Biasing an amp, uhhh, what is biasing?

[badpenguin]

I read all the time about how a tube amp needs to be Biased when you do a tube change. I've never done that in 40 odd years of owning tubers and am curious as to what it is. Yeah, I know, somewhat of a stupid question, but hey I'm a former bassist.

[Mr.Grumpy]

Tubes act like valves to electric current.  A very small amount of current can control a valve that throttles the much larger current passing between the cathode and plate. In a preamp circuit, a single tube has to amplify both the "up" and "down" currents, so they do that by setting the tube to be "50% ON" with no signal. The BIAS is set by placing a negative voltage to the tube's grid to set it as close to the middle of it's linear amplifying range. This is class A amplification, Imagine a spigot that's halfway on all the time, not very efficient and it does use up electricity which has to come from a transformer and power supply...so class A is used in preamp circuits and some low-power power amps.

The more efficient and powerful class B operation requires two tubes operating in pairs, one to amplify the "up" signal and another to amplify the "down" signal. The trick is that each tube has to be turned on a little bit, imagine leaving a faucet on at a trickle, but considerably less than the 50% of class A. This "trickle" of electric current is the bias, and the amount of flow is measured in milliamps (thousandths of an amp) through the tubes plate. The plate is where the louder amplified signal appears, but has to be separated for the very high "B+" voltage that sucked the electrons to the plate in the first place. In tube amps this is done by the speaker transformer. 

If the tubes in a class B amp were biased to be all the off with no input signal, there will be ugly "crossover" distortion which doesn't sound good on a clean amp. Some people think turning the "trickle" up higher (hotter vs. colder bias) makes an amp sound better, but turning up the bias makes tubes run hotter and shortens the life of power tubes. 

Famous illustration showing the inner construction of a power beam tetrode similar to the 6L6 type Yes, there are two grids, I believe one grid is for biasing and the other for the incoming signal, but I'm not sure. 

 

[Mr.Grumpy]

I've biased a solid state bass amp and my Music Man HD-212, in both cases there is a small trim pot on the circuit board that's only accessible by opening the amp up and sliding out of it's case. If there's no pot inside then the bias is set by the value of a fixed resistor (one of those brown or tan tubular things with the colored stripes). As far as I know, the preamp sections in all guitar amps are fixed bias. If an amp has adjustable bias, it's for the power tubes because it's more critical. 

Worrying about bias is for high-gain djenty metal people, and maybe those tone connoisseurs out there. 

Transistors work similarly to vacuum tubes and also have to be biased according to whether they are in a class A or class B amplifying circuit. 

This post has been sponsored by...COFFEE!!.....weed....and the ghost of WRGKMC. 

Edited June 29, 2022 by Mr.Grumpy

[daddymack]

Also, some amps are 'self-biasing' [cathode biased] like AC30s, and 5E3 Deluxe...most [not all] amps under ~35 watts are self-biased. When you get into the four power tube zone, then it gets important.

I don't know if I would say that it is only for the high gain crowd or tone connoiseurs...anyone who wants their tubes in their 40-100W amp to last more than a few gigs will benefit from understanding the need to bias the power tubes...my Marshall SL 100 was a PITA to get the bias right...and I did fry some EL34 tubes very quickly when I did it too hot...that was the first amp I actually had to bias myself, and it was a learning curve. Back then there was no internet [1977] and few manuals available. I was fortunate that I worked with a lot of techs and engineers [computer/test equipment industry] who were helpful to a point...tube tech was out of vogue, but the basics still applied.

I still have one amp, my Vibrolux Reverb, that needs to be biased. The rest of my amps are 20W or lower and self biasing...thankfully.

[Hoot Owl]

On 6/29/2022 at 10:51 AM, daddymack said:

Also, some amps are 'self-biasing' [cathode biased] like AC30s, and 5E3 Deluxe...most [not all] amps under ~35 watts are self-biased. When you get into the four power tube zone, then it gets important.

I don't know if I would say that it is only for the high gain crowd or tone connoiseurs...anyone who wants their tubes in their 40-100W amp to last more than a few gigs will benefit from understanding the need to bias the power tubes...my Marshall SL 100 was a PITA to get the bias right...and I did fry some EL34 tubes very quickly when I did it too hot...that was the first amp I actually had to bias myself, and it was a learning curve. Back then there was no internet [1977] and few manuals available. I was fortunate that I worked with a lot of techs and engineers [computer/test equipment industry] who were helpful to a point...tube tech was out of vogue, but the basics still applied.

I still have one amp, my Vibrolux Reverb, that needs to be biased. The rest of my amps are 20W or lower and self biasing...thankfully.

I hear you, it's nice to not have to worry about biasing. But for some people, they might want to be able to fine tune their bias. As you know, fixed bias amps can be adjusted hot to cold. But I have a MESA Lonestar, fixed bias, with no bias pot (which is why you are supposed to buy your tubes from MESA - they are all speced to work at one bias setting). Some guys don't like that. They say MESA biases their amps too cold, so they put in a pot so they can run the tubes warmer. The warmer you run them, the shorter their life, however.

[daddymack]

Dumpster diving this week, eh? This is a 2 year old thread...but...frankly, anyone who believes that their Mesa is biased too cold needs to buy a Dumble.