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Help: 65 Princeton Reverb Kit Build Problem

Discussion in 'Amp Tech Center' started by trber, Dec 28, 2019.

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Try to make it look like the picture from King Fan. It appears he does not have the heater wire installed yet.

    Nice presentation King Fan!
     
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  2. trber

    trber Tele-Meister

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    Wow, thanks for all this! I had to return to my day job today, but I'll go through the post looking closely, first chance I get! Nice!

    Sent from my SM-G955U using Tapatalk
     
  3. trber

    trber Tele-Meister

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  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Correction to this post. "Phase Inverters JJ 12at7"
     
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  5. trber

    trber Tele-Meister

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    HOLY $H!!!TTT....I THINK WE'VE GOT IT!!!!

    You're a genius! So, when I read this my curiosity got the better of me. On my lunch break I ran home because how hard is it to pull a tube and play? Easiest test so far!

    Guess what! I pulled v2, and beautiful Princeton overdrive! Go figure.

    I need to find out how to properly repair this, and then clean up the mess, but I think this is the problem!

    There is if course the varied bias on the power tubes, but that nasty overdrive sound seems resolved!

    Who'd have thought?

    I also will follow up with Mojotone. Can't understand why they have not seen this before, and....

    Why my amp and not others?

    Interesting!

    More later, gotta get back to work!

    Sent from my SM-G955U using Tapatalk
     
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  6. King Fan

    King Fan Poster Extraordinaire

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    Cool. Looks like a game-winning touchdown in double OT by @Lowerleftcoast!!

    I see the TGP thread he found refers back to a TDPRI thread:

    https://www.tdpri.com/threads/ab763-blocking-distortion-troubleshooting-w-pix.386807/page-2

    Scanning through both threads I seem (???) to see three possible solutions mentioned (I may easily have missed one, or included one that didn't work).

    1. Simple swap for a different 12AT7?
    2. Simple insertion of a ?? 20-50pf cap from reverb driver plate to ground?
    3. Lead dress to keep leads to plates well away from the wire across the V2 socket???
    The last of these is not at all clear to me and may not be a real problem or solution. If it's just a matter of a bad 12AT7, that might explain why it's not common. If it's a matter of sometimes needing a cap there to prevent runaway, maybe that's not a common occurrence either.

    Anyway, here on page 14, I'm gonna go chill the champagne, lay out the ski goggles, and put plastic sheeting over everyones' lockers.
     
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Nice. I have to agree with King Fan. Your patience is saintly. I bet you will be ready for your next build in about two weeks.

    I will be doing a family thing. Not champagne probably tequila. I'll check back in a few days. I figure about page 17?
     
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  8. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Hammond/Fender tube reverb is like a 5W Champ shoehorned inside your Princeton and turned up to '10' all the time driving the input transducer in the spring tank. Maybe a 12AT7 full on is more like 2W, but I'm trying to paint a picture. It's a lot.

    This is all to say that all the preliminary troubleshooting was important to eliminate confounding variables and stand on solid ground. The folks who suggested anti-blocking distortion and anti-oscillation measures knew what kind of problem this amp had pretty much from the start, but I think it was good to get all the other stuff out of the way.

    Now you can do a bunch of careful chopsticking with the reverb driver and any leads nearby while the amp is idling and listen for any changes in the self-noise. That can be a helpful clue.
     
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  9. dan40

    dan40 Tele-Afflicted

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    Congratulations trber on narrowing down the problem! Lowerleftcoast's suggestion of removing v2 was a great idea and all of this troubleshooting that you have been doing will really pay off for you in future builds. You have gained a lot of experience through this project that many folks don't bother to learn.

    Hopefully a new tube or some lead dress adjustment around v2 will solve the issue. I really like using NOS Mullard cv4024 tubes in this spot because they are mil spec and not extremely expensive yet.

    How is your reverb tank mounted? Is it place inside of the usual Fender bag laying in the bottom of the cabinet?
     
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  10. trber

    trber Tele-Meister

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    Gentlemen, thank you for the compliments on patience, but my patience would be useless without your direction and guidance. In a perfect world, I'd be able to buy you a round of whatever your preferred beverage, champagne, tequila, you name it, in person and hear your stories, particularly how you got to be such bad-ass amp troubleshooters. Now that would be cool. It's truly impressive and humbling that you spent so much time to help a perfect stranger, fumbling along, trying to fix his amp build. Think about that.

    Since I can't be there, throw one back with me...I'm toasting!

