About a Fixed Bias

feojack · May 11th, 2004, 10:17 PM

OK, I'm a bit thick on this. I have a complete set of new tubes for my Showman top. (68 -70 model) I was reading from the Fender Amp Field Guide on ampwares.com that these have a fixed bias.
I'm taking it to a friend to have it recapped & put the tubes in for me. With a fixed bias, there's nothing for him to adjust, right?
Pro & cons of a fixed bias? I know I've read it somewher here before but since I'm as lazy as a 15 year old dog in August, I thought I'd just ask for a fresh answer rather than ruin my already bad eyeballs looking for something that I don't know exactly where tis.
Cheers, feojack

Mik · May 11th, 2004, 10:30 PM

Untechnical answer from informed-nuff-2b dangerous-Mik:
Fixed bias means your new set of output tubes will likely require the amp to be rebiased, your tech should know this and the easiest action to rebias now and future is to install a bias adjustment pot.

Mik

Madmaestro · May 11th, 2004, 11:09 PM

I would have someone check it. My new Classic 30 was burning out my J.J.'s tubes until I finally put in a bias adjustment.

Teleologist · May 12th, 2004, 09:11 AM

Fixed Bias means there's a set negative DC voltage applied to the power tube grids and the cathodes are grounded(or very close to it). You bias such an amp by adjusting this voltage so the tubes draw the amount of current you want. It has nothing to do with the presence of a bias pot. 'Healthy' power tubes of the same type from the same manufacturer and same lot selected at random will vary quite a bit(generally they follow a bell curve).

Fixed bias amp designs can produce more power for a given set of tubes. Preamp tubes are cathode biased using a resistor between the cathode and ground and don't need to be rebiased. Cathode biased power amp designs are much more tolerant of wider variation in tubes and only rarely need adjustment.

May 11th, 2004, 10:17 PM	#1 (permalink)
feojack Tele-Meister Join Date: Apr 2003 Location: Houston,TX Posts: 240	About a Fixed Bias OK, I'm a bit thick on this. I have a complete set of new tubes for my Showman top. (68 -70 model) I was reading from the Fender Amp Field Guide on ampwares.com that these have a fixed bias. I'm taking it to a friend to have it recapped & put the tubes in for me. With a fixed bias, there's nothing for him to adjust, right? Pro & cons of a fixed bias? I know I've read it somewher here before but since I'm as lazy as a 15 year old dog in August, I thought I'd just ask for a fresh answer rather than ruin my already bad eyeballs looking for something that I don't know exactly where tis. Cheers, feojack __________________ "Those who danced were thought to be quite insane by those who could not hear the music." - Angela Monet

May 11th, 2004, 10:30 PM	#2 (permalink)
Mik Tele-Afflicted Join Date: Mar 2003 Posts: 1,416	Untechnical answer from informed-nuff-2b dangerous-Mik: Fixed bias means your new set of output tubes will likely require the amp to be rebiased, your tech should know this and the easiest action to rebias now and future is to install a bias adjustment pot. Mik __________________ a "motor-bike with a touch of blood in it is better than all the riding animals on earth, because of its logical extension of our faculties, and the hint, the provocations, to excess."-T.E. Lawrence

May 11th, 2004, 11:09 PM	#3 (permalink)
Madmaestro Tele-Meister Join Date: Mar 2003 Location: Lethbridge, AB, Canada Posts: 182	Check the bias... I would have someone check it. My new Classic 30 was burning out my J.J.'s tubes until I finally put in a bias adjustment. __________________ The smoker you drink the player you get~Joe Walsh

May 12th, 2004, 09:11 AM	#4 (permalink)
Teleologist Tele-Meister Join Date: Nov 2003 Location: Chicago Posts: 382	Fixed Bias means there's a set negative DC voltage applied to the power tube grids and the cathodes are grounded(or very close to it). You bias such an amp by adjusting this voltage so the tubes draw the amount of current you want. It has nothing to do with the presence of a bias pot. 'Healthy' power tubes of the same type from the same manufacturer and same lot selected at random will vary quite a bit(generally they follow a bell curve). Fixed bias amp designs can produce more power for a given set of tubes. Preamp tubes are cathode biased using a resistor between the cathode and ground and don't need to be rebiased. Cathode biased power amp designs are much more tolerant of wider variation in tubes and only rarely need adjustment. __________________ Keep on Twangin'