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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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    After the Trem started working, I played with it some and it's just too danged fast. So, started looking for info on how to slow it down. Found some info here on this site.....damn, for a Telecaster site - you guys do a lot of amp work.

    I've ordered parts to slow down the Trem, put in a bias pot, and install the NFB switch.. Everything is here and just waiting on me to start taking the amp apart.

    Now to get off my butt and go do it......

    While I'm sitting here typing.....is there any way to dial back the reverb intensity a little? A lot? Anything past 3 is just stupid....it would be nice to have a little play in it so that it's not either off or on. Seems like every Fender amp I've seen is like that....
     
  2. KBarW

    KBarW TDPRI Member

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    Ok, I've torn into it have done the following....

    1. added a bias pot as per Uncle Doug
    2. Changed to Tremolo caps for a slower Trem
    3. Installed an NFB switch as per Robinette's site.

    Here's what it looks like - please tell me if you see anything amiss....

    Mounting bracket
    IMG_2977.JPG

    Trem caps
    IMG_2978.JPG

    NFB Switch.....put heat shrink around the resistors...hope that's not a problem.
    IMG_2976.JPG

    NFB installed.....switch end
    IMG_2979.JPG

    NFB.....coax to board and ground
    IMG_2980.JPG

    Bias Pot wired up.....
    IMG_2981.JPG

    Ok, so all of that is done....

    I haven't fired it up yet as I'm still lost on what voltages at what points in the circuit I should be looking for to bias this amp.

    I've watched videos - Uncle Doug's video is great right up to the time he starts to actually test the bias and the camera stays on the multimeter and never does he show exactly where he's putting the probes.

    So, I just got through doing more research and ended back up here reading an old thread about how to bias a PR.....based on most the comments there, I probably shouldn't have even put in the bias control because I don't have the knowledge to keep from killing myself.

    Thus, I'm at a total loss as to what to do. Can anyone just tell me very simply - what pin or point on the amp am I measuring and what should it be? Or, do I just set the pot in the middle, play it and make adjustments based on what I hear? Trying the tremolo to see that it's working fine, but the amp doesn't sound too cold?

    Sorry for the long post.....just trying to figure this out.
     
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  3. dan40

    dan40 Tele-Afflicted

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    If you have 1 ohm resistors installed from pin 8 to ground on each 6v6, you could simply take a DC voltage reading across each resistor. The mv reading that you would get will equal the actual current draw (ma) of each tube. You could then plug in those numbers along with your plate voltage into Rob's bias calculator and find out where your tubes are idling. You can raise or lower the dissipation with your bias pot. You can also take this reading with a bias probe plugged into the sockets if you don't have the 1 ohm resistors on the sockets. You can also use another method called the "output transformer resistance method" which is detailed on Rob's site. Either way you choose to go, you simply need to take the necessary measurements to find the idle dissipation of the tubes and then raise or lower it with the bias pot. This page at Rob's site details the procedures and will also link you to a bias calculator which will help with the math.

    https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm
     
  4. dan40

    dan40 Tele-Afflicted

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    For a first startup you will want to set the bias cold and then adjust up from there. Power up your amp without power tubes installed and then take a reading at pin 5 of each 6v6 socket. This will be a DC voltage reading and it will appear as a negative number. Set your pot to the highest negative number you can get. -50 is higher than -40, so just dial up the highest bias voltage you can and then power down and reinstall the power tubes. If you do not get a - voltage reading at pin 5 of each socket, you may have miswired the pot and you should not install the tubes yet. Without the proper bias voltage at the sockets, the tubes will run full tilt and melt down pretty quickly.
     
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  5. KBarW

    KBarW TDPRI Member

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    Thanks, I just read that a little while ago....there was also instructions on how to read the output transformer voltage at the center tap and pin 3 in mA.....I guess that's what Uncle Doug was doing in his video. Because he was manipulating the bias pot and showing the mA go up and down. Although, that's a two-hand operation and little more risky.

    I did go ahead and fire it up and measure the bias voltage at the Trem pot. My Mojotone charts showed that should be -31 volts. With the pot, I could go from about -28 to -41. So, at least the pot is working properly.

    I adjusted while playing and really didn't hear a huge difference. Now, when I cranked on the Tremolo, I could really hear the difference in the intensity as I turned the pot. If I understand correctly, the Tremolo goes away as the amp runs hotter - so I went to that point and backed it off to where the Trem was engaged, but not overwhelming (man, you can get it to really pump big time if you turn it back too much). This ended up back around the -31 so I just set it there and have left it at this point. I know that I wasn't actually setting the proper bias for those specific tubes, but everything was working properly.

    Deciding whether to dive back in try to actually check the true bias. Maybe I've done enough.

