Help adding a bias knob

[GuitarGoat]

I just discovered this website after being directed here for some help with a project. I recieved an old DuKane tube PA with two 5881 power tubes, a 5U4 rectifier tube, 5 12AX7 tubes, and 2 12AU7 tubes. I'm trying to convert it to a guitar amp head, and I'm thinking if I leave the power section alone and convert the preamp section to be JCM800-like it's doable. There's no bias pot on this though and I'm pretty sure it's rather necessary. I've attached the schematic to it. I circled the area where I'm almost positive the bias pot will go but I'm not sure exactly where it should or the value. I'm just pretty new to all this and I'd like to get any help I can get. Thanks in advance!

Jake

[defaced]

You are correct, that is where the bias pot would go. Reduce the value of R3 to say 68k, and wire a 50k linear pot between R3 and ground. Once you get this built, you may have to experiment with the value of R3 that will let you get the range from the bias pot you need. Wire the pot like the attached schem.

Someone else on here drew the schem, I take no credit.

[GuitarGoat]

Thank you very much. Another question...will a Marshall JCM800 preamp be added to this schematic with no other altering?