User`s manual

Biasing

Biasing is very easy, and doesn't require removing the chassis from the cabinet. You simply use

a voltmeter (any inexpensive digital multimeter will do), set it for the smallest DC voltage range

(typically 200mV). Then:

(1) Unplug any guitar cords going into the input jack. Turn the two volume controls all the way

down (other control positions don’t matter). Unscrew the lock nut on the bias pot on the rear

panel, and turn the bias control all the way down (counterclockwise). Turn the Power switch on,

wait a minute or so until the tubes have warmed up, then turn on the Glory switch. Wait a few

more minutes for the tubes to reach their stable operating point.

(2) Put the negative probe lead into the black tip jack (leaving the positive probe in one of the red

tip jacks - it doesn't matter which one), and adjust the bias pot for approximately 30mVDC (that's

30 millivolts, not 30V!).

Note: For the 2-EL34 30W amps, use the same procedure as above, but adjust the bias pot for

60mV, which corresponds to 60mA per side.

(3) Put the red meter probe leads into the other red tip jack and check for 30mVDC (or 60mVDC

for 30W amps). If the reading is higher than 30mVDC (or 60mVDC for 30W), adjust the bias pot

down until the meter reads 30mVDC (or 60mVDC for 30W). If it is lower than 30mVDC (or

60mVDC for 30W), leave it set where it was. Note: if the two readings differ by more than 5mV

to 10mV, this indicates that the two tubes are too far out of match, and they probably should be

replaced with another set that are better balanced. However, it will not harm the amplifier to use

unbalanced tubes, as long as the bias pot is set for 30mV (or 60mV for the 30W) on the highest

current side. Check the voltage from the black tip jack to the left red tip jack and then to the right

red tip jack, and adjust the bias for 30mV (or 60mV for 30W amps) on the highest reading side.

An imbalance can sometimes even sound good! Too high an imbalance, however, can cause

high levels of hum in the output and loss of low end response.

(4) Once the bias is set, tighten down the lock nuts on the bias trimpot and you're ready to play.

Note: the 30mV setting (60mV for the 30W amp) should be considered a maximum setting. It is

perfectly acceptable to bias at a lower setting. This will result in longer tube life, lower

background noise, and a cleaner tone, although if you go too low the increased crossover

distortion may become objectionable. You may prefer the tone with the tubes biased colder, so

feel free to experiment, it won’t hurt anything as long as you don’t bias the tubes too much hotter

than the recommended setting.

Note: It is normal for tubes to drift a bit, particularly when new, until they have had several hours

of playing time on them. The bias will also vary a bit with variations in AC line voltage (a Variac is

used to set the AC line voltage to 120V at the factory before biasing the tubes). Small variations

like this shouldn't cause any problems, so don't worry about constantly checking the bias and

driving yourself crazy trying to keep it at exactly 30mV. If a tube is biased too hot, the plate will

glow a dull red color. The plate is the large grey metal "box" structure seen inside the tube.

Don't confuse normal filament glow with plate glow. Plate glow can usually be seen as a "hot

spot" near the center of the plate, sometimes accompanied by a "hot" smell and some "ticking"

noises as the metal expands. Continued operation with glowing plates will soon result in failure of

the tubes, possibly damaging screen grid resistors or other amplifier components. The HT fuse

should prevent any major damage to output or power transformers, or other expensive

components.

InvaderMKIIManual.doc 12/08/11 Page 12 of 17