1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Need some help with Princeton Reverb Build

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

  1. King Fan

    King Fan Poster Extraordinaire

    Posts:
    5,734
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Just read thru this thread. Wow.

    @KBarW , you deserve a lot of credit. A working PR is a huge achievement for a first amp build. Also, credit for absorbing a ton of info and making sense of it -- and for stating your questions clearly, admitting your mistakes, and responding to suggestions gracefully and intelligently.

    And the folks who pitched in here get a special award. So smart, patient, insightful about what needed to be shared — and so decent and humane in the sharing. I see this often here, but this was an all-out great example, and without the insistent 'wrong answers' and internecine flame wars that sometimes mar our efforts.

    Now, KBarW, a brief coda: A 10kl bias pot and a couple solder joints would make your amp a masterpiece. I like the approach described here by Doug Hoffman. Although it's clear he wrote this up in the dot-matrix era, it still works perfectly.

    upload_2020-2-26_19-21-25.png
     
    dan40, Whatizitman, KBarW and 2 others like this.
  2. trber

    trber Tele-Meister

    Age:
    54
    Posts:
    207
    Joined:
    Jun 4, 2016
    Location:
    NorCal
    Couldn't agree more... great, helpful people. Thrilled to hear another amp up and running. Nicely done everyone!

    Sent from my SM-G955U using Tapatalk
     
    Last edited: Feb 26, 2020
    KBarW likes this.
  3. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Thanks for the kind words.....I appreciate all of the help here. Everyone has been wonderful. That example you gave of the bias pot might actually be something I could do - that drawing was very clear.

    What I'm still not clear on is what exactly is a "good" bias setting for these amps - I get it that it can vary, but it would be helpful to understand what the difference in sound is between say -30 and -40? Does one give you more clean headroom?
     
  4. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    A little follow-up.....

    I took it to practice tonight (getting a new drummer up to speed) and it was really fun to play through. All my testing thus far in the garage has been with my Tele - which is the guitar I'll be playing through this amp. But, tonight I used my LP (that's what I play in this band) and it sounded really good. The reason for taking it was to see if it could keep running for 3 hours without letting out the smoke. It passed with flying colors.

    If I decide to do any mods, it will be a bit of an undertaking as I put a solid baffle in this cab with a separate grill frame....the fit is such that I'd have to take the speaker out to get the amp back out. So, it might be a while before I undertake something like that.

    As far as this forum - I really like the attitudes here and everyone was very helpful and patient with this newbie.

    I only visit one forum regularly - it's the BFM speaker building site. Everyone there is like the folks here - we try to help the new guys make good decisions and help them through their builds.

    I ended up here because I saw a blurb somewhere else about you guys being helpful and knowing your stuff. I was already a member here from some work I did on my Tele a few years ago.....so, I'm glad that I jumped in here.

    I rarely play my Tele as I play in a Rock and Blues Band and it's LP all the time through my Boogie Mark V:25, but I've got some country gigs coming up and my usual setup with my ES335 and a little Roland Cube just wasn't as country as I'd like.

    The reason I rarely play the Tele is that, even though I'm a big guy - I have short fingers and the Gibson scale just fits me better. So, I finally broke down and put a Gibson scale neck on my Tele a couple of weeks ago - HUGE difference for me. I can play the Tele now....that kinda' pushed me toward building the Princeton - they should play nicely together.

    Anyway, that's who I am and what I do. There's a build thread for this amp over on the BFM forum if anyone is interested in seeing pics of the cab and amp being built.

    Thanks for all the help.....I feel like I'll be lurking here more now.
     
    Last edited: Feb 27, 2020
  5. Whatizitman

    Whatizitman Friend of Leo's

    Posts:
    4,243
    Joined:
    Feb 18, 2018
    Location:
    WV
    Uncle Doug? I didn’t know that. I figured he was in AZ.

    I am a huge fan. Was just watching his videos on bias and plate resistors tonight.

    OP, even if you decide to enlist a pro to help, do yourself a favor and watch these series. He has a gift for explaining things in ways even I can understand!




     
    KBarW likes this.
  6. Whatizitman

    Whatizitman Friend of Leo's

    Posts:
    4,243
    Joined:
    Feb 18, 2018
    Location:
    WV
    Yes. More negative charge on the screen means less current flow. So a “colder” bias. This allows for more headroom, or more “room” for the tube to increase current flow with a signal applied before saturating completely. Bias voltage must be within a specified range negative or positive (less negative) for a given tube, or the tube could “shut off” or red plate, etc...
     