    The NFB switch is interesting. Definitely changes the dirt as you move it around. I did the 3 way with Light, Normal, and Heavy. The Light seemed to have the best clean headroom.

    The Tremolo is definitely a little slower by changing out the caps. I thinks it's workable now. I'm really looking for a slow Trem to just use with chords. I think this will do for that application.
     
  6. trber

    trber Tele-Meister

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    Nice bias pot bracket! Did you make it? I'd like to do the same on mine.

    Sent from my SM-G955U using Tapatalk
     
  7. dan40

    dan40 Tele-Afflicted

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    If you power up the amp in a dark room and let it idle for a few minutes, you should be in a safe zone if you do not see any reddish orange spots develop on the plates of the power tubes. If you do notice and hot spots beginning to form, you need to raise the bias voltage up a bit more to cool the bias off a little. I usually like to set mine in a cooler range of 50-60%.
     
  8. KBarW

    KBarW TDPRI Member

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    This is where I get confused....seems like minus is more.....

    So, if I'm at -31v at the trem pot.....would going to -35 be raising the bias voltage meaning running cooler?
     
  9. KBarW

    KBarW TDPRI Member

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    Ha! Wish I'd have taken a picture of what I started with. I keep pretty much any kind of brackets and small metal parts .... have been for years. I dug around and found this bracket that was shaped kinda' like a U but with turned out legs....like an Omega. And it had a swiveling plate riveted to the top. I liked the size and the foot angle, so I drilled out the rivet, chunked the plate, the started straightening out the U shape in my vise to get it flat. The hole on the foot was already perfect. Drilled out the hole for the pot, then used a hack saw to lop off the rest of it. Then to the grinder to clean it up a bit.... kinda' looks like what Uncle Doug used on his video.

    Actually, I was pretty proud of that little bracket - even showed it to my wife.....boy was she impressed....(insert eye roll here).
     
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  10. trber

    trber Tele-Meister

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    Yeah, that's what I thought. It looked like Uncle Doug's! Nice work! Your wife should appreciate your bracket fabricating prowess, says I! Lol.

    I will eventually get back to working on my princeton and make something like that, but in the meantime, I'm thrashing about with work and life under quarantine. Some day things will allow for bracket fabrication. [emoji1]

    Nice work, bud!

    Sent from my SM-G955U using Tapatalk
     
  11. BigDaddy23

    BigDaddy23 Tele-Holic

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    The way to think of it is like this:

    The lower the number (ie closer to zero volts) > the more current it will draw and the hotter the tubes will run (increased plate dissipation).

    Take a plate voltage reading at different bias voltage settings (you should be doing this anyway to calculate your plate dissipation) - as the bias voltage gets closer to zero, the plate voltage on your output tubes will drop but the current draw will increase (hotter bias). The opposite will happen when you move the bias voltage further from zero (ie more negative voltage = cooler bias).
     
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  12. KBarW

    KBarW TDPRI Member

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    I guess the quarantine stuff isn't that big a deal for me.....I'm retired. The only thing that's really changed for me is that I'm missing playing gigs and going out to eat!

    I've missed two country gigs that I specifically built this amp for.....along with my normal Rock gigs....
     
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  13. KBarW

    KBarW TDPRI Member

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    Ok, I'm gonna' play with this some tomorrow and see what kind of changes I see....and I'll fire it up in the dark and see how it looks.

    Thanks for the help.
     
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  14. BigDaddy23

    BigDaddy23 Tele-Holic

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    Remember to use your ears and not worry too much about the numbers provided you are not going silly and ignoring the general <70%max PD number - With mine, I started at the 70%max PD point and worked back, incrementally cooling the bias and listening to the result on both the tremolo and the sound/feel of the amp. I don't recall the actual number offhand but it was low 60's% where I liked it best with my power tube set (5U4GB rectifier).
     
  15. trber

    trber Tele-Meister

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    Sorry you're missing you're gigs and eats! Fortunately, you're ok and are getting to the princeton. [emoji122] Best with the bias project. You'll get it sorted!

    Sent from my SM-G955U using Tapatalk
     
  16. KBarW

    KBarW TDPRI Member

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    Well, I have this nagging feeling that this pandemic is going to kill me.... no, not the virus.....but, I think after being cooped up this long that my wife is gunning for me....

    I was outside the other day for a while....when I came back in, she said that she missed me. I thought was really sweet until I went back out and saw the bullet hole in the fence where I was sitting.....hmmmm....what did she really mean?
     
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  17. KBarW

    KBarW TDPRI Member

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    The tube I was waiting for showed up today, so I installed it and put it all back together. Just spent the last couple of hours playing it.

    So, here's the final rundown on the project....

