BFPR build trem socket voltages vary

Discussion in 'Shock Brother's DIY Amps' started by Porkchop Jones, Sep 15, 2020.

  1. Porkchop Jones

    Porkchop Jones TDPRI Member

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    Hi all, I've got a Mojoton BFPR underway and have hit a bit of a snag. The voltages feeding the 1 and 3 pins of the Trem/phase inverter socket bounce around. They don't stay fixed. It seems like it's rolling through 4 or 5 voltages somewhat in the range of the correct voltage. I get no noticeable tremolo out of the amp, but everything else seems OK (haven't tried the reverb).

    All the other voltages on the amp are within range of the listed numbers. I've poured over the diagram and everything seems in order, accept that I had some problems when I first powered up the amp and had arcing between the 1 and 2 pins of that same socket. I discovered that I had missed the jumpers from the 2-7, 3-8 pins on the Reverb Send socket. After correcting that, the arcing on the Trem socket ceased.

    Could I have caused some tube or component damage? Any ideas?

    DSC_3944-a.JPG
     
  2. EmWeAh

    EmWeAh TDPRI Member

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    That is some neat wiring! Well done!
    Have you tried swapping in a new 12AX7 in V4? Also make sure output tube bias is setup correctly, since in a Princeton it is a bias modulation type of tremolo. Usually the colder the bias the more bias wiggle is experienced. Regarding the initial arcing: sometimes conductive traces are left behind on a socket that can cause issues, please inspect that socket closely for no leftovers.

    P.S.: welcome to the forum!
     
  3. Dacious

    Dacious Poster Extraordinaire

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    The trem section is an oscillator - it should move around, as it's always on. The trem control is a nice knob that mixes wet 'trem' signal to dry.
     
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  4. Porkchop Jones

    Porkchop Jones TDPRI Member

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    Thanks and thanks EmWeAh. I did swap the 12ax7's in the 1 and 4 position and voltages did change. I've got another 12ax7 from my Champ that I'll swap with the 12ax7 that was in the socket when the arcing occurred. That should tell me if I scorched the original tube. Is the bias that you're talking about the 2.05v lead that goes to the #3 pin on V4 (just to make sure I'm on the same page are you calling the trem/phase inverter socket V4? And does that mean that V1 is the preamp socket? And in addition then are the two 6v6's V5 & V6?)

    PS. I've been lurking the forum for a couple of months while I built a Champ which I love! I got through that one OK so I decided to up the game and try a BFPR. I'm not an electronics engineer so I can get lost with too much jargon.
     
    Last edited: Sep 16, 2020
  5. Porkchop Jones

    Porkchop Jones TDPRI Member

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    Pin values I took at initial testing

    Rectifier
    4 = 329v
    6 = 329v

    6v6
    2 =
    3 = 379v
    5 = -28v
    7 =
    8 =

    6v6
    2 =
    3 = 379v
    5 = -28v
    7 =
    8 =

    Trem/Phase
    1 = varies
    2 =
    3 = varies
    4-5, 9 = 6.65v
    6 = 193v
    7 = 19v
    8 = 55v

    Reverb recover/mix
    1 = 163v
    2 =
    3 = 1.25v
    4-5, 9 = 6.65v
    6 = 163v
    7 =
    8 = 1.3v

    Reverb send
    1 =
    2&7 =
    3&8 = 6.65v
    4-5, 9 = 6.65v
    6 =

    Preamp
    1 = 159v
    2 =
    3 = 1.3v
    4-5, 9 = 6.65v
    6 = 164v
    7 =
    8 = 1.25v
     
  6. schmee

    schmee Doctor of Teleocity Silver Supporter

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    make sure you have a switch in the trem switch RCA jack. The trem is active unless you do (even intensity at 0) and that may be why it jumps around...
     
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  7. EmWeAh

    EmWeAh TDPRI Member

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    Please have a look at Rob Robinette's fantastic site and read especially through the AA1164 and the how to bias pages. Your 6V6s seem to run a bit on the high side which may (or may not) be the reason for the weak tremolo.

    Edit: you might want to consider adding a pot for an easier approach to adjust bias (instead of changing fixed value resistors).
     
    Last edited: Sep 16, 2020
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  8. Porkchop Jones

    Porkchop Jones TDPRI Member

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    Touchdown schmee! Got the foot pedal attached along with reverb and it all seems to work. With the pedal attached and in the off position the voltages freeze at 226v for pin #1 and 2.18v for pin #3.

    The trem doesn't seem to be very intense though, I will check out the bias on Rob's site.

    Thanks guys! It appears this thing is operational.
     
  9. dan40

    dan40 Tele-Afflicted

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    My PR runs about the same plate voltage and I like to set the bias at 20ma per tube. When the bias is too hot in this circuit, the trem intensity will be very low. Bringing the bias back into a cooler range should help with that. Hopefully none of the components in the trem section were damaged during the arc over you experienced.
     
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  10. Porkchop Jones

    Porkchop Jones TDPRI Member

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    OK, so I've spent a couple of days playing the amp and researching the bias situation. Is the recommended approach to put the adjustable bias pot in there via Rob's scheme? See below(upper left corner).

    [​IMG]
     
    Last edited: Sep 21, 2020
  11. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Yes absolutely to Robrob’s approach. A bias pot is a very nice addition to the original/vintage PR circuit.

    +, If you can rig the bias pot and bias test points so you don’t have to remove the chassis, that’s a great mod. (Plenty of online info on how to do that)
     
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  12. King Fan

    King Fan Poster Extraordinaire

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    ^^^ this. External access to test points and a bias pot make checking/setting bias a 1-minute operation.

    Let us know if you want to do external test points (they're perfectly safe, but you'd have to drill a few small holes in your chassis). BTW that's a very tidy build -- nice!!!

    If you happen to be a bias probe type of guy, you could still benefit by an externally-accessible pot (also perfectly safe) and the easiest way is to run the Fender-style 10KL pot in the (useless) ground switch hole. Doug Hoffman and others sell the pot, and Doug's old dot-matrix drawings showing a 10K pot are probably the 'standard textbook' on a bias pot in a PR.

    upload_2020-9-23_7-24-27.png
     
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