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Thread: Rectifier tube question

  1. #1
    RDK
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    Rectifier tube question

    I'm working on my first amp. It's the amp build in the back of the guitar amp handbook.
    My question is about the rectifier tube in the schematic. One end of the tube (I believe pin 2 and 4) are the plates and are tied to xfmr. The other pins 6 and 8 (filaments) are also tied to another tap of xfmr.

    My question is :how is this possible. Doesnt the rectifier send DC voltage out? Wouldn't that collide with the ac voltage coming out of the xfmr? In the schematic pin 6 and 8 branch to xfmr and the circuit board.

    Any help in understanding this?

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    The yellow wires are typically 5 volt rectifier heater wires. The red ones are high voltage AC from the power transformer. The rectifier produces high voltage DC output (aka B+ or HT - High Tension) from the high voltage AC produced in the power transformer (PT).

    There are step up and step down windings in your PT.

    There is generally another low voltage output available, that is the 6.3 volt AC circuit for your heater filaments in your power tubes and preamp tubes. These wires are typically the green ones.

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    Senior Member Enzo's Avatar
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    You should note that the 5vAC for the heater is not connected to ground. It is floating. Think of those high tension power lines going across your town. The wires have thousands and thousandsd of volts on them. And yet small birds sit on them without harm. The reason is that the birds are not grounded, they offer no circuit for current to flow. Your 5v does not offer a circuit path for that DC to go anywhere through it. SO while it may be at +500v to ground, the thing is still free to flutter back and forth at 5VAC, just like my birds are free to ruffle their feathers even though they are sitting on 50,000 volts.
    Education is what you're left with after you have forgotten what you have learned.

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    RDK
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    Quote Originally Posted by Enzo View Post
    You should note that the 5vAC for the heater is not connected to ground. It is floating. Think of those high tension power lines going across your town. The wires have thousands and thousandsd of volts on them. And yet small birds sit on them without harm. The reason is that the birds are not grounded, they offer no circuit for current to flow. Your 5v does not offer a circuit path for that DC to go anywhere through it. SO while it may be at +500v to ground, the thing is still free to flutter back and forth at 5VAC, just like my birds are free to ruffle their feathers even though they are sitting on 50,000 volts.

    Thank you so much! Both responses helped a lot!!!

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    Quote Originally Posted by Enzo View Post
    You should note that the 5vAC for the heater is not connected to ground. It is floating. Think of those high tension power lines going across your town. The wires have thousands and thousandsd of volts on them. And yet small birds sit on them without harm. The reason is that the birds are not grounded, they offer no circuit for current to flow. Your 5v does not offer a circuit path for that DC to go anywhere through it. SO while it may be at +500v to ground, the thing is still free to flutter back and forth at 5VAC, just like my birds are free to ruffle their feathers even though they are sitting on 50,000 volts.
    Does this explain why (when no center tap ground is present) a DC voltage (17v from the power tube cathode, for ex.) can be introduced to the green heater wires thru (100ohm) resistors - That this DC voltage has no path to ground in that heater circuit?.

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