    KBarW and koolaide like this.
  7. King Fan

    King Fan Poster Extraordinaire

    Posts:
    5,734
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    We want to think in terms of % max dissipation, not just current; plate voltage interacts with current. Amps like the PR (non cathode-biased) sound best and work safest between 50 and 70% dissipation. All we need is to figure out what to measure and plug into Rob's bias calculator mentioned above. Let's measure one tube at a time. For each...

    calc.jpg

    Step 1. Click on your 6V6 type (12w unless you're running JJs) and enter the DC voltage you measure between 6V6 pin 3 and pin 8. (It's easiest to use mini-grabber probes for this.) In our example, the voltage is 390VDC. Hit Calculate. Repeat this step with measured p-to-c voltage every time you change tubes or bias.

    Step 2 (easy way). Measure plate current using a cheapish 'octal bias probe' like those from Eurotubes or the 'bay.

    Step 2 (free way). On Rob's how-to-bias page, go to the bit called Measure Bias With the Output Transformer Resistance Method. You'll measure VDC to ground for each of your pins 3 and then for the OT center tap -- often a red wire -- which you can probe here on the PR.

    OTCT.png

    Subtract plate voltage from CT voltage for each tube. Now turn off the amp and -- while warm -- measure resistance between that same OT CT and each 6V6 plate (pin 3). Divide voltage drop by resistance, multiply by 1000 for mA.

    Rob's example: 1.347 volt drop and 72.0 ohms between the center tap and tube plate = 1.347 / 72.0 = .0187 amps or 18.7 milliamps.

    Step 3: Plug the mA value from step 2 into Rob's 'plate current' calculator.

    Our example: If we plugged 18.7 mA into the calculator with our 390VDC p-to-c above, we'd get this:

    upload_2020-2-27_8-37-35.png
     
    BigDaddy23 and AlbertaGriff like this.
  8. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Ya' just sent my head into a tailspin. Thanks for taking the time to post that.....this thread will definitely be a primer for me to do anything else with this amp.
     
  9. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    I watched Parts 1 and 2. That taught me more than I've ever known about how amps work.....of course, I pretty much started at knowing nothing.
     
    Whatizitman likes this.
  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,286
    Joined:
    Dec 30, 2019
    Location:
    california
    Most importantly, bias needs to be in a "safe" range. That is why @King Fan and others here would like you to check the bias. If a tube receives too much current it "will" red plate and fail. The failure can take out other parts of the amp before the fuse blows. Not a happy day. Until you know the amp is safe, keep an eye on your tubes to make sure there is no red plating.

    To try to answer your question...

    According to Dr. Z "...when the bias is set cooler you get a richer clean sound but the overdriven sound isn't as strong or smooth..." "...if you turn the bias hotter you don't get quite the clean sound but the overdrive sound is strong and comes on fast..."

    I'm not so sure I would describe a hotter bias with "overdrive sound is strong and comes on fast" but a hotter bias does change the overdriven texture. (Not "always" in a pleasing way to my ears, but it does change it.)

    I don't find changing bias as a matter of changing headroom, it is more of tuning for the way the amp feels and responds to the guitar signal.

    Tube life is a consideration as well.
     
  11. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    I understand what everyone is saying, but they've been putting out these amps for 50 years with a fixed bias, right? Shouldn't they be fine if built to the specs? Maybe not optimal, but stable? Not arguing, just trying to understand.
     
  12. AlbertaGriff

    AlbertaGriff Tele-Afflicted

    Posts:
    1,593
    Joined:
    Mar 2, 2016
    Location:
    Canada
    I would say the question of bias came up because your trem wasn't working. And the trems functionality is directly impacted by your bias.
     
    trber likes this.
  13. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    ahhhhh......now I get it. Damn, I hate to have to take it all apart to check it....and have to watch a video on how to check the bias. :)

    In my lack of understanding, I thought getting the -31 reading on the output of the bias board (as called for in the manual) meant it was where it was supposed to be.
     
  14. televillian

    televillian Tele-Afflicted

    Posts:
    1,091
    Joined:
    Oct 3, 2013
    Location:
    Clarkdale, AZ
    any chance of hearing it in action ?
     
  15. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Sure, come on by! :)

    Not sure what you'd hear from an iPhone video or even how to put a video on this site, but if I can fit it in tomorrow, maybe I can give it a try.
     
    televillian likes this.
  16. King Fan

    King Fan Poster Extraordinaire

    Posts:
    5,734
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Heh, you don't have to do anything. We're folks who like to build and improve amps. Heck, 95% (maybe 99%?) of guitarists don't know how to bias their amp -- even if it has adjustable bias. Your weak trem *is* probably bias-related, but unless your amp is eating 6V6s, you don't have to change it. Are you running JJ power tubes? Not the best-sounding, maybe, but way sturdy -- and they drop your dissipation just being 14W tubes. (Hmm, does this affect the trem? Not sure.) Maybe you'll get the hankering to open her up one day and add a bias pot -- if you do, look into putting 1% 1ohm 'bias-measure' resistors on the 6V6 cathodes.

