Technical information

STEP 13: Installing bias resistors, heater wiring (part 1).
Locate (4) 1Ω 2W metal oxide resistors, color code BROWN-BLACK-GOLD-GOLD. These are referred
to as “bias resistors” in Metroamp kits because they are utilized to simplify measuring and adjusting
bias of the output tubes. In a stock 2203 amp, pins 1 and 8 (cathode and suppressor grid) of the
output sockets are grounded. We replace this connection with a 1Ω resistor. Thanks to Ohm’s Law,
measuring millivolts across this resistor tells us the current flowing through the tube in milliamps. This
will be discussed in detail in the testing section.
Install one resistor for each socket by attaching one lead
to the ground lug. The other lead is routed to terminal #8
(through the hole and wrapped around) and then on to
terminal #1. Assure solid solder joints at all three
connections.
Next up is perhaps the most tedious task of the entire amp assembly: wiring the tube heaters. It takes
practice and sheer diligence to end up with evenly twisted wires. Don’t be surprised if you end up
redoing your first attempts. We have included Topcoat™ wire (stranded wire that is fused into a single
conductor) to help you out. It will stay in place and stay twisted better than typical stranded wire.
Locate the 22g RED and BLACK wire. Cut (3) 6” pieces of RED and (3) 9” pieces of BLACK. These
will be used to daisy chain the heater connections for the output tube sockets. I prefer to work from
RIGHT to LEFT when wiring heaters, but you can work from LEFT to RIGHT. Which ever way yields
the best results.
Strip ¼” of insulation from the end of a RED wire and attach to PIN 2 of an octal socket. Solder in
place and route towards the rear of the chassis and on to the next socket. Next, attach a BLACK wire
to PIN 7 and route as shown below. Assure good, solid solder joints here, since if one fails each tube
downstream from the failure will no longer heat properly.
Try to make a minimum of two full
twists of wire between output tube
sockets.
TECH NOTE: Heater wires are twisted
because the AC voltages that they carry
are out of phase and the interaction
between the wires will allow phase
cancellation of noise. It also helps
prevent AC induced hum from leaking
into the amplifier signal.