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Princeton Reverb Issue

Discussion in 'Shock Brother's DIY Amps' started by Mark the Moose, Jan 25, 2021.

  1. Mark the Moose

    Mark the Moose Tele-Afflicted

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    (Redirecting a previous thread)

    Last year I built a Weber 6a14HP, which is a 35 watt Princeton Reverb. I wanted to try dialing it back to the original 14 watts, so I bought an OT spec'd from the original 6a14 kit. The PT was the same, just needed different taps. The bias resistors were a little different, so I changed them. Swapped the 6l6 tubes for 6v6. Finally there was an extra filter cap attached to the cap can. I can't find any other differences. Did the work yesterday, but no sound. While chopsticking and checking voltages, the tremolo started to thump really hard. I found that with the thump a brief instant of guitar sound would come through. Other than that, nothing. My B+ with the bias pot as hot as it will go is 379v, pretty low. Comparing bias resistor values to RobRob's layout, I found the values to be different than the Weber layout. Might that be the issue?

    Where to go next?

    I've included the 6a14 which is the target layout I'm working towards. Then the 6a14HP layout, which is where I began with and worked fine, just stupid loud. Then Rob's layout, for reference. 6a14_layout.jpg 6a14hp_layout.jpg AA1164_Princeton_Reverb_Layout_Mods.png
     
  2. D'tar

    D'tar Friend of Leo's

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    What is your bias voltage at the power tube grids? Sounds like you initiated the oscillator circuit with your meter indicating you may have a bias voltage issue.

    A full voltage will be required for troubleshooting help.
     
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  3. Mark the Moose

    Mark the Moose Tele-Afflicted

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    I'll check it this afternoon. The oscillation occurred one time and I've not been able to recreate it.
     
  4. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    What @D'tar said. :)

    And FWIW, I’d try to follow Rob wherever possible. Weber makes great speakers, but their layouts (and many other kit layouts) often have issues.

    BTW for target voltages, if you ignore the GZ34 thing, the Fender schematic isn’t bad.

    FC9FFDF4-65A0-414E-B0EF-691C13E19841.png
     
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  5. D'tar

    D'tar Friend of Leo's

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    I would recommend pulling the power tubes if you suspect any bias voltage issue.
     
  6. Mark the Moose

    Mark the Moose Tele-Afflicted

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    Quick note: I retained the copper cap version of the GZ34/5AR4 rectifier. Should I pick up a 5u4gb before proceeding? I wouldn't think the difference would be significant enough, but maybe?
     
  7. schmee

    schmee Doctor of Teleocity

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    If you want it to be Princeton sound, Return the PI section and the Bias Splitter/output coupling caps section of the board on the left to being per the Princeton layout. That's what makes it a Princeton, rather than a Deluxe really.
    The larger Power Transformer is a great thing. I have a stock Princeton with big PT now and it's superb.
     
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  8. Mark the Moose

    Mark the Moose Tele-Afflicted

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    I believe my OT is actually a DR transformer, which is what Weber includes in it's 6a14 PR kit. I will try changing the bias board configuration to match Rob's layout and see what happens.
     
  9. schmee

    schmee Doctor of Teleocity

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    yeah, my current '71 PR has a Vibrolux PT and a Deluxe OT. I left the Princeton layout in all the rest. It's killer. The PT is what gives it headroom. If you do the exact same with a small PT it's much less clean headroom.
    That amp has been many things in the last 10 years, kinda my test platform. It was my own version of a 6L6 powered PR for a while with various mods.
    What I discovered through all that is it's the PI section that makes a Princeton a Princeton. Loud but a hint of grit.
    Everything from the Deluxe on up is the same....a BF Long Tail pair PI.
     
  10. Mark the Moose

    Mark the Moose Tele-Afflicted

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    I rewired the bias section as Rob does, but I have a couple of questions related to my power transformer. First, I have 540v and 680v available. The 35w uses the 680, the 14watt indicates 540. Which makes more sense to use for 6v6 tubes and a DR OT. Second, the blue/red wire is connected to the bias board in the weber layout, while Rob connects his directly to the high voltage leads at the rectifier. If I'm using the 540v leads, as Weber shows, should I connect the top left bias board connection to the rectifier and ground the blue/red...or leave it unused?
     
  11. Mark the Moose

    Mark the Moose Tele-Afflicted

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    w025130sch.JPG
     
  12. D'tar

    D'tar Friend of Leo's

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    How was bias voltage supplied before? If using a bias supply like the blu/red above, then use the same here. Using the HT supply is a way of getting bias voltage without the bias tap. You would need to adjust your resistor values when changig from one to another.
     
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  13. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Somebody smarter than me is gonna have to think about the HT taps. Altho 340-0-340 is just a tad high, 270-0-270 sounds really low. Did you say you weren't getting much B+?

    As for bias, somebody smarter than me *did* answer. :) Think of it this way -- your bias tap brings 45V to the bias board; the Rob / Fender setup (original PTs didn't have a bias tap) brings 340V.
     
  14. schmee

    schmee Doctor of Teleocity

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    I used the bias tap off the PT in my current situation. I have used the wire off pin 4 (?) before though.

    I'd use the 680v tap. That would be 340 on the tube plates. A BF Princeton spec's 340 volts I think. I think a Deluxe is nominally about 330V. But actual vintage run much higher. The Vibrolux PT I used has something like 660/330 V tap and I like it. you could try the lower voltage though, it might be more like some tweed amps that way.
     
  15. Mark the Moose

    Mark the Moose Tele-Afflicted

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    Restored my bias circuit. Found a broken ground wire in the preamp. I have sound now, but also an oscillation which increases in rate in conjunction with the tremolo speed control. Haven't been able to check voltages and check the bias, later tonight.
     
  16. Mark the Moose

    Mark the Moose Tele-Afflicted

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    I switched the PT leads to the rectifier and turned the bias control all the way up, the oscillation is gone...or mostly so. If I sit close to the speaker I can just barely hear the tremolo working. Chopsticked the wire to to tremolo switch and it quieted right down. So now I have a functioning, nice sounding amp...

    However, voltages are low (I still have the 540v taps connected).
    B+1: 375
    B+2: 368
    B+3: 327
    6v6 Plate Voltage 375

    Should I smile and play my amp or should I try to get voltages up? I could try the 680v PT leads or I could change the bias resistors. Afford me your two cents.
     
  17. D'tar

    D'tar Friend of Leo's

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    If it were mine, I would like to know where the bias is set. You may be fine. I would play it for a while but curiosity will prevail so, in Your case, perhaps you could do a little recording so you can compare results. However, if its not obvious which taps to use once you've biased and heard them both, I would opt for the lower voltages. YMMV
     
  18. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    As usual, I can't improve on that! B+ 375 isn't necessarily bad.

    Couple details, I can't quite tell. Do you now have plenty of trem when it's turned up? How's your adjustable bias working?
     
  19. Mark the Moose

    Mark the Moose Tele-Afflicted

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    Trem works great. No red plating. Breakup begins around 4 on the volume. Turning down the bias control knocks my voltages down to B+ at 355. Maybe tonight I'll try to figure out my actual current settings.
     
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