    On a wider scale, though, your experience reflects one reason I'm against the (way common) idea that all we need to build an amp is a kit with 'good instructions' -- that if you can follow a recipe, you'll get a great amp. Not entirely. Grandma added 'just a little something' to the Fanny Farmer recipe to make her prize-winning chocolate cake...
     
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,286
    Joined:
    Dec 30, 2019
    Location:
    california
    Amps utilizing a "self bias" circuit work differently than a "fixed bias" circuit. The amount of voltage that passes thru the power tube in a "self bias" circuit "varies".

    Amps utilizing a "fixed bias" circuit, the bias supply voltage (lets say -36v) is "fixed" and does not vary like the "self bias" circuit.

    "Fixed bias" does not mean the bias is never adjusted or that it is set and then forgotten. The term is used to describe "a" circuit for stabilizing the voltages/currents of a tube.

    Fender set the bias very cold, back in the day. Almost any power tube could be used and not red plate. For the case of the old fenders your observation "Shouldn't they be fine if built to the specs? Maybe not optimal, but stable?" would most likely be accurate.

    If Mojotone supplied a bias resistor pair that would set the bias very cold, most any pair of power tubes would be fine. We wouldn't be having this "too hot" conversation.

    I can appreciate you not wanting to open the amp up again. Red plating is "the" major concern here. The second concern would be the heat generated in the power transformer. Does it get too hot to touch? Keep a watchful eye. Most of us agree the bias on your amp is running hot. We don't know how hot or if it is "too" hot.

    (The princeton kit supplies a gz34 rectifier and a 22k bias dropping resistor. The old schematics show a 27k bias dropping resistor when using a gz34 rectifier. The 27k would deliver a cooler bias than the 22k. 27k was still too hot in the princeton kit TDPRI member @trber built).
    (IMO. That kit supplies a bias resistor pair that could very easily create a bias hot enough to red plate the power tubes. The kit should include a bias pot or at least supply a few different resistors to choose from. Having a representative tell you a bias supply of -31v is ok when the plate dissipation of the tube is unknown is very risky. These are just my opinions. OMMV.)
     
    King Fan and KBarW like this.
  18. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Yep to the JJ tubes. That's what they supplied.

    And I agree with your statement....I certainly knew it wasn't going to easy, but with tons of background in soldering wires and being able to follow instructions - I felt I could take it on. Truthfully, what seemed to throw me into a tailspin was the lack on my part of reading the part where it said to measure the DC output in the manual. I was looking at the layout diagram and misread what I should have been looking for.

    So, ultimately - once I started reading the correct voltages, everything fell into place and the reverb got fixed as soon as I hooked up the tank. Much of my fears were my own fault.....even though I felt like I followed the instructions, I got a little ahead of myself.

    That said, they certainly could have made it easier in a few places in the instructions....and I passed that along to them. We'll see if they make changes.

    But, someone with no experience soldering has no business even attempting one of these.

    Overall, I'm glad that I messed up a little - because of that, I found this forum and now know more than when I started and I just might have to continue working on the amp.
     
    King Fan and dan40 like this.
  19. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    To be clear, the -31 was the output at the bias board.....the range they gave at the tubes was -40 to -25. So, at -30 it was in range.

    I do think Mojotone sells the majority of their kits to experienced builders and so a lot of questions I posed to them and all of you were probably not what they see every day. But, I will say that they did answer questions and I really thought they were gonna' tell me tough luck. So, I have no complaints.......

    But, because of you guys - I now have lots of questions. The bliss of my ignorance has been shattered with wondering whether my tubes are blowing up every time I turn it on.... Gee...thanks. :)
     
  20. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    On another note...... There's a luthier who lives about 20 miles away from me who just finished building a 5E3....it's his first amp also. We're gonna' do a little shootout next Wednesday to see how different they are. I know his will blow past mine in volume as I play with two guys who use 5E3s and they are stupid loud at 3....the Princeton isn't going to be as loud at 6, but it will be cleaner and that's what I'm looking for.

    And on another 'nother note..... Another friend who's a luthier/amp repair guy just brought me an old Princeton cab that had been cut down to be just a head....I think they did it with an axe. I'm going to put a new bottom on it and try to fix all of their mess. Then I'll route the edges and give it back to him the re-tolex for his customer.

    Just funny, that I haven't touched a Princeton in my life, now in a week, I've built one and I'm working on another one.
     
    corliss1 likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.