    Lowerleftcoast, great find. I am reaching out to Mojotone to share the links and story. They really need to know about this, cause apparently they don't!

    In the meantime, I have a lot of reading to catch up on here, some work to do on my reverb problem, a bias discrepancy problem to sort, and parts to order! I am really looking forward to hearing this amp in all its glory, and now I can think about future mods and my next amp build...maybe a 5E3 in a couple of weeks! ;)

    I'll be reporting findings and results as they happen.

    Cheers and gratitude everyone!
     
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  11. trber

    trber Tele-Meister

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    Thanks Dan! I'm pretty pumped...just need to finalize everything and figure out he real solution for me, possibly your tube choice. Thanks! My reverb tank, a mojotone tank: https://www.mojotone.com/amp-parts/ReverbTanks_1/4AB3C1B-Reverb-Tank-Fender-Marshall-Ampeg-2-Spring

    It's mounted on the bottom of the cabinet.

    I am pleased with my newly acquired skills, and will be really fired up when I can play the Princeton! Can't wait!

    Thanks on all.
     
  12. trber

    trber Tele-Meister

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    Great input Snfoilhat! I am going to get chopping asap! I am glad I learned as much as I did, but just wish it took a bit less time! LOL. I'll be checking those leads for sure.

    Cheers
     
  13. trber

    trber Tele-Meister

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    Thanks for condensing the findings! I am going to read the two threads carefully and try the path of least resistance...Hopefully, a lead dress or tube swap will do it. To be safe, I'll be ordering those caps, as well!

    Thanks, King Fan!
     
  14. trber

    trber Tele-Meister

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    This is great, and thanks for the comic relief. I was losing my sense of humor for f's sake. First chance I get, I am learning to bias my amp, and I'll post up results. I want to get that discrepancy sorted, but no, I don't have any 1%s yet. I am planning to order a bunch of stuff from Tubeampdepot, and have been in communication with their tech guy, and they don't carry any 1% resistors other than a Dale's at 10 ohm, which just changes the math a bit as I understand it. They have some other 1ohms that are supposed to be good, but not rated at 1%. Any idea where to source some of those 1%ers? I have not had any luck yet.

    Thanks...results to follow!
     
  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Did someone say 5e3? If you don't mind I'll insert some eye candy. I don't have it anymore. I traded it for a "Big Blonde". A '64 blonde showman.

    DSCN1316.JPG DSCN1319.JPG

    5e3 installed in a '30s Philco. Yes master volume in the standby hole.
     
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  16. King Fan

    King Fan Poster Extraordinaire

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    Too bad, I thought Tubedepot carried the 1R in the past. But never fear, Doug Hoffman has them, listed as the very first item on his resistors page. Do you need some other stuff? in general he's my favorite supplier, ships instantly, 2-day, at reasonable cost, and carries *almost* everything you need, all of it amp-specific so you aren't sorting through thousands of options.
     
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  17. dan40

    dan40 Tele-Afflicted

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    Biasing your amp will be a breeze once you get a couple of good 1 ohm resistors in place. Just remember that as you increase the ma (current) that the tubes's are drawing, the plate voltage will come down. If you decrease the current through the tubes, plate voltage will rise. You have to take the plate and cathode readings several times as you adjust the bias because the changing plate voltage will affect the calculations. You can also just purchase power tubes from a seller that uses a specialized rating system for their tubes. This way you can purchase tubes with the same bias point every time and you won't need to constantly readjust for every new pair of tubes.

    I asked about the mounting because it needs to be placed in the bag and then the bag will be loosely attached to the bottom of the cab. Direct mounting of the tank to the cabinet floor can cause problems but I'm sure that Mojotone would have supplied the correct bag. Hopefully the oscillation problem will be an easy fix. Every amp is different and even a tried and true layout can cause problems occasionally that require extra steps to be taken. Fender was also known to add suppression caps in some of their Silverface amps to combat the same issue.
     
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  18. trber

    trber Tele-Meister

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    Wow...That's a unique amp! Thanks for posting, and I'm taking notes: master volume. Nice! Enjoy your Big Blonde, too. [emoji106][emoji106]

    Sent from my SM-G955U using Tapatalk
     
  19. trber

    trber Tele-Meister

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    Yes, I do need other items and am working on getting a user name and account set up. Not very intuitive! Thanks for the lead and you're the second person in this thread to mention his site. It must be worth a look! Cheers

    Sent from my SM-G955U using Tapatalk
     
  20. trber

    trber Tele-Meister

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    Thank you, sir. Great help and information here!
     
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