Mesa owners... Still maintain "fixed" bias?

[cellardweller]

Mesa owners ~ How many of you have had a bias potentiometer installed on your Mesa?

If you have not had a bias pot installed, do you use the tubes supplied/tested by Mesa?

Where else might you have you bought tubes from?

 

[dirtythermos]

I figure Mesa knows more about amps than I do, so I leave my amp alone and buy their tubes. I got it because I like the way it sounded, and have no real desire to change it.

Here is the manual for my amp: http://www.mesaboogie.com/manuals/Dual Cal DC-3.pdf

Read the section called "Bias Adjustment" on page 19. It's very informative.

Since a bias supply needs to put out the right voltage and never vary, