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Help: 65 Princeton Reverb Kit Build Problem

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

  1. trber

    trber Tele-Meister

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    I am finding that some of the things that a layperson would think are givens--the amp should just work, cuz the build is done, right?--are not! What a great learning curve and build up of knowledge re: amps, troubles, troubleshooting, and maintenance for me. Thanks Dan!
     
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  2. dan40

    dan40 Tele-Afflicted

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    Hoffmans forum is a wealth of info with a bunch of very smart builders and techs! If you want to purchase from his store, you don't need to set up an account unless you want to. His service and shipping speed is top notch but he has started to discontinue some of his larger items (transformers and chassis) as he gets closer to retirement.
     
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  3. trber

    trber Tele-Meister

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    Thank you! I finally got is sorted, and my order is on the way. I also joined the forum, so I can dive in there. I have to say, this Princeton project, difficult as it has been for me, has really bonded me to the amp, and I look forward to working on it, getting it "tuned" correctly, and enjoying the process. I like learning about it, and amps in general, the more I learn, the more I want to know! I am glad to have the resources.

    It's funny, when I come across someone's build pictures, I have a much deeper understanding and appreciation of what they have done. Dare I say it, I like looking at pictures of people's chassis! LOL.

    Thanks, Dan. Great group here, you all are.
     
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  4. trber

    trber Tele-Meister

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    Hey King Fan or anyone else willing to chime in, I am posting some updates, and hopefully, someone can help clear up some confusion.

    I received my parts order today and didn't have time to do anything else besides install the 1ohm, 1%, 1/2watt resistors from pin 8 to ground and take some readings. Here's what I found.

    My voltage across pins 3 to 8 was 416v on each tube. I got the readings off of pins 5 from each power tube @ -41.5v. I also got readings off each 1ohm resistor at 12.5 and 12.7. So, the tubes are apparently running much more closely than they seemed before. Maybe it was because I was using a 1ohm 5% resistor, as some have suggested.

    Here's my confusion. I used the bias calculator above and came up with:
    Plate Dissipation of 5.2w and Plate Dissipation % as 43.2%, which is nowhere near what's recommended, as I understand it. PD should be 8.4 and PD% at 70%. Why is everything so far off?

    Reminder: I added a 10k resistor in series with the 22k resistor on the bias board to try to get some numbers sorted out previously. Is this 10k causing/contributing to the strangely low bias?

    I have not installed a bias pot yet, as I wanted to see if things were OK without it after replacing the 1ohm resistors. Apparently not!

    So, if I install the bias pot and adjust it within it's range of 10k, am I likely to get this sorted out? It seems like these numbers are really only close to half of what they need to be, but I also don't have a sense of how things change with the bias pot, so maybe it will be within adjustability range.

    I also tried calculating bias differently using a formula and came up with 20ma which is 60%, so I am doing something wrong here.

    Any comments or feedback would be appreciated.
     
    Last edited: Jan 12, 2020
  5. trber

    trber Tele-Meister

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    BTW, did you add a bias pot to the ground switch location? I don't see one anywhere else?
     
  6. King Fan

    King Fan Poster Extraordinaire

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    Yeah, it's the Fender style you can adjust from outside with a slotted screwdriver. You'll notice the twisted yellow/white bundle heading back across the chassis floor.

    To measure bias now, you simply need plate-to-cathode voltage (and as you'll understand, unlike cathode-biased amps, here that's basically plate to chassis) and the mV across your 1R. Yep, your bias is too low. I'd revert your bias board to the correct values, but I'd also install the bias pot.

    You're saying you have a 32K bias tail? This bleeds less negative bias voltage to ground, so sends more negative voltage to your power tubes, so cold is expected. What's your first (bias adjust) resistor value? What size pot will you use?
     
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  7. trber

    trber Tele-Meister

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    Thanks, I did! That's what made me wonder if it was a bias pot. If I were to locate mine here, I'd have to extend the lead from the board to the new location and J hook it. Any cons with this? I could also just locate it near the board as I have seen done as well.

    Will do. I can remove the 10k and try reinserting the 22k, but I bought a new 22k replacement resistor for just this reason, so I'll see if I can leave the old one and...picking a bias pot location will get the process solidified.

    Not sure what this is, "first (bias adjust) resistor value". Apologies, but I am not clear on all amp specific terminology.

    I have a 10k sloted style and I also purchased a 25k Alpha linear taper pot, both via Hoffman's webstore. The Alpha pot has a solid shaft, so will this be a safety problem for biasing without a knob as in will it be "hot" with voltage? I wasn't sure if the 10k would cover the range I ultimately needed so ordered the 25k, too.

    Thanks, and looking forward to putting this thing back together to play rather than work on! ;)
     
  8. King Fan

    King Fan Poster Extraordinaire

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    Now I'm really confused. Shoot us a pic of your bias board.

    I used the 10kl for safe external adjustment, but Rob's 25kl would be nice if you mount it near the board.
     
  9. trber

    trber Tele-Meister

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    Here's the bias board. I was intending to follow lowerleftcoast's directions on an earlier post but may have misunderstood the information. [​IMG]

    Sent from my SM-G955U using Tapatalk
     
  10. trber

    trber Tele-Meister

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    Update:
    After removing the 10k resistor above and inserting the 22k it's attached to, I got the following readings.
    -27.8v at pin 5
    368v across pins 3 to 8
    v5= 27mv across 1ohm
    v6= 28mv across 1 ohm
    PD= 9.9w
    PD%=82.8

    So, too hot if 70% PD% is the goal.
    I'm feeling like the 10k bias pot might get the job done, but would like confirmation if anyone has a sense of it.

    Also, according to the robrob schematic here, he's running a 10k resistor to ground off the 25k pot and has no resistor on the bias board any longer, so to mimic this for my situation, I'd use a the 10k slotted pot with what resistor value to ground? 25k?

    I realize I'd have less sweep on the pot than with his 25k pot, but to get the combined resistance, would that be the wise approach?

    Thanks!
     
  11. King Fan

    King Fan Poster Extraordinaire

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    You're getting it. Doug Hoffman runs a 27K tail resistor with a 10K pot.

    Re component names, it helps to see they relate to function. You can figure the 'tail' part; the first resistor on the bias board is sometimes called the 'bias adjust' resistor cuz without a pot that's the one you (traditionally) keep swapping to adjust your bias.
     
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  12. trber

    trber Tele-Meister

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    Thanks! 27k it is!
     
  13. dan40

    dan40 Tele-Afflicted

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    The 27k will likely get you really close to your desired bias. I like my PR biased a bit cooler, somewhere between 50-60%. It helps the trem to function better and eases the burden on the power tubes a bit.
     
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  14. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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  15. trber

    trber Tele-Meister

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    Thank you, Dan. I have a Marshall amp that I biased lower than recommended and appreciate what it did for that amp. Cleaner at lower volumes, and tube life never hurt anything!

    Now that I have adjustable bias, I can give this a try. Looking forward to it and thanks for the recommendation.

    Cheers
     
  16. trber

    trber Tele-Meister

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    Hi, I believe it's there, upper right, no?

    I looked closely at the layout he uses and decided to put the bias pot in the ground switch location. I may move it later, but really wanted to get things buckled up to play the darned thing!

    I have found a great appreciation for the robrobinette website. Really interesting and informative.

    Thanks!
     
  17. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Ooops! Sorry for the noise, missed it.
     
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  18. trber

    trber Tele-Meister

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    No worries! If you miss stuff as often as I do, it's just routine! Lol. Thanks for asking.

    Sent from my SM-G955U using Tapatalk
     
    Last edited: Jan 13, 2020
  19. trber

    trber Tele-Meister

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    Dan,
    Out of curiosity, do you use pedals on your amp or ever push it into od from volume alone? If so, how do you find the overdriven sounds with your bias as low as it is? Where does it start to break up? Thanks, as trying to understand more about this lower bias setting.
     
  20. dan40

    dan40 Tele-Afflicted

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    I'm mostly a home player so I don't get to crank my PR up as often as I would like. Unfortunately I really don't hear much of a difference in the amp's overdriven tone when set anywhere between 50-70% dissipation. The amp does have a bit more noise at idle and the trem does not sound as nice when set at 70%, but I cannot tell much difference in tone when compared to a lower setting. The amp does seem to feel a bit spongier at higher bias levels though which may be caused by the higher demand on the power supply.

    There are many folks here that do hear and feel a difference between those different bias settings though. I'm sure if you started a new thread on that topic you would get plenty of replies from the fellas.
     
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