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Need some help with Princeton Reverb Build

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

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I looked back at the numbers trber pulled from his princeton.
    At -42v he had a cold bias reading of ~ 15.2mA. (The bias resistor to ground was ~32k)
    At -36.1 he had a hot bias reading of ~ 28.2mA. (The bias resistor to ground was ~27k)
     
    Last edited: Feb 26, 2020
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  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    The more negative the bias the less current will slow through the tube.
     
  3. KBarW

    KBarW TDPRI Member

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    Ok, another weird issue.....putting in the reverb tank. I understand signal flow....out to in - in to out....however, the plans say to wire IN on the tank to the IN on the amp....and OUT to OUT on the other wire.....

    Is this some kind of situation where they are trying to tell you on the tank where the wire should go? This is not how signal flow works in my world. Or, did Mojotone's instructions get it backwards?

    I'd really like to know before I cut the wires to length, solder the ends and find out that it's backwards. Anybody run across this?
     
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    https://robrobinette.com/Tube_Bias_Calculator.htm

    How about we put that new multimeter to the test and get the bias numbers.

    You will be testing high voltages. From what you have described I think you have worked with high voltages and know how to be safe.

    With the amp warm and shut off. Discharge the filter caps. Get the resistance of the two sides of the output transformer. Measure resistance (ohms) from pin 3 of each power tube to the point on the board closest to the power tubes. (probably 3 red wires and a 1k resistor meet here). Each side should measure in the 100 to 400 ohm range. Write the ohm figure down for each side. Be accurate.

    Check the plate to cathode voltage. Attach the black probe to pin 8 and the red probe to pin 3 of one of the 6v6s. Put the meter on DC volts. Turn on the amp let the voltage stabilize and write down the result. It should be around 420VDC. Shut down amp and discharge caps. Repeat with the other tube.

    Check the voltage drop across the output transformer. Shut off amp. Discharge filter caps. Attach red probe to the point on the board closest to the power tubes. Attach the black probe to pin 3 of one of the 6v6. (same locations used to get the resistance of the output transformer). Turn on amp. Let it stabilize. The DCV should be 2 to 3 volts and may bounce a little. Get as accurate reading as possible. Repeat for the other tube.

    The voltage drop across the output transformer divided by the resistance of that side of the output transformer = the bias current. (Voltage drop/OT resistance=bias current).

    Look at the bias calculator listed above. Select 6v6gt and enter "Your" "Plate to cathode voltage". Look at the class ab fixed bias line. Compare your bias current with the bias current displayed on the calculator.

    Any change to the bias current will change the "plate to cathode voltage". All readings will have to be repeated when a change is made.
     
    Last edited: Feb 26, 2020
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    @trber may have the answer to this question.
     
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  6. KBarW

    KBarW TDPRI Member

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    Okay, I want to make sure I'm getting this. The video I watched said the more negative, the hotter the tubes run (i.e. -40 runs hotter than -30). I'm assuming that more current equals hotter tubes - is that correct? If so, then what you're saying and what the other guy said is opposite?

    starting around 4:00.

     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    He's talking milliamps at that point of the video. We're talking voltages so far in this thread.

    milliamps = current. So, more mA = more current.

    Higher/more negative voltage = less current flow. So -40 would be a cooler bias than -30, allowing less current to flow in that tube. That negative voltage is like a valve - it controls what's going on in the tube.
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I watched the video. The video agrees with @corliss1. -40v is a cooler bias than -30v. That agrees with the numbers I listed above.
     
    Last edited: Feb 26, 2020
  9. KBarW

    KBarW TDPRI Member

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    I guess what confused me was that he was using the same range of 25-40, just like the negative range for DC voltage. So, -30 is indeed hotter than -40.

    Thanks
     
  10. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yes, he happened to the in the same number range of 25-40, but he was in mA and we are in DC V.
     
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  11. KBarW

    KBarW TDPRI Member

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    Beginning to think I need to just play guitar and not build amps. :)
     
  12. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    It really just depends on what you're into. Everyone has to start somewhere, so learn what you can and go from there!
     
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  13. KBarW

    KBarW TDPRI Member

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    Well, it's finished. Everything is working. Temolo is a little weak, but I'm really a Reverb guy anyway. Just glad to have it done.

    Many thanks to all of you for the knowledge and the willingness to share it.

    IMG_2930.JPG IMG_2931.JPG IMG_2929.JPG IMG_2932.JPG IMG_2930.JPG IMG_2931.JPG IMG_2929.JPG IMG_2932.JPG
     
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  14. KBarW

    KBarW TDPRI Member

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    Don't know why the pics posted twice......
     
  15. KBarW

    KBarW TDPRI Member

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    At 62 years old, it's something I've wanted to do. Now, I've done it. Don't know if I'll do it again or not. But, glad I took on the project.
     
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  16. BigDaddy23

    BigDaddy23 Tele-Holic

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    Looks great - well done! Did you end up adjusting the bias via resistor swap on the bias board or installing the adjustable pot? What numbers did you arrive at? If you have the pot in there, it is a simple adjustment to get the tremolo to have more swing.
     
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  17. dan40

    dan40 Friend of Leo's

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    Congratulations on completing the project! I know that installing the bias pot seems like a pain but it will make a big difference in the sound and performance of your new amp. Every new pair of power tubes that you install in the future will draw a different amount of current. Some pairs may be running extremely cold and others may be on the verge of redplating. Having the pot will allow you to optimize the current which will help your tubes last longer and the amp will play at it's best.
     
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  18. KBarW

    KBarW TDPRI Member

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    At this point, I just wanted it finished and working. Maybe I'll do more in the future, but right now - my brain is fried. Gonna' play a couple of gigs with it over the next two months and see how it goes. Gonna' take it to practice with my Rock and Blues band tonight just for grins.
     
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  19. BigDaddy23

    BigDaddy23 Tele-Holic

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    Super - It's great to have something you made in service. I'm sure it will sit well with your band!

    This is a fantastic circuit and the tremolo is really something to behold when it is working well. I understand what you mean by the brain being at capacity with some of this amp building palaver! Perhaps when you have played it for a while and then want to come back to understanding bias adjustment, repost here. This is a useful bunch of folks here who will talk you through the process. Perhaps consider posting a couple of gut shots of the power section of the board (Power tranny, bias board, cap can and power tubes and the power tranny end of the circuit board) and we'll be able to mark up the photos to tell you what to do. Given you have been in taking measurements already, the pot addition is an easy mod to employ once you get your head around it. You can set it at say a moderate bias, test the tremolo and sound by ear and adjust from there. On face value, biasing seems like a bunch of mysterious hocus pocus but once you work through the process you will find it easy.........building a Princeton Reverb as your first amp is way harder!
     
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  20. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Great! When you think about it in a few weeks, you can always tweak the bias to see if that helps the trem at all.
     
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