AC30TB and AC30TBX Owner Service Information

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It isn't hard to change tubes on a Korg/Vox AC-30TB or AC-30TBX. Here is how.

Disconnect the power cord from both the amp and the wall socket.

Lay the amp grill down on the floor. Note that there are four flat head philips screws on the upper back panel, indicated at left by the four red arrows.

Remove these four screws and pull the upper back off the amplifier chassis.

Never remove this panel with the amp plugged in!

This is what the AC-30TB and AC-30TBX look like with the upper rear panel removed.

Be careful to not touch or disturb any of the wires or circuit board traces inside the chassis. It is possible for components inside the AC-30 to hold a charge and shock you if you touch the internal wiring. Keep your fingers toward the outside of the chassis and you will be fine.

The chassis must be removed from the cabinet to change tubes. The amp should still be lying face down. Remove the four large flat head philips screws on the side of the amp, pull the chassis straight up out of the amp.

The fit of the chassis into the cabinet is often quite tight. I recommend you carefully remove the chassis, keeping it level to avoid further binding of the chassis in the cabinet.

This is a the location of tubes on an AC-30.
The EL-84 power tubes are secured by a retaining clip. The tube at right has it's retaining clip released. Each tube makes about 8 watts, so the combination of four EL-84 tubes give the AC-30 it's 30+ watt output.
This is the rectifier tube, a GZ-34. If your amp is blowing fuses, replace this tube, it is likely the culprit. It is held in place by a big spring loaded cap. Remove this carefully to avoid breaking the tube.
Fuses are located on the right side of the circuit board.

Does your amp have an annoying low level hum that actually gets quieter when you turn up the volume controls? This is a "lead dress" issue, and you can fix it yourself.

Please remember, you are working around voltages that can electrocute you, so if you are not a competent amp tech, show this to a local service center and have them perform the work.

The picture at upper left shows the pairs of red and black wires that carry the 6.3 volts to the filaments of your preamp tubes. If these wires are not positioned exactly right, your amp will hum at idle.

In the lower picture, you will see how I used a electrical tie wrap to position the right red wire. This eleimnated the hum in this particular amp.

This is a trial and error thing. When you find the right location, carefully tie wrap the wire or wires into a permanent position.

I have seen many AC-30s with loose speaker wires. The location of these push on speaker clips that often become loose are shown in the highlighted area at left.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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