AB763 - stuck

Discussion in 'Amp Tech Center' started by Curlydog, Dec 7, 2019.

  1. Curlydog

    Curlydog TDPRI Member Silver Supporter

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    After successfully building a 5e3 I decided I had so much fun I would try an AB763

    got all the way up to putting in the rectifier tube and blew the fuse quickly - I built a light bulb current limiter(and waited for the bulb to arrive) replaced the fuse and fired it up, trying to follow Rob’s troubleshooting tips and the voltage step in stewmac’s instructions-
    The bulb lights dimly and I get 250volts at the eyelet the red wire from the output transformer connects to - the StewMac guide indicates I should see 450V .

    slightly confused on next troubleshooting step as it doesn’t seem to fit nicely onto Rob’s guide - should I follow the stepson #1 and isolate the transformer output pairs to isolate a short?
     
  2. timfred

    timfred TDPRI Member

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    With or without the rectifier tube?
     
  3. Curlydog

    Curlydog TDPRI Member Silver Supporter

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    With the rectifier tube in place
     
  4. D'tar

    D'tar Friend of Leo's

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    Pictures will put a lot more eyes on things.

    Grab a fresh copy of your layout and schematic. Go over the circuit and highlight your layout, each wire, connection and component as you confirm routing and placement.
     
  5. Snfoilhat

    Snfoilhat Tele-Afflicted

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    A GZ34 rectifier pulls 5VAC at 1.9A? 2A? That's 10 watts. That should not be perceptibly lighting a lightbulb unless the light bulb's wattage is very low.

    How many watts is the bulb in your limiter?

    You want a bulb power that won't go to full brightness even when your amp has all tubes in and is idling -- because full brightness is supposed to be the visual signal of a short or drastic overdraw of current.

    I have found 150W to be useful across a large range of amp sizes.


    Lower than expected voltages are a good sign of higher than expected currents (and vice-versa).

    Do you have a bleeder resistor or capacitor-voltage-balancing resistors (like in a Twin Reverb's first couple electrolytic capacitors)? If you got the resistances too low, they would load down the power supply.

    Good for you testing the power supply before just slotting all the tubes in and hoping it worked right the first time. Good luck
     
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  6. Vibrolux59

    Vibrolux59 Tele-Meister

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    An AB763 what?
     
  7. King Fan

    King Fan Friend of Leo's

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    All excellent advice.

    Are you measuring voltages with the limiter in place? They'll be low.

    Hang in, share all the info you can -- you'll get this.
     
    Last edited: Dec 7, 2019
  8. Dacious

    Dacious Poster Extraordinaire

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    Sounds like you might have a short in your recto
     
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  9. King Fan

    King Fan Friend of Leo's

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    Also you say you got to the 'rectifier only' step? What did you see on the light bulb and measure on the meter when you did Rob's step 2, the no-tubes step?

    "Remove all tubes from the amplifier. Place the amp's Standby switch in "Standby" if it has one... turn the Amp Power Switch to "On". Be prepared to immediately turn the amp off and watch for arcing and smoke.

    If you are using a light bulb current limiter when you turn the power switch on the limiter light bulb will initially glow as the power transformer comes up to voltage and then dim because without the tubes in place no current is being used. If you are using a high wattage light bulb you may not even see the bulb glow. If the bulb burns near its rated brightness then there is a short in the power cord, power transformer or their connections.

    Carefully measure the AC voltage on all of the power transformer connections.

    With no tubes installed and no load, all the following voltages may be higher than specified.

    You should have wall voltage at the fuse and power switch (red meter probe on the fuse or power switch and the black probe on the chassis).

    You should have around 5 volts AC on the rectifier tube heater wires (measure wire-to-wire, not wire to ground).

    You should have around 6.3 volts AC on the preamp and power tube heaters. On the preamp tubes measure pin 4 to pin 9 and pin 5 to pin 9 across each preamp tube socket and pin 2 to pin 7 on the power tube sockets to make sure every tube socket has heater voltage.

    The 6.3v pilot light should light up. If it doesn't the bulb may be burned out or there is no 6.3v AC at the bulb socket terminals.

    You should have high voltage AC at the rectifier input (tube or solid state rectifier). I recommend you measure the high voltage wire to ground (not wire-to-wire) on both power transformer high voltage secondary wires because measuring wire-to-wire can expose the meter to over 700 volts AC RMS. If your power transformer is rated at 325-0-325v it means you should have around 325 volts AC from each wire to ground but 650 volts wire-to-wire."
     
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  10. Curlydog

    Curlydog TDPRI Member Silver Supporter

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    It is a mojotone kit so deluxe reverb
    250 watt photobulb
     
  11. King Fan

    King Fan Friend of Leo's

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    Is that photobulb *incandescent*? Even if it is, 250 watts may be too high, and if it was too high, you'd get a dim glow even with too much current flow...
     
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  12. dan40

    dan40 Tele-Afflicted

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    You can still find a standard 60 watt incandescent bulb at most stores. You will also find 100 watt incandescent bulbs listed as "rough service" for use in drop lights and work lights at most hardware and auto parts stores.
     
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  13. Wally

    Wally Telefied Ad Free Member

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    Subscribed...
     
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    If the amp was ok with no rectifier tube but then the fuse blew with only the rectifier tube installed then either the tube is bad or more than likely there is a short beyond the high voltage DC output from the rectifier.
     
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  15. FenderLover

    FenderLover Poster Extraordinaire

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    Pull the 5Y3 from your 5E3 and eliminate the rectifier as your problem first.
     
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  16. Peegoo

    Peegoo Tele-Holic

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    Double check the orientation of your electrolytic filter caps to ensure you have the polarization correct.

    Installing them backwards can make the fuse pop on power-up.
     
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  17. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    Absolutely need pictures to see where you are in the build. Have you just built the power section and testing it, or did you wire everything up and then try to power it (which makes troubleshooting VERY difficult - that's why I always suggest building in stages and testing after each section is wired.
     
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  18. Curlydog

    Curlydog TDPRI Member Silver Supporter

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    OK team, sorry so long to get back, finally had time to spend on it this afternoon - 1)the fuse blow issue gone - my bad - had a short - fixed it all good 2) the half voltage issue - fixed it - I was cheating and using the stewmac instructions on a MOJOTONE build - assuming it was all the same - not exactly - they don't use Mojotones multitap transformer - I had what I thought was two black leads but one was black with a dark green stripe - when I should have been using the white with the solid black - I now have 6.3 amps at all the heater wire pairs at each of the tube sockets(add only 3 before - assume no big deal and moved on, shouldn't have - I won't have a chance to put the rectifier tube in and check B+ voltage until tomorrow(really too excited that I figured this out and don't want to be poking around with HV while this "amped" up - bad Dad joke I know). Thanks to Uncle Doug's great videos on transformers, Rob's great website and all the responses above - I know I have work left but confident with you all out there I will get there!
     
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