in with the VA3 grid leak.... ground is ground?

    EDIT: Something with the switch? Does this indicate something is off inside the amp?
    Here's an image of me connecting the shield/ground from the reverb switch to the nut on the switch body—this cuts the buzz enough that 7 or so becomes usable on the reverb dial. Still has some buzz though. What is going on here? (trem is quiet—amp is really quiet—when reverb is off.)

    IMG_2112.jpg
     
    Last edited: Mar 17, 2021
  7. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Don't know if this helps- it's the layout I used. The reverb works well until the pot is at 8, where it gets a bit hummy. I'm told "they all do that, sir"!

    Re1.JPG re2.JPG
     
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  8. pfarrell

    pfarrell TDPRI Member

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    Rewired per the above (Thanks! @Uncle Daddy)—I feel the noise floor of the whole amp went down a notch—the above diagram also makes a lot more sense in my brain. Reverb is A LOT quieter. Light buzz (have to be standing near amp to hear it) kicks in around 4, becomes unusable or really pretty loud—easily heard across a room—around 7 or so—again—inside the footswitch itself if I attach the pedal wire shield (grounded to switch lug) directly to the switch body—the buzz cuts in half (not totally gone, but the level at 8 is now similar to the level at 4 w/o direct switch body ground). Quite a bit of other noise/microphonics can be be made by messing with the switch cable. And while I had it all apart again I verified continuity of my copper shielding tape on the roof—guess what McMastercarr sells has conductive adhesive!

    I think I will make my own switch box. SPST switches it seems? In the mean time—it definitely feels like a hack—but i can add a lead to ground the switch body in the switch I have....
    I'm also going to try some other 12AT7s—swapping tubes has made a huge difference in noise in every amp so far...

    Fantastic amp all things considered.

    PF-AA1164-Build_17.jpg
     
    Last edited: Mar 18, 2021
  9. pfarrell

    pfarrell TDPRI Member

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    Figured I'd update this with a little reverb buzz report... I directly grounded the reverb push-button to it's own ground—since with an alligator clip, that had reduced the buzz by 100%. My "fix" wasn't really as effective as the alligator clipped version...So before I made my own switch from scratch, with LEDs etc... I purchased an official Fender footswitch—made in Mexico—suffice it to say—it's different inside—and better made—all the grounds are tied directly to the housing with a solder blob—which is bare metal inside—the shield around the reverb wire is 100% more substantial than the aftermarket—black powder coated item. My diagnosis was correct, but I thought the pushbutton switch body was the issue—but really it was just the only bare metal that was in contact with the housing's bare metal around the switch hole—due to the powdercoating—which wasn't enough to really do the trick...with my piece of wire attached.

    Amp is really quiet now—reverb is very silent until around 4—then some light buzz shows up (100% less with new footswitch)—and isn't an issue at all when actually playing. Without any experience with any other amps—I guess I'm calling it normal. I'm still going to swap out the volume and treble pots for better ones—that has the potential to really be a big deal and further lower the noise floor. It's still not as deadly quiet as my 5E3 build. But reverb is sweet and so is tremelo.

    The amp sounds fantastic. Blessed to own it really. All of them.

    Next up might be something EL84 based... Matchless style. Need better guitar skills. Only about 80 hours so far—mostly acoustic.
     
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  10. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    I have some Neutrik isolated RCA jacks that I'm putting in my latest build to get rid of the "they all do that" hum. They'll have a common ground at one of the ground nodes on the board, rather than relying on the chassis.
     
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  11. jmp81sc

    jmp81sc Tele-Meister

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    Glad you got it sorted, and also a good resource for future Fender Reverb amp builders. Noisy revervb seems to be theme with the reverb and trem foot switches and tank connectors.
     
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  12. pfarrell

    pfarrell TDPRI Member

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    Yay! It did occur to me to isolate the jacks—and secure them their own grounding scheme. I mean—that's what I've done in every audio power amp and preamp—... worth trying...
     
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