Thanks! Can you please explain how to do this mod?
*** standard disclaimer***If you haven't worked on amps before, just take it to a tech. There are LETHAL voltages present inside an amp, even when it's unplugged, and you could hurt yourself if you don't know what you are doing!!
You didn't say if you are getting one of the new 6505's that are made in China, or a used USA one.
I have a 1992, and just sold a 2012, both made in USA and they were identical on the inside, circuit-wise and layout-wise. I haven't seen inside a Chinese one, so they MAY be different.
The schematic is from 1992 for the original 5150, but like I said, they never changed. The 15k R68 circled is the resistor that you either swap out to a 10k , or you can replace it with a 10k pot, with a 5k on the wiper so you can swing the value from 5k - 15k.
The first pic shows my '92 5150, looking from the front to the back on the left side of the chassis. You can see 3 new, black 10uf bias caps hot-glued to the board, and the 10k resistor to the right of the caps just above the large cap can. That 10k is the resistor that was 15k originally.
The second pic is the 2012 6505, but on this amp, I put in a 10 turn, 10k pot with a 5k on the wiper. The 2 yellow wires, where the 15k was, go over to the blue pot in the upper left of the pic. I drilled the chassis so I could adjust the bias from the top without pulling the chassis. The big blue resistor from the first pic is not in the second pic because I installed a choke in this amp as well.
To do both of these, I lifted out the main PCB and soldered to the bottom of the board. The other possible way to do this would be to solder a 30k resistor in paralell with the 15k on the board from the top of the board. That will give you 10k, and it will work fine like that, but looks a little un-professional when you do this kinda stuff.
No matter what method you use, when you're done you still MUST check the bias with a meter to make sure you're not cooking the tubes, but using a 10k vs 15k puts you in a good range where the majority of 6L6's will be biased properly.
Hope that helps!!