Help with my Princeton Reverb homebuild :)

Discussion in 'Shock Brother's DIY Amps' started by Uffepungen, Jul 20, 2020.

  1. Uffepungen

    Uffepungen TDPRI Member

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    Hi all!

    I used to have an account here years ago, but I can't remember my login details. I hope I can get help from you knowledgeable people still.

    About ten years ago, I built a PR from Marsh amps (Mojotone kit I think?). I have gigged with that amp for years, but always had some problems with it. Recently they are getting worse. Here they are:

    - The sound cuts out and distorts at very low volume
    - Reverb cuts out and hums very loud
    - Sometimes when I engage the Tremolo it sounds "hard" and it's not useable

    I opened up the amp yesterday and resoldered everything to match pictures of PR's that apparently work fine. I replaced the 6V6's and it stopped cutting out (I guess that solved one problem, I will replace them with good NOS tubes this week).

    I also tried different NOS 12AT7 tubes at V2, and nothing really helped.

    I moved the reverb tank away from the cab (and speaker), and it took away the hum! With the reverb tank still in place of the cab, I could NOT get rid of the hum when I moved the reverb cables. The hum reappeared when I put the tank back in the cab, probably from the speaker.

    I use red and white RCA cables from TV's, are they bad for this application?

    I can't believe this sort of hum is to be expected, but I just can't figure out why the hum gets louder when I put the reverb tank in the cab? I know people have PR's that are quiet. Why does my reverb tank hum so much while in the cab?

    Also, the "send/return" cables affect the hum, and I cleaned them with isopropanol and resoldered the connections, but still when I touch the cables it can make a difference.

    Does this make sense to anyone here? How would you guys troubleshoot this amp? I patiently await your knowledge, if you have the time :)
     
  2. Uffepungen

    Uffepungen TDPRI Member

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    Sorry guys, I forgot to tell you that I have 12" Weber silver bell alnico speaker in the cab. But I don't think that matters?
     
  3. Jorgen83

    Jorgen83 Tele-Meister

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    TV RCA cables are unshielded and yes that will lead to some these problems with the reverb (noise, not the cutting out).

    As far as the other problems go, it’s impossible to say without at least some detailed photos of how your amp was built.
     
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  4. Uffepungen

    Uffepungen TDPRI Member

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    Jorgen, thank you for your answer! I should get shielded cables for the tank and try it. I didn't realise TV cables were unshielded, I feel stupid now. The reverb tank though, is it a big antenna?

    I will also post some pics of the amp, even though I feel shame for my wiring (I can take criticism, but be gentle please) :D
     
  5. tubegeek

    tubegeek Friend of Leo's

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    TV RCA cables unshielded? Hmm. Not in my experience. But possibly cheap ones are poorly shielded.

    What reverb tank are you using? Most are only grounded to one of the cable shields, the other is grounded at the amp end only.

    Check the connection of the female RCA's to the chassis, make sure they are tight.

    Try rotating the tank 90° (still parallel to the floor, just front-to-back instead of right-to-left.) If that makes a difference then I think the sensitive reverb coils are coupling to the power transformer or the filter choke.

    The reverb recovery tube is a fairly high gain stage, make sure your leads and grounding are well laid out and your solder joints are still well connected.
     
  6. dunehunter

    dunehunter Tele-Holic Double Platinum Supporter

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    If this was your first amp build, I think we would all expect your solder joints and wiring to look a little less than perfect; better looking work comes with practice (additional builds). On my first build, my wiring and joints were...not that good. It worked but I'm not sure how it would hold up over time.

    Second to @Jorgen83 ; you need shielding for your cable and those standard RCA cables you're using do not have it. A braided cable similar to that which you find in Gibson guitar wiring--or a shielded coaxial cable should do the trick, I would think.
     
  7. tubegeek

    tubegeek Friend of Leo's

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    Where are all you guys getting unshielded RCA cables from? I've literally never seen such a thing.

    Agreed that @Uffepungen wants shielded cables, though, for sure.

    Also just remembered something: does your reverb have a footswitch? And is it dual (trem & reverb)?
     
  8. dunehunter

    dunehunter Tele-Holic Double Platinum Supporter

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    I have a feeling, he's referring to those stereo pairs that come standard with a TV; at best, poor shielding.
     
