Aiken Amplification Invader User Manual download pdf (Page 10)

Tube layout and replacement

The Invader uses four ECC83/12AX7 tubes, one ECC81/12AT7 tube, and two 6BQ5/EL84 output

tubes (four EL84s in the 30W version). It also uses a single 5AR4/GZ34 rectifier tube.

The preamp tubes are housed in removable "bayonet" metal shields. The shields can be

removed by pushing down on them and rotating the shield to the left, then pulling it upwards off

the tube. The tube can then be pulled out of the socket. The output tubes have spring wire

retainers that can be slipped off to one side, allowing easy removal of the EL84 tubes.

The preamp tube function is as follows, starting at the side closest to the input jack: preamp 1

(12AX7), preamp 2/cathode follower (12AX7), reverb driver (12AT7), reverb recovery (12AX7),

and phase inverter(12AX7). The chassis top view layout is shown in the diagram above.

You may freely sub any 12xxx type tube in the two preamp and phase inverter positions. It won't

hurt anything, and you may find some different tones you like. The 12AU7 is a good lower-gain

tube, as is the 5751. Other choices are 12AY7, 12AT7, or 12AZ7. In addition, different

manufacturer's tubes will sound different. Some people prefer the tone of various Ei, Sovtek, or

other tubes, and some insist on NOS tubes for a true vintage tone. Feel free to experiment!

The 30W model may be safely operated with either the two inner or the two outer EL84 tubes

removed for half-power operation. It is recommended that the rear-panel impedance selector be

set to the next lower setting (i.e. 8 ohms when using a 16 ohm speaker), but it won’t hurt anything

if you forget to do this.

Note: vacuum tubes, especially output tubes, can get very hot in normal operation, and may

cause burns if removed before they have a chance to cool down. Be sure to wait several minutes

before replacing tubes to allow them to cool down. Do not, under any circumstances, remove or

replace the tubes with the power on, as this could cause a deadly electrical shock should the

glass break, exposing the high-voltage plate terminals. Also, be sure to pay attention to the

orientation of the tubes when putting them back in the socket. The tube pinout is keyed so it can

only be placed in the socket in one direction, and if the tube is forced in incorrectly, it can damage

the tube or the ceramic socket. The octal rectifier tube has a phenolic key that fits a

corresponding keyhole in the socket. If this key is broken off, it will allow incorrect placement of

the tube in the socket, which could result in damage to the amplifier. Be sure to carefully

examine the tube before replacing it.

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Aiken Amplification Invader User Manual Page 10