Name: Steve Kennedy
Email: steve@pacair.com
Subject: Biasing HD 130
Thread: 46
Time: Mon, 7 Jan 2002 20:57:07 UTC
On the schematic, look at R56 and R61. These are the emitter resistors for Q1 and Q2, the drivers that drive the output tubes.
For each pair of output tubes, the combined bias current of both tubes has to pass through one of these resistors. This means that 25-30mA of current should flow through each of these resistors when the amp is properly biased.
25-30mA of current through a 3.9 ohm resistor gives you a voltage drop of approximately 10mV as a nice round value.
Since one end of these resistors already goes to ground, just measure the voltage at the "high" side of both these resistors (at the transistor emitters). They should both measure about 10mV at this point when the amp is properly biased.
Any more than about a mV or 2 of mismatch should be investigated. If the tubes are new and matched then the resistors may require changing (due to value drift with age) or the two driver transistors are leaky suspects.
Make sure that proper bias is set in the high power switch position. BE CAREFUL (standard boilerplate warning)!!!! This high plate voltage can KILL you when you least expect it!
Steve