Name: Steve Kennedy
Email: steve@pacair.com
Subject: 212 HD 130
Thread: 46
Time: Mon, 22 Oct 2001 00:01:54 UTC
As long as you supply a readable schematic for your amp you should be able to take it to any competent tube amp technician.
On your amp, bias is adjusted by using the bias trimpot (marked TR-1 on the schematic). Using a VOM, measure the voltage across each of 3.9 ohm cathode resisitors (R56 & R61 on the schematic) while adjusting this pot. Since you want 15mA of bias current per tube at idle, a total of 30mA will be passing through each 3.9 ohm cathode resistor. This means a voltage measurement of about 115-120mV (0.120 Volts DC) when the bias is set properly.
If a matched quad of tubes is used, then the voltage measurement across these resistors should be fairly close... if not, then you have a problem.
Steve