    1. The new bias pot really didn't do anything for me because I really never could figure out exactly where to set it. So, I turned it hotter until the trem started going away, then pulled it back some. Turned off the lights and left it on for a while - nothing glowing, so that part's done.

    2. The Negative Feedback switch wasn't really doing much for me until I got it all buttoned back up and got to play with it some. It let me stay pretty danged clean all the way up past 7 on one of the settings, I labeled 'em with C-clean, D-dirty, N-normal.

    3. Swapped out the capacitors in the Tremolo circuit as per some info I found on another thread in the forum. That definitely helped to slow down the Trem to a manageable level. I can actually see using it some on slower ballads with ringing chords.

    4. I've always felt like Fender reverbs were just too danged aggressive. Either on or off without much room to dial in what you want....and anything past 3 got stupid. So, once again I found a post somewhere (I don't even remember where now) about swapping to a 12AU7 instead of the 12AX7 in the reverb driver. That was worth the $10 and the wait. The reverb is much tamer now. I was running it around 4 and it was nice with room to back off to 3 without it disappearing.

    So, this project, along with the short-scale neck on my Tele has opened up a whole new sonic world for me. I think some chicken-pickin' is in my future. I've played the Tele more in the last month than I have in the last 10 years.

    As I reached over the back of the amp to turn it off - a though jumped into my head.....if I ever decide to open that thing back up again, I think I just might move the On/Off switch and the NFB switch to the front panel. Might help break up the blank spot on the front panel where there is normally the Princeton logo. And it would damn sure be easier to turn the thing on and off.

    I want to say many thanks to all of you who have jumped in and helped me on this project. I started out as someone who knew nothing about building an amp, but wanted to .... and now I still know nothing about building an amp, but I did.... :)

    You guys have been great.

    Thanks,
    Bruce Weldy
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Congratulations Bruce on successfully building one of the coolest amps ever designed.
     
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  19. Muttlyboy

    Muttlyboy TDPRI Member

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    Congratulations on your Princeton Reverb Build.
    I'm also building a Princeton Reverb clone.

    A couple years ago, someone on the forums walked me through the process of biasing my output tubes.

    I'll do my best to describe the process...

    1) With the amp off and filter caps discharged, measure the resistance of the output transformer primary halves.

    In my case it was Red wire to blue wire =151.1 ohms
    Red wire to brown wire= 150.3 ohms

    Write those numbers down.

    2) Prepare yourself to work safely for probing a live circuit with over 400 volts dc.

    3) With the amp powered up, all controls on the front panel set to "zero", and the speaker plugged in...

    Measure the Voltage to ground at the plates of your Power tubes (pin 3 on the power tubes, or the Brown output transformer wire on one tube, and the blue output transformer wire on the other tube)

    In my case it was 426 volts dc on both tubes.

    4) you will now measure the Voltage across each half of the primary side of the OT...

    Positive probe on the Red OT wire, negative probe on the blue OR wire (pin 3 of one of the output tubes)

    In my case, I measured 3.18 volts.

    Measure the Voltage across the Red OT wire and the Brown OT wire

    In my case, I measured 3.22 volts


    5) A little arithmetic...

    Divide the Voltage measured across the "blue to red" side of the output transformer by the Resistance measured across the "blue to red" side...

    In my case that's 3.18v÷151.1ohms=0.0210 (21 milliamps)

    6) do the same process for the "brown to red side...

    In my case 3.22v÷150.3 ohms =0.0214 (21.4 milliamps)

    So that's the current running through each tube...

    7) now to multiply the current × plate voltage for the dissipation in watts...

    Again in my case, 426×.021= 8.9 watts,

    And 426×.0214=9.1 watts.

    I've been told two different watt numbers for maximum plate dissipation for 6v6gt tubes...12 and 14...since I'm not exactly sure which one to go with, I use the number 13 for my max plate dissipation in watts.

    Final step...

    9 watts÷ 13 "max watts"= 0.692 x 100 = 69%.

    70% is typically recommended as the safe upper limit to run the output tubes at.

    You can run a bit more, or you can probably run them cooler down to 50% or so...thats an adjust to taste kind of a thing.

    As you run the tubes warmer (less negative bias voltage, and more current) the plate voltage will decrease.

    As you run the tube cooler with less current, the Voltage will increase.

    Every time you make an adjustment to the bias pot, you will need to check plate voltage versus the Voltage read across the OT halves...then do the math for current in amps, then do the math for the dissipation in watts.

    After you've done it a few times you kind of get a feel for where things are going as you make adjustments
     
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  20. KBarW

    KBarW TDPRI Member

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    A lot of the guys have hit on all of these points, but you put it all together in a step by step. I've printed this out and it will go with all of my Princeton documentation. If I open it up again, I may give it another go. Right now, though - it sounds pretty danged good.

    Thanks!
     
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