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
—Michael R., Youngstown,Ohio
There 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,
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
1) Unplug the amplifier from the wall.
2) Place the standby switch in the
3) Carefully remove the chassis from
the cabinet. Do not touch the internal
4) Clip one end of a jumper wire to
the metal chassis of the amp.
5) Clip the other end of the jumper
wire to pin 1 of any 12AX7-style tube
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.
To 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
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.