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Need some help with Princeton Reverb Build

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

  1. koolaide

    koolaide Tele-Afflicted

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    LLC is correct. Uh Uncle Doug has a video on light bulb current limiter. And no I am not Uncle Doug, but over the years, through YT he has taught me a ton.(as have many on this forum) Thanks UD and others!
     
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  2. KBarW

    KBarW TDPRI Member

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    I've gone back through all of the steps for testing with my new multimeter that actually goes to 600VDC....my old one didn't and that's what might have been scaring the hell out of me.

    However, while most everything seemed within tolerance, the second to last point on the bottom right hand of the board (according to the layout diagram) should be 1.3V, but it's reading 162V. Backside of the two caps at that point go to the Bass tone control. The last point on the board right next to it is reading the proper 1.3V.

    That's just a wee bit out of tolerances.....

    Looks like it getting narrowed down. Any ideas?
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    That point would have both DC and AC voltages. The AC at that point would be 1.3 VAC when a AC test signal is applied to the input. The capacitors (going to the bass pot) are part of the tone stack. They block the DC from continuing through to the pots, but they allow the AC "signal" to move through the circuit. At that test point there is still high DC voltages.

    It is normal to have "both" AC and DC voltages in certain parts of the amplifier. On more modern schematics the test points give a clue as to what voltage to look for. An AC voltage is denoted in an oval shaped test point and a DC voltage is denoted in a rectangular shaped test point. Unfortunately for princeton schematics, only the 65 princeton reverb RI schematic has the voltages listed this way.
     
  4. KBarW

    KBarW TDPRI Member

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    I'm gonna' go test that right now. Then I'm gonna stick my Tele in that thing and see if it holds tight or lets out some smoke!
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    @trber has recently finished a mojotone princeton kit. One point of his experience was dealing with the bias of the 6v6s. You will need to check the bias of your 6v6s. Rob Robinette has a site with a bias calculator. Read through his info on bias and post your findings.

    You do not want to run too much current through your power tubes. It can damage the tubes.
     
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  6. KBarW

    KBarW TDPRI Member

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    Well, it makes noise......Geoffrey Hill (guitarist for Randy Rogers Band) heard it and came down to give it a listen. He liked it....especially when I turned it up to 7 and it got dirty. I was hoping for a little more clean headroom, but so far so good.

    However, I got nothing out of the vibrato at all....zip, nada....switch was working. I did put in a TRS switch instead of the supplied RCAs.

    The other issue was the reverb - I don't have it hooked up yet, but when I turn up the Reverb knob, it starts a slow motorboating.

    Sounds like something is crossed between the Tremolo and the Reverb...don't know how, but I'll start going over the wiring diagram one more time......
     
  7. KBarW

    KBarW TDPRI Member

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    One thing that was a bit out of spec was the Power tube pin 5 voltage. It was -30 instead of -40. Don't even have a clue what could cause that.
     
  8. KBarW

    KBarW TDPRI Member

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    It's a fixed bias amp......what can be done? Change out the components for different values?
     
  9. koolaide

    koolaide Tele-Afflicted

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    Last time I swear- Uncle Doug has several videos on Bias and specifically one on fixed bias in a Fender- I think a Princeton. I suggest you binge on UD.
    Peace Out!
     
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  10. dan40

    dan40 Friend of Leo's

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    The lack of tremolo could be caused by the bias being too hot. Lack of headroom may also indicate a hot bias. You will need to measure the bias and change out a resistor if you do not have a bias pot installed. Here is a great page on adding a bias pot to yours if you need to. Either one of the two bias circuits could be added to your amp to allow you easier adjustment.

    https://el34world.com/charts/Biascircuits.htm
     
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  11. KBarW

    KBarW TDPRI Member

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    Thanks, but that's a bit above my head at this point. I may have to take it to someone should I need to do this.
     
  12. KBarW

    KBarW TDPRI Member

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    Great educational tools. Thanks for the info. Don't know that I'll try something like this.....will probably go to one of the amp builders around here if I need this.
     
  13. koolaide

    koolaide Tele-Afflicted

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    If you don't know-UD is in Tx.
     
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  14. trber

    trber Tele-Meister

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    KBarW, I had the same problem with high bias and installed a bias trim pot. It was really not complicated, but I honor if you think it's not something you'd like to tackle. I actually saw a way to get it done without having to drill the chassis, which I am willing to do, but didn't have the time to get into when this came up for me.