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  9. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    It seems the one brain cell I have left remembers speaker cables from that era sporting RCA jacks.
     
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  10. tubegeek

    tubegeek Friend of Leo's

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    "That era" - pleistocene? But yeah, there was some stereo equipment that was hooked up like that. Cheap, horrible equipment.

    I probably peaked in the pleistocene era myself.
     
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  11. D'tar

    D'tar Friend of Leo's

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    You've come a long way my friend!
     
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  12. dan40

    dan40 Tele-Afflicted

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    The power tube bias also affects the tremolo operation in the PR. I'm not sure what you mean when you say that the trem sounds "hard" but a weak sounding trem could be coming from the bias being set to hot.
     
  13. Dacious

    Dacious Poster Extraordinaire

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    Back to the swamp with you! :lol:
     
  14. Dacious

    Dacious Poster Extraordinaire

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  15. Uffepungen

    Uffepungen TDPRI Member

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    Guys, thank you so much for your support. This forum really is the best for troubleshooting.

    I recorded a video where I show the problems I have. BTW I know my footswitch is labled wrong, the soldering is correct though (I have opened it and checked it out).

    I wasn't able to reproduce the tremolo problem, maybe it was fixed when I changed 6V6's? Also, I've ordered NOS 6V6's yesterday, and I will of course put them in and bias them accordingly. It doesn't seem like the speaker cable affects the reverb hum, I thought it would.

    I hope this poorly produced video gives you something to go on? Don't mind the mess in my living room :p



    I will post pics of the innards of the amp if it helps you guys, I just felt too lazy today to unscrew the horrible fender chassis mounts.
     
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  16. zook

    zook Friend of Leo's

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    The output of the reverb tank should be on the opposite side of the cabinet as the power transformer.
     
  17. Uffepungen

    Uffepungen TDPRI Member

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    It still hums, thanks though, I'll be sure to mount it like that later :)
     
  18. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Time for the traditional 12AX- tube swapa-rama experiment-a-pallooza, but! before that I’d get a new rectifier tube and measure all key voltage points and post those here.
     
  19. King Fan

    King Fan Poster Extraordinaire

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    Hang in there, you'll get this.

    As you did, it's good to re-check the whole problem list after each 'fix' and update the team on what remains. It's now hum and trem?

    One clue you're following is these faults developed after some years in a working amp. Fatigue, like jacks or plugs or socket springs or solder joints, or components with a lifespan, like tubes or caps?

    If the hum isn't the cable (usually, the plugs) RCA jacks are also kinda funky, and IIRC the reverb tank has delicate wiring to dinky transducers. Finding the hum will need folks who know more about this than I do; I just know RCAs are a pain when they don't work.

    The trem could be bias as noted. Do you have a bias pot? If it isn't bias, then we'll start a different hunt. When you open it up can you shoot some good pics and measure voltages?
     
  20. Uffepungen

    Uffepungen TDPRI Member

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    Thanks for your dedication guys. I tried the amp again today and the status is:

    - The amp is quiet enough with the tank out of the cab.
    - With the tank in the cab (which makes it hum), the hum gets louder with increased "Reverb" levels.
    - Moving the reverb tank RCA cables doesn't seem to change anything. They sometimes disconnect the reverb though, I think that points to bad connections, but cleaning and resoldering them hasn't helped. The reverb works though if I rock the RCA cables so I guess I can live with that.
    - Since I put new 6V6's in, the amp hasn't been cutting out. I have a set of good quality NOS 6V6's on the way that I will install.
    - I have switched out the V2 12AT7, the hum doesn't go away. It sounds better with Mullards than JJ's though.

    I will open it up and take voltage readings and pics when I install and bias the new 6V6's. I did however check everything a couple of days ago, and no voltages stood out too much. BUT some resistors seemed out of spec, so I will order a set and replace those. I will probably also order a new rectifier tube per advice in this thread. I'll get better RCA cables while I'm at it.

    If nothing reduces the hum, I wonder if I just have to live with reverb hum? It's not that bad on "3", which is where I usually keep it anyway :p

    I have a bias pot installed, and the tremolo sounds nice after biasing. The problem I had before was that the tremolo sounded loud and "hit" the sound I was getting, but I don't have that problem right now :p
     
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