FUQ (eff -yu - cue) - FAQ's with an EDGE

Tagged As: Laney Guitar Amp

Question:
I'm trying to find out if there's a way to set the bias on my 70's Laney Klipp guitar amplifier. I've got a schematic of the head, and I think I've been able to find out that it uses a grid bias but I've been unable to find if there's a way to set the bias. I'm sure you people out there have better understanding of schematics and could point me in the right direction.

Answer:
You are correct. There is a fixed-biased amp with no variable adjustment. The 56k resistor could be replaced with a higher / lower value to decrease / increase plate current as needed. Do not place a resistor in parallel with the existing 56k resistor. This will only reduce bias voltage and make the amp run hotter when it may in fact need to run cooler. Before fooling with the value of this resistor, I would first perform some preventive maintenance to the bias circuit. Replace the bias current rectifier with a 1N4007-type diode. Also, replace the 15uf bias capacitor with a 22uf - 47uf cap. This will increase reliability of the amp. BTW, the direction of the bias rectifier appear to be backwards on the schematic...