Where to solder CT's?

Discussion in 'Amp Tech Center' started by DucDone, Oct 7, 2019.

  1. DucDone

    DucDone TDPRI Member

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    Hi. I always wonder where the CT's actually need to be soldered in a tub amp? Could someone enlighten me? See pic.
     

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  2. kbold

    kbold Tele-Holic Silver Supporter

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    To the tube, if the tube needs the centre tap. Depends which tube.
    For 12AX7, pin 9 is heater centre tap.
     
  3. DucDone

    DucDone TDPRI Member

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    Thanks. On the pic there are two CT's, the grey CT is going to? And the Black CT? The heater filaments are going to pin 7 and 2 on the EL34 (in my case) and from there to 4/5 and 9 on the 12ax7's. How do I find out if CT's are needed?
     
  4. kbold

    kbold Tele-Holic Silver Supporter

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    12AX7: centre tap is needed to pin 9. CT Black is heater CT.
    EL34: no centre tap (El34 just wants 6.3V untapped)

    The Grey CT is the high V cct: search online for "EL34 amplifier circuit" will show you connections. For an EL34 the Grey CT would go to High voltage PS (typically 400 to 500 V)
     
    Last edited: Oct 7, 2019
  5. trobbins

    trobbins TDPRI Member

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    The heater is a 6.3V heater. Typically the heater green wires go to the valve heater pins ie. EL34 pins 2 and 7, and 12AX7 pins (4,5) and 9. The CT can go to any appropriate reference voltage - usually that is 0V gnd, but is also commonly taken to an elevated DC level, or not connected at all (in which case there is often a humdinger pot or resistors across the heater) - but there are many options.

    The CT grey wire usually goes to the negative terminal of the first filter capacitor (and not to a 0V point somewhere else), but this needs to be confirmed with the schematic diagram.
     
  6. boredguy6060

    boredguy6060 Friend of Leo's

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    The HT schematic you posted shows the heater center tap and high voltage center tap.
    Typically these are soldered to a solder ring that is attached to one of the bolts on the transformer.
    You can also use a terminal strip to ground these center taps.
    I’ve used both. As long as it’s a solid ground point then it doesn’t matter.
     
  7. Wyatt

    Wyatt Tele-Afflicted

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    This depends on that type of amp your building.

    Heater C.T. (Black on your diagram)
    • Cathode bias? Connect the Heater CT to the cathode of one of power tubes, this will DC elevate the ground reference for lower noise
    • Fixed bias? Ground to the chassis. Bolt a lug to the chassis and solder to it, make sure to use a keps nut or lock washer. Some people use the PT bolt, I don't because there is vibration present.
    • If you connect the Heater CT to Ground, do NOT install a faux C.T. (two 100-ohm resistors) like Fender used to use. If using a faux C.T., do not connect the Heater CT, instead just shrink tube it off and leave unconnected.
    High Tension C.T. (Gray on your diagram)
    • Depends on the rectifier circuit, but with this PT, you'll probably Ground to the chassis. Bolt a lug to the chassis and solder to it, make sure to use a keps nut or lock washer. Don't use the transformer bolt because it may vibrate loose.
    You can Ground the Gray and Black wires to the same lug but power cord ground wires always receive their own dedicated ground lug bolted to the chassis. That's what CE, UL, and others mandate.
     
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