Aiken Amplification Invader MKII User Manual page 11

Output power reduction by using 6V6 tubes (30W models only):
It is possible to reduce the power of the 30W Invader to around 18W to 20W by changing the two
EL34 output tubes to 6V6 tubes. The amplifier will need to be rebiased before using, to insure
the tubes are running at a safe idle current . In order to familiarize yourself with the biasing
procedure, you should read the next section on biasing before making this change.
The proper procedure for changing from EL34 to 6V6 is as follows:
(1) Turn off the amplifier.
(2) Unscrew the locking nut on the rear panel bias potentiometer and turn the screwdriver bias
adjustment fully counter-clockwise (anti-clockwise for our friends across the pond).
(3) Remove the EL34 output tubes (Careful! They may be extremely hot if the amp has been
on!)
(4) Install the 6V6 output tubes
(5) Set the impedance selector to half the normal setting. For example, if you have a 16 ohm
cabinet connected, set the impedance selector to 8 ohms. If you have an 8 ohm cabinet, set
the impedance selector to 4 ohms. If you have a 4 ohm cabinet, set the impedance selector
to 4 ohms as well. If you are running two 16 ohm cabinets, set the impedance to 4 ohms. If
you are running two 8 ohm cabinets, set the impedance to 4 ohms as well. You cannot safely
run two 4 ohm cabinets.
setting, i.e. matching the cabinet impedance. You may like the tone better that way, or not.
Either way, it won't hurt anything.
(6) Turn on the power switch and wait a few minutes for the tubes to warm up.
(7) Bias the amplifier as indicated in the next section on biasing, but instead of setting the bias
adjustment to a max range of 60mV on the test jacks, set it no higher than 30mV, typically
around 25mV to 28mV for best tone.
(8) Tighten back down the bias adjust locking nut, and you're ready to go.
To convert back to EL34 tubes:
(1) Turn off the amplifier.
(2) Unscrew the locking nut on the rear panel bias potentiometer and turn the screwdriver bias
adjustment fully counter-clockwise (anti-clockwise for our friends across the pond).
(3) Remove the 6V6 output tubes (Careful! They may be extremely hot if the amp has been on!)
(4) Install the EL34 output tubes
(5) Set the impedance selector to match the impedance of the cabinet you are using. For
example, if you have a 16 ohm cabinet connected, set the impedance selector to 16 ohms. If
you have an 8 ohm cabinet, set the impedance selector to 8 ohms. If you have a 4 ohm
cabinet, set the impedance selector to 4 ohms. If you are running two 16 ohm cabinets, set
the impedance to 8 ohms. If you are running two 8 ohm cabinets, set the impedance to 4
ohms. You cannot safely run two 4 ohm cabinets.
(6) Turn on the power switch and wait a few minutes for the tubes to warm up.
(7) Bias the amplifier as indicated in the next section on biasing, to a maximum of 60mV, typically
around 45mV to 50mV for best tone.
(8) Tighten back down the bias adjust locking nut, and you're ready to go.
Other tube types:
You can also use any of the following tube types in the 30W Invader: 6L6, 5881, KT66, 6550.
Bias them the same as you would for EL34 tubes, as described in the next section on biasing.
InvaderMKIIManual.doc
Note: you can try setting the impedance selector to the normal
12/08/11
Copyright 2003-2011, Aiken Amplification, Inc.
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