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 a maximum of 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 a

maximum of 60mV, which corresponds to 60mA per side.

Note: For the 2-EL34 50W amps, use the same procedure as above, but adjust the bias pot for a

maximum of 45mV, which corresponds to 45mA 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, 45mVDC for 50W),

adjust the bias pot down until the meter reads 30mVDC (or 60mVDC for 30W, 45mVDC for 50W).

If it is lower than 30mVDC (or 60mVDC for 30W, 45mVDC for 50W), 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, 45mV for 50W) 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, 45mV for 50W 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, 45mV for 50W) 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. Our “optimum” bias point is around 25mV to 28mV

for the 18W and around 45mV to 50mV for the 30W, and 25mV to 45mV for the 50W amps.

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 the setting you chose. 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

IntruderMKIIManual.doc 06/17/13 Page 11 of 17