Magazine article Guitar Player

Gizmo Alert: Svetlana's Bias King Pro

Magazine article Guitar Player

Gizmo Alert: Svetlana's Bias King Pro

Article excerpt

Replacing output tubes typically

requires a readjustment of your

amphtier's bias setting. Biasing determines

how hard your power tubes are going

to be working, so it's important that this setting

be within an acceptable range in order to get

the best tone without exceeding the tubes' maximum

plate dissipation rating. Numerous methods

exist for performing this arcane task, but

one favored by many amp builders involves

measuring the current flow (in milliamps)

through the tube when the amplifier is idling.

The Bias King Pro ($259) makes this job

relatively easy, thanks to an integrated design that

mates a digital current meter with a pair of sturdy,

molded, hardwired socket adapters. Here's how

it works: With the amp turned off, simply plug

your new tubes into the adapters (which are

grafted to short coil cords) and then insert the

adapters into the amp's output-tube sockets.

Switching on the amp activates the meter display

(no batteries are needed), and once the amp is

fully warmed up, you can see how much cathode

current is being drawn by each tube by using the

socket selector switch next to the display.

The Bias King's four-page manual briefly

explains biasing and operating classes, and details

how to determine maximum plate current draw

for 6V6, 5881, 6L6GC/WGB, SV6L6GC, EL34/

6CA7, and SV6550C output tubes in Class AB1

(most Fender, Marshall, and similar designs) and

Class A amps that use the same output-tube

sockets. This initially involves measuring your

amp's cathode-to-plate voltage with a DC voltmeter

(something you may need a tech's help

with), but once you have this figure, you can

tweak the bias to your heart's content without

fear of exceeding the maximum plate dissipation

rating for any tube your amp can digest. …

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