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5E3 Build w/ B+ Gone Awry

Discussion in 'Shock Brother's DIY Amps' started by Huddy, Nov 1, 2020.

  1. Huddy

    Huddy Tele-Holic

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    I built a 5E3 today... What would cause B+ to spike when flipping the standby to "Play" mode? I'm not really sure where to start. All was good with Lightbulb limiter during initial startup. It plays. My bias is way way high though. I've never heard up the voltage going UP.

    Wall Voltage is 120. 355-0-355 Hammond 290BBX PT and puts out 380-0-380 in real life.

    Rectifier Pin 8 w/ a TAD 5Y3 in "Standby" position is 362 VDC; which seems a smidge lower than it should be.

    Pin 8 in "Play" position jumps up to 526 WITHOUT Tubes. With JJ 6V6GTs it's 420.

    Thanks in advance.
     
  2. Engine Swap

    Engine Swap Tele-Afflicted

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    How is your standby switch wired - did the original Fender circuit have this?
     
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  3. Huddy

    Huddy Tele-Holic

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    It did not originally have a standby. It's just a basic Carling SPST On/Off between Pin 8 of the rectifier and the first filter cap.
     
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  4. Paul G.

    Paul G. Friend of Leo's

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    All tubes in...What is the voltage on your 6V6 plates? What's the voltage on your 6V6 cathodes?

    What's your plate dissipation?

    Honestly, a voltage chart would be helpful.
     
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  5. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    362 volts is about perfect for a 5E3 with all the tubes in place. You wouldn't want it much higher than that.

    I'd rip out the standby switch altogether. No need for it whatsoever, and it's clearly causing issues here.
     
  6. Huddy

    Huddy Tele-Holic

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    If I jumper from Pin 8 straight to the first filter cap and leave in standby mode - what I'd think the equivalent to removing the switch would be - the B+ without tubes is still 521 VDC.
     
  7. Fluddman

    Fluddman Tele-Holic

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    The B+ will always be high with the tubes removed.

    Its the voltage drop that occurs when current starts to flow that drops the voltage.

    Cheers
     
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  8. Huddy

    Huddy Tele-Holic

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    That's not really the question.

    Pin 8 Rectifier - Standby "Don't Play" - 362 VDC
    Pin 8 Rectifier - Standby "Play" - 526 VDC.

    Is that normal? What's the difference to pin 8 if there are not tubes drawing down the B+ if the Standby switch is NOT in play mode?
     
  9. Huddy

    Huddy Tele-Holic

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    Getting one together now ;-)
     
  10. tubegeek

    tubegeek Friend of Leo's

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    But the rectifier's not connected to a cap with the switch open?

    So nothing charges up and it looks like your meter is reading the RMS (average) voltage. This is correct with no caps connected.

    Pin 8 is not connected to any caps based on what you've said.

    BE CAREFUL: With the standby open and no tubes, you have no way for those caps to drain.
     
  11. Huddy

    Huddy Tele-Holic

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    I feel like it's something beyond the switch because the if I jumper pin 8 to the first filter cap with the standby in "don't play" mode it's as if the switch isn't there, right?

    And I wouldn't think there'd be a difference between the standby being in "don't play" vs "play" without tubes, unless there was a fault somewhere.
     
  12. tubegeek

    tubegeek Friend of Leo's

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    I edited my reply, check it out. What's going on is normal.
     
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  13. Huddy

    Huddy Tele-Holic

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    Okay! Fantastic. Glad to know I didn't screw something up... or... at least not related to this. I've just got to sort out the cathode bias resistor Thanks so much as always!
     
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  14. D'tar

    D'tar Friend of Leo's

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    The rectifier circuit is incomplete until in the play position.....
     
  15. Huddy

    Huddy Tele-Holic

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    Any ideas on a good 5Y3? The TAD 5Y3 is putting the same unloaded voltages as a TAD GZ34... which is about 1.4 of 380 at 530ish.
     
  16. tubegeek

    tubegeek Friend of Leo's

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  17. Huddy

    Huddy Tele-Holic

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    Haha! Okay. ;-)

    These tubes are cooking as is. I can't help but think if my B+ was in the 360ish ballpark my bias would be pretty normal.

    Screen Shot 2020-11-01 at 10.48.34 PM.png

    I don't think I've ever had to bias with a cathode biased amp. When I adjust the cathode resistor, does the plate voltage change like when you adjust a trimmer pot on a grid biased amp?
     
  18. NTC

    NTC Tele-Meister

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    I have a 5f11ish thing I made with an unknown 380-0-380 PT and get about 430V at the plates and I am using an NOS 5Y3. Well, it was NOS when I got it 20 years ago... Those are real high voltages for a 5E3.
     
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  19. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    It can be eye-opening how high the B+ will go in standby, or without tubes installed. That's why it's good to use filter caps rated somewhat higher than the voltages you expect to see in regular service, especially the first one in the chain...

    The TAD 5Y3GT/6087 appears to be indirectly-heated. A real 5Y3 (or perhaps the JJ 5Y3S) would help bring the voltages down a little.

    Boggles my mind that NOS 5Y3s are up to $30 now, but that still might be a good deal compared to new ones that don't meet the original spec or might not be as reliable: https://www.tubesandmore.com/products/vacuum-tube-5y3gt-rectifier-full-wave
     
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  20. Nickfl

    Nickfl Friend of Leo's

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    Your bias is hot but your calculation is a little off. Remember the cathode resistor is showing you both plate and screen current, so your can subtract about 8-10% to compensate for that. Also, plate voltage is plate to cathode so you need to subtract the cathode voltage from the plate to ground reading. So, your plate dissipation is more like 17-18w. Still high, but not as high as you thought.
     
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