Installed a balanced variable bias mod in my Bassman 70
Posted: Tue Apr 26, 2022 5:06 am
This is something I needed to do because the amp is slightly too much of a load for my Mass 100 to handle under full attenuation. My current living situation doesn't give me the ability to run this amp wide open so attenuation for silent recording is a must. I was contemplating buying a Powerstation and would love to have one but I don't play out, haven't in years and likely never will again so dropping nearly $1k on another toy when I can solve the problem with a couple of bucks worth of parts is ridiculous. If I'm going to spend money on something it needs to be a guitar. Welp, long story short it worked out just as planned and it's nice to be able to play this amp without worrying about setting my attenuator on fire. 
Some mod details, vintage toob upgrades and a tone test clip below.
https://youtu.be/v7kw-orblAM
This amp has 500V on the plates. There aren't too many 5881's I will be able to run in this sucker. I'm using a tough old set of Sylvania's right now. I'll pick up a set of JJ's when they are back in stock. Re-issue Tung-Sols would roast in minutes. The tube balance control came in very handy because the tubes weren't well matched. Between that and the adjustable bias pot I installed I was able to dial them in to 33ma on one side and 32.5ma on the other which is damn near perfect in my book.
Some mod details, vintage toob upgrades and a tone test clip below.
https://youtu.be/v7kw-orblAM
This amp has 500V on the plates. There aren't too many 5881's I will be able to run in this sucker. I'm using a tough old set of Sylvania's right now. I'll pick up a set of JJ's when they are back in stock. Re-issue Tung-Sols would roast in minutes. The tube balance control came in very handy because the tubes weren't well matched. Between that and the adjustable bias pot I installed I was able to dial them in to 33ma on one side and 32.5ma on the other which is damn near perfect in my book.
