Fixed Bias?

[idsnowdog]

Fixed bias means that there isn't a trim pot for adjusting the bias right? I have heard that instead there are fixed resistors that would need to be replaced in order to change to a different tube like 6L6 to kt88. Also that tubes that are 6L6's need to be tested and matched to the amps bias settings.

 

[drew_half_empty]

Re: Fixed Bias?

correct

 

[wixedmords]

Re: Fixed Bias?

Both "fixed" and "cathode" biasing circuits may have bias pots.

Both types of circuits may need to have their bias adjusted.

"Fixed bias" most of the time needs to be biased. "Fixed" does not mean "no nead to bias". The "fixed" means that the circuit operates at a fairly exact, "fixed" measurement, where cathode biased operates in more of a range. Many cathode biased amps don't need to have the bias adjusted, but it is still good procedure to check to see that the bias is with a safe operating range.

 

[kmcguitars]

Re: Fixed Bias?

Fixed bias normally means the amp has a bias resistor. Many Fender amps are considered Fixed bias/Adjustable or in other words any Fixed amp can be made adjustable by adding a trim pot. Hoffman amps has a good tutorial on his website on added a trim pot to Fender Non-adjustable circuits.

 

[Lake Placid Blues]

Re: Fixed Bias?

Actually the terms mean the opposite of what most think:

"Most guitar amps are biased in in one of two ways;fixed bias and auto/self bias. Self bias refers to to an older type of biasing scheme that uses a high current resistor between the cathode and ground and constant negative grid voltage. As the name implies, in self biasing amps the tube it's self determines the bias, based on it's condition and operating circumstances. In a fixed bias amp the cathode is grounded, and the bias voltage is controlled by a bias pot on the grid circuit. These two terms can be confusing, since fixed bias is adjustable and auot/self bias is not. ...."
Famous Amp Tech (page 57)

I don't usually argue the point since most people intutively think it's the otherway around, but fixed is actually the correct term for adjustable bias, not cathode bias.

 

[ES350]

Re: Fixed Bias?

I have heard that instead there are fixed resistors that would need to be replaced in order to change to a different tube like 6L6 to kt88.

It's probably gonna take a little more than that (rewiring the sockets, etc) to go from 6L6 to KT88, including considerations of current demand on the PT and reflected impedence on the OT.