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Shock Therapy

June 1, 2010
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I WOULD LIKE TO LEARN MORE ABOUT amplifiers, but I know there are potentially lethal voltages inside. I would rather not have a shocking experience, so what is the proper way to drain an amplifier of stored electricity so that one may work on it without getting shocked?
—Michael R., Youngstown,Ohio

gp0610_gearfixjumperThere are a few ways to drain an amplifier of stored voltage. On all vintage blackface and silverface Fenders with 6L6 output tubes, one only needs to unplug the amplifier from the wall and put the standby switch in the “play” mode. In a minute or two, all the electricity will be drained.

On amplifiers that use a 12AX7, 12AU7, 12AT7, 12AY7, or any other 9-pin 12AX7-style preamp tube, you can easily drain the electricity by following this sequence:

1) Unplug the amplifier from the wall.
2) Place the standby switch in the “play” mode.
3) Carefully remove the chassis from the cabinet. Do not touch the internal components!
4) Clip one end of a jumper wire to the metal chassis of the amp.

 

 

 

gp0610-gearfixdrain5) Clip the other end of the jumper wire to pin 1 of any 12AX7-style tube (Fig. 1).
6) Wait a minute or two for all the current to drain. When you think the voltage has drained, you may double-check by connecting a jumper wire from the chassis to the “plus side” of each filter cap. This will drain out any residual low voltage that may still be in the cap.

How do you know which pin is pin 1? Notice the spacing of the pins on the bottom of a preamp tube socket. There are spaces for 10 pins, but since only nine pins are used there is an open spot, which is called the “key.” When you are looking from the bottom of the socket, the first pin clockwise from the key is pin 1 (Fig. 2). Then the other pins would follow in sequence as you move clockwise—pins 2, 3, 4, 5, 6, 7, 8, and 9. Note that when looking at the socket from the top, pin 1 is the first pin counter-clockwise from the key.

 

 

 

 

gp0610_gearfix_9pinTo drain electricity from amps that don’t use 12AX7-style tubes, you would still unplug the unit from the wall outlet and put the standby switch amp in the play mode. Then, after removing the chassis from the cabinet, you would attach a jumper between the plate of any preamp tube and the chassis. For example, most vintage Ampeg tube amps use 6SL7 tubes. Pin 2 and Pin 5 are the plate leads of a 6SL7 tube. So your jumper would connect to either of those pins on one end and to the chassis on the other end. Remember to be patient and wait a couple of minutes to give enough time for complete discharge.

If you want to test an amp to make sure there is no voltage left in it, attach the leads of a voltmeter between the chassis and the plate lead you just drained. You should get a zero voltage reading. If not, go back and drain some more and re-check in about a minute.

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