So... I've been messing about with the Last Witness 6505.
You can check out Last Witness by CLICKING HERE
The first cool thing about this amp, is the 'Witness' on the front.
My man Sykes brought this to me for an Adjustable Bias mod. Here is the 'before' shot. That little resistor is going to get sliced out of there, and then we'll bang in a trimpot and another fixed resistor. The trim pot allows the bias voltage to be adjustable, while the fixed resistor acts as a safety device. These amps are biased notoriously cold and this is a pretty common mod.
This is it. Job done. The trim pot and resistor are joined in a way that stops a total break in the circuit should the wiper in the pot become disconnected. This means that the bias voltage won't vanish and your valves won't fry. It's a bit awkward to get in there, but you just need to be careful and use insulated tools, and watch your fingers. That big black tube is a high power resistor, and it has the plate voltage across it... which will make your hair stand on end if you touch it!
Now... another thing we found with this amp, was a bad valve. This valve has in turn blow one of the 100ohm screen grid resistors. After finding this some more surgery was needed.
100ohm is pretty low in value for a screen grid resistor, and since we've got the bias mod, changing to a more usual value of 470ohm for 6L6 valve types was definitely a good idea. We could even go up to 1Kohm but 470ohm should be cool. I also spotted the amp was running set for 220/230v, I'm guessing the switch was knocked at some point. If you stick ~240v into an amp set for 220v, you're going to get higher than expected voltages inside the amp. When you consider that some mains in the UK can be 250v (the legal maximum) you can imagine the problems this might lead to! Well maybe you can't... but be honest... doesn't look good does it? So putting the switch back to 240v and using 470ohm screen grid resistors with the adjustable bias, means this amp should be running a bit smoother and sounding like mini atom bombs are going off all over the strings on the axe whacked into it! Peace