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Need some help with Princeton Reverb Build

Discussion in 'Amp Tech Center' started by KBarW, Feb 24, 2020.

  1. KBarW

    KBarW TDPRI Member

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    Building a Mojotone Princeton Reverb amp. All the way down to the testing. I've run into a big problem - I'm reading 833 volts at the power tubes. The rectifier is reading 328 volts just like it should. The diode board is correct with the proper polarity.

    I've looked over my wiring on the transformer section and everything looks good.

    I checked the eyelet board with the power tubes removed and every point I checked was reading about double what it should be.

    I'm experienced in soldering, having built tons of connectors, cables, rack panels, etc., but I'm a total novice at amp building.

    Anything of the top of anyone's head that I could check?
     
  2. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    Did you switch your meter to DC Volts to measure at the power tubes?
     
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  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Alright, so move up from the rectifier. Check the AC coming into it from the power transformer. If that seems reasonable then check the DC at the first filter cap node, which will also be connected to the rectifier. If those two areas aren't right, nothing else will be right.

    Move out from there - second filter cap node, and so on.
     
  4. BigDaddy23

    BigDaddy23 Tele-Holic

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    Step one
     
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  5. KBarW

    KBarW TDPRI Member

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    No, I didn't. The layout diagram just says 380v. Do I assume that every voltage is DC unless it specifically says AC?
     
  6. KBarW

    KBarW TDPRI Member

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    Thanks for that.....now if I only knew what you mean by filter cap node. The voltage from the transformer measures properly at the rectifier.

    It's embarrassing at my age and experience in PA systems and playing to be such a newb at this.
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Filter cap nodes are usually lettered on some schematics, with A being the very first cap that connects to the rectifier.

    So ignore the "node" thing - what's the DC volts at the first filter cap connected to the rectifier tube?
     
  8. dan40

    dan40 Tele-Afflicted

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    The voltage from the PT to the rectifier will be AC. Everything after the rectifier will be DC. The 6.3v secondary for the tube's heaters will be an AC voltage at every socket. Be aware that voltages will be high until you get power tubes installed.
     
  9. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    833?? No mater what your VOM is set for, that seems odd. My PR builds have been around 414-422VDC with a 630VAC secondary.
     
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  10. Whatizitman

    Whatizitman Friend of Leo's

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    No. You need to know what currents are AC or DC in an amp, and why. A schematic assumes this is fundamental and known.

    The rectifier changes AC to DC for the tube power supply. The big caps further ‘smooth’ the DC. Power coming to the tube plates will be DC. The signal circuit will be AC. Sound is an alternate current form (AC). Heater supplies can be AC or DC, depending on the amp.
     
  11. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Mojotone’s wiring diagram now lists desired-voltages at a bunch of different test points. See here: https://system.na3.netsuite.com/core/media/media.nl?id=3393932&c=923962&h=1a2d186b30f62b14658d&_xt=.pdf&vid=sfseFGyuAlFmiGXM&chrole=17&ck=WMT1fWKMAqfLk70t&cktime=175724&promocode=&promocodeaction=overwrite&sj=wQoirDS18J968UFOTH2GgswYo;1582603724;406816000&gc=clear

    What are yours at all those same test points? If you can post a pic of the above image with your voltage measurements overlaid, that would be super helpful. (Mojotone doesn’t show desired voltages on their PR schematic, idk why).

    Also, what kind of meter do you have and what is it set to?

    You will totally love this amp when you are done. Lots of us here have built and fixed these.
     
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  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Everyone starts somewhere. The people here will be helpful. It may be a little overwhelming having several people asking many questions but you should be able to get your questions answered. Hang on, you will learn a lot.

    I hope the readings you get with your meter on DC make sense. This is not always the case though because some meters have difficulty reading high voltages. Your reading of 833 Volts is most likely inaccurate and just the meter having trouble recognizing a DC voltage on the AC range.

    Post your voltages. I will be interested in your findings.
     
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  13. trber

    trber Tele-Meister

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    Yes, check the mojotone diagram as it has labeled voltages, AC and DC. I concur... this amp is a hoot when you get things sorted. I just went through a bunch of troubleshooting problems in December. It'll come together and you will be psyched when it does!

    I just got done playing mine for the last hour or so. So great.

    Sent from my SM-G955U using Tapatalk
     
  14. koolaide

    koolaide Tele-Afflicted

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    I would recommend that you see YouTube (Uncle Doug Chanel) How tube amps work- Part 1 and 2 first. Then review his content as needed. The first two videos will help you understand AC vs DC and what is where in thee amp, and give you a basic understanding. Great foundational stuff. Short and not too boring.

    Good luck with your project.
     
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  15. KBarW

    KBarW TDPRI Member

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    That's the layout diagram I'm using. Unfortunately, I was taking readings in AC throughout. Now I know that only those that say AC are indeed AC. I'm about to start over with my testing and checking voltage in DC at the appropriate points.

    I'm trying to understand the meaning of B+ and secondary voltages. Wish I was more educated in this, but I guess I just assumed I could follow the instructions and it would be fine. I'm very meticulous in everything I build....or at least try to be.

    I'll go start the process over with the appropriate setting and see what happens....

    Oh, I do have one question.....when I was testing voltages (incorrectly) with the power tubes removed, after a while the fuse blew. Would this be typical with the power tubes out?

    Thanks to all of you for the suggestions...
     
  16. KBarW

    KBarW TDPRI Member

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    Wow....just watched both videos.....really clears up a lot. At least I now know what the heck a "heater wire" actually does.... I'm going to do a little more testing now that my understanding is clearer.

    Thanks
     
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  17. KBarW

    KBarW TDPRI Member

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    This is the cab I built for it.....waiting patiently.

    IMG_2893.JPG
     
  18. trber

    trber Tele-Meister

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    Nice! 10 or 12 inch?

    Sent from my SM-G955U using Tapatalk
     
  19. KBarW

    KBarW TDPRI Member

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    12" Putting a Cannabis Rex in it. Or, I'll put that in my Boogie and move the Lil' Texas Neo into the Princeton.....I'm looking for the Princeton to marry up with the Tele for clean country. The Boogie gets a LP for my Rock and Blues Band.
     
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  20. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    No.
    1 You may have touched something in your testing.
    2 You may have something wired wrong.
    3 Without the tubes installed, the voltages can exceed the filter cap voltage ratings causing them to fail.

    I would suggest building a "light bulb limiter" to help with amp testing. How to video is available on you tube. It is a handy tool for ailing amps and startup of new builds.
     
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