Good resource on how to bias a guitar amp

kid · 31st December 2006

I am beginning to learn about electronics. I've rebuilt my Scott 299B home stereo. I've built a bunch of Seventh Circle Studios preamps. These are relatively simple projects. I want to be able to bias the studios Fender Twin Reverb, and a Blues Junior as well other amps. I recognize that the voltages can kill me but there must be some learning material on how to do this properly and safely. I just got the Aspen Pittman's "The Tube Amp Book" but this does not tell me how to bias an amp. In fact it states that I should bring the amp to a qualified technician to do this. Any suggestions?

dementedchord · 1st January 2007

sorry i cant refer to the book..cant seem to find my copy at present but as memory serves there's a fairly detailed description in there somewhere... most of the twins had what was called a balance adjust for the bias (so the sides pulled evenly) but was easily modded to a bias adjust (the mod may be in the book as well) so check to see if it's already been done on yours... the primary reason they suggest to take it to a pro is to cover their own ass cause there are voltages there that'll light you up... ( i grabbed a 600v line on an old hammond once that threw me cross the room... it'll make a believer outta ya) as far as readding the bias one thing i used to do is... if you look at the sockets you'll see there's one pin held to ground... take precision 1 ohm say 1 wt resistor and put it between there and ground instead... you can then read the bias directly from the pin... makes it way easier to compair individual tubes... IME if you want the thing to really sing rebuild the whole output section with 1% tolerence resistors no use useing matched tubes if the circuit it's self isn't matched...

31st December 2006	#1
kid Gear Head Joined: Sep 2004 Location: New York Posts: 51 Thread Starter	Good resource on how to bias a guitar amp I am beginning to learn about electronics. I've rebuilt my Scott 299B home stereo. I've built a bunch of Seventh Circle Studios preamps. These are relatively simple projects. I want to be able to bias the studios Fender Twin Reverb, and a Blues Junior as well other amps. I recognize that the voltages can kill me but there must be some learning material on how to do this properly and safely. I just got the Aspen Pittman's "The Tube Amp Book" but this does not tell me how to bias an amp. In fact it states that I should bring the amp to a qualified technician to do this. Any suggestions?

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1st January 2007	#2
dementedchord Lives for gear Joined: Apr 2006 Posts: 885	sorry i cant refer to the book..cant seem to find my copy at present but as memory serves there's a fairly detailed description in there somewhere... most of the twins had what was called a balance adjust for the bias (so the sides pulled evenly) but was easily modded to a bias adjust (the mod may be in the book as well) so check to see if it's already been done on yours... the primary reason they suggest to take it to a pro is to cover their own ass cause there are voltages there that'll light you up... ( i grabbed a 600v line on an old hammond once that threw me cross the room... it'll make a believer outta ya) as far as readding the bias one thing i used to do is... if you look at the sockets you'll see there's one pin held to ground... take precision 1 ohm say 1 wt resistor and put it between there and ground instead... you can then read the bias directly from the pin... makes it way easier to compair individual tubes... IME if you want the thing to really sing rebuild the whole output section with 1% tolerence resistors no use useing matched tubes if the circuit it's self isn't matched...