It's alive! Brownface Princeton Reverb

Discussion in 'Shock Brother's DIY Amps' started by Uncle Daddy, May 6, 2021.

  1. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    It works, it didn't catch fire, and I didn't electrocute myself. Three out of three! I wanted something a little different from my 12"PR with Deluxe transformers, so I went for a 10" and the correct 8ohm O/T, with a very useful 25k mid pot on the back in the external speaker jack.

    Got a nasty hum on the reverb to work out though. The verb is simply huge on this- halfway up is a massive abbey tone, but the hum kicks in around 3 on the dial. It may have something to do with the isolated reverb sockets I tried, but I still have to do some chopsticking which will happen when I set the bias. I did the deeper trem mod too, and it's nice, but I'm hold out till the bias is set to pass final judgement.

    Thanks to everyone who helped out with my inane questions!

    Pics-
    P1130276.JPG P1130277.JPG P1130278.JPG P1130281.JPG P1130282.JPG P1130275.JPG P1130284.JPG
     
  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Very tidy build, looks great.

    Now. I don't know *anything* about reverb hum, but you're right, ground is a potential issue, and yeh, isolated jacks are 'different.' Where do they ground (looks like on the preamp bus?) -- and where does the preamp bus ground to chassis? Does it make a difference if the footswitch is plugged in and turned off?
     
  3. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Looks great! The PR I built a couple years ago had some reverb hum issues and it ended up having to do with the NFB & reverb lead crossing or touching or something. Just took enough poking and pushing around to find it and get rid of it.
     
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  4. Chicago Matt

    Chicago Matt Friend of Leo's

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    Sweet amp!
     
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  5. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    I've been having a lil fiddle with it, and it look like it's the tank. The hum disappears entirely with the footswitch engaged. I tried another tank, and there's a lot less hum. Both tanks are sensitive to orientation, the original much more so.

    The reverb jacks ground onto the preamp bus, as you noticed, which grounds to the chassis via the input sockets.
     
  6. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Wondering if I made a mistake grounding all the jacks- just came across this, similar issue with a Blues Junior.

    Is it an 8EB2C1B? Only one RCA jack shell is grounded. If both are grounded, you get a ground loop through the tank. Other part numbers have other grounding schemes.
     
  7. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    So it looks like the "out" on the tank grounds the tanks shell, so I'm guessing I'll have to get back inside and de-isolate that return socket on the amp chassis?
     
  8. dan40

    dan40 Friend of Leo's

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    Great looking build!! You got me itching to build another one now.
     
  9. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    If the chassis is cranked tight into your cab, and the top of the cab has shielding in it, and you are playing 6+ feet away from the amp, does the nasty hum still persist?
     
  10. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Yes. It starts around 3 to 4, and gets louder further round the dial
     
  11. Commodore 64

    Commodore 64 Friend of Leo's

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    Dude, that's a sweet build. Lead dress, test points, protection diodes, it's all there!

    Some reverb tanks are for horizontal, some for vertical positioning, too.

    I've built a Fender stand-alone reverb. And that's the only thing I've ever built with reverb. So I'm not much help regarding stuffing the circuit in a PR.
     
    Last edited: May 7, 2021
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  12. ruger9

    ruger9 Poster Extraordinaire Ad Free Member

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    That's a dream amp. To my knowledge, only one builder has ever added reverb to the brown Princeton: Marsh. The Buffalo Reverb. Always wanted to try one, but only 1 online demo, and not much said about about Marsh in general.

    I wonder if the Lil' Dawg guy does any custom work?
     
  13. Jowes_84

    Jowes_84 Tele-Meister

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    Besides grounding: shielded wire for the reverb signal circuit and maybe up that reverb mix resistor to 1meg?
     
  14. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I'm not sure... you’re trying to repro the function of the original jacks, where each one was grounded by design?

    Before I went to too much trouble I might post a 'help: reverb hum' thread. Sometimes NAD threads don’t draw in all the smart eyeballs out there...
     
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  15. Greg70

    Greg70 Tele-Meister

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    I researched reverb hum when I did my Mojotone PR build. I was concerned because it seemed to have a lot of hum during initial testing. I saw a thread on another page where the solution was isolation jacks on the reverb circuit to avoid a ground loop. Once I got it all together I figured out that my issue was that I had the chassis on the test jig with the tank sitting immediately under the PT. How much larger is your PT than a stock Princeton would have? The PT can induce hum into the tank. The other thing that I learned was that hum is inherent to the PR reverb design. Even the new PRRI's have reverb hum. If you start googling you will find there are a LOT of PR's out there with reverb hum- both old and new ones. But once I started actually playing my guitar through the amp I realized that in practice I never have the reverb up high enough that the hum is an issue. Reverb sounds best around 2.5 and hum kicks in around 4.

    I also read on that same thread that metal oxide resistors (220k?) on the reverb loop can induce hum. I'm not sure how true that is. I have a metal oxide one on mine and left it in place.
     
  16. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Mine hums even with the tank out of the cab and some way away. I'm going to rework the isolation sockets; I think I may be getting a ground loop through the reverb tank. With the reverb knob dialled right down, the amp is very quiet with just the normal tube hiss.

    My other PR sounds much like your description, with a hum that kicks in around 4 and by 7 makes the reverb unuseable. I was trying to improve on that with the isolated jack sockets, but it's not there yet.

    The 25k mids knob works well though.
     
  17. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Big improvement, but still not quite there. I de-isolated one of the jacks, and swapped the tanks around. The metal shielded reverb cables weren't helping either, so I swapped them for modern RCA cables. I also wrapped the tank in foil. Together it's reduced the hum, but I'm wondering if I may have a wrong value component- the verb is huge and splashy around 4-5, which is where the hum begins to really show. I tried a 12au7 in V2 but it set off some oscillation, so I'm back with a different 12at7 to rule out tube issues.

    Apart from that, it sounds pretty good. Very quiet on idle.
     
  18. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Did you already try moving the return jack’s cap to ground to the cathode of the same stage’s ground? At the chassis/jack is how it’s usually done, which can add noise.

    I had problems with my verb noise too. I also had to flip the tank. For some reason it also helped. I forget what else I may have done.
     
  19. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Tried a different tank and cables- same hum. Tried another V3 tube, some improvement. I noticed I indexed the heater filaments wrong on v4, but there's zero heater hum.

    What would changing the reverb mix resistor do?
     
  20. Wally

    Wally Telefied Ad Free Member

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    just to keep things straight.... This is NOT a 6G2 Princeton reverb added. It is a BF Prin Rev. The key difference would be the tone stack. This amp has the BF tone stack whereas the 6G2 has a simple one-pot tone circuit that does not eat gain the way the BF stack does. Hence, the 6G2 is a meaner sounding amp. The Marsh amp you mention has that simpler 6G2 type of tone circuit...and therefor is a different amp.

    Nice looking amp, Uncle Daddy. Kudos!
     
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