    Here is what I found helpful:
    Uncle Doug Video (true!) on installing a bias trim pot, which I watched numerous times. You'd need to find the bias pot to install, and then, I ran it across the chassis to the spot where the ground switch is located--pretty sure that's the slot I placed it in. No drilling, I just J-hooked a longer lead to get from the bias board to the switch location. I believe I saw KingFan's chassis had a similar setup, as shown in a picture he shared on my help thread.


    Scroll down this page to get to the "Suggested Mods" diagram and see his location of the bias pot by the board, upper left.
    https://robrobinette.com/AA1164_Princeton_Reverb.htm

    You can find all this info and more on this thread...careful, it's loooooooong! Lol.
    https://www.tdpri.com/threads/help-65-princeton-reverb-kit-build-problem.996822/page-11

    After installation and adding some resistors across the appropriate sockets, it is a relatively matter of fact couple of measurements with the multi-meter. Not sure if that is of any help, but I'd say you could do it, but it might take a bit of research to clarify the steps. I liked the location on Uncle Doug's installation, but I did not have a means to fabricate the bracket he used. I liked his spot, though. Pretty slick.

    Congratulations on getting it up and running. I think you'll really enjoy that amp!
     
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  15. KBarW

    KBarW TDPRI Member

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    Thanks for all that info. Maybe I'll try to tackle this at some point. I wonder if getting the bias from -30 to -40 will give me more clean headroom? Not sure exactly what the bias affects other than how hot the tubes run.
     
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  16. KBarW

    KBarW TDPRI Member

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    Well, I heard back from AJ at Mojotone and he said the -30 is fine at the power tubes and that the 1.3V at the second to the last point on the board was a typo in the plans and the value I got was good.

    No to see if I can figure out why the Trem isn't working.

    I'm going to hook up the verb today and see if that has any effect on the Trem. Hopefully, hooking it up will fix the motoboating that is happening with it unplugged and turning up the Reverb knob.

    Getting closer.

    Thanks for everyone's help thus far.
     
  17. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    If the reverb wasn't hooked up you could get all kinds of weird noise as that reverb circuit would just be amplifying whatever stray signal it was getting. If it's still behaving oddly once the pan and cables are connected, then there are more things to check.
     
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  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    There are two ways to "adjust" the bias. One is to change out the components (one or two resistors). The other is to add a pot to be able to vary the resistance of one of those resistors. (I like the former many on the forum prefer the latter.)

    The amp is still known as a "fixed bias" amp. An adjustable bias pot allows you to easily pick a new "fixed bias supply voltage". A "cathode bias" amp acts differently.

    The power tubes will red plate (get too hot) when too much current flows through the tube. The negative voltage you noticed on pin 5 of the 6v6s is used to change the amount of current flowing through the tube. As the voltage on pin 5 becomes more negative it draws less current to flow through the tube (and vise versa).

    The bias supply on the princeton is found on the small board near the pilot light. On that board is a diode to change the voltage from positive to negative. There is a capacitor to "smooth" the voltage. Lastly, there are two resistors 100k and 22-27k which together make a voltage divider. The voltage divider bleeds some of the voltage to ground and lets some of the voltage go to the tube. These two resistors are what we are focused on to set the bias.

    There are two schematics for your princeton. One schematic depicts a 5u4gb rectifier tube. The resistor in the bias supply on this schematic is 22k. The other schematic depicts a gz34 rectifier tube and the resistor in the bias supply is 27k. The rectifier tubes supply different amounts of voltage to the amp, so the resistors need to be different to provide the required voltage to the bias supply.

    The mojotone kit provides a 22k regardless of the rectifier. In the real world the value of this resistor should be optimized to provide a bias voltage to keep your amp happy. (You will find the princeton tremolo is affected by this bias as well.)

    Part of your education is to get your head wrapped around adjusting the "bias voltage divider". (aka adjusting the power tube bias).

    Well maybe. Do yourself a favor and check the bias.
    On the princeton it will impact the tremolo. Too hot is bad for the tubes and may be bad for the transformers. Too cool is subjective. Some say it can sound lackluster. Your ears know what they like. Some people adjust bias by ear. YMMV.
     
    Last edited: Feb 26, 2020
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  19. KBarW

    KBarW TDPRI Member

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    If you'll indulge my ignorance once again, since we are dealing with a negative number - is -40 hotter than -30, or is it hotter as it approaches zero? I'm assuming the second as the instructions say to shut down the amp if the measurement is closer to zero than -25....guessing that's getting to hot.
     
  20. KBarW

    KBarW TDPRI Member

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    Well, just watched a video that said the opposite of what I thought. -40 is hotter than -30. So, since I'm at -30, the Trem shouldn't be effected by having the bias too hot.....correct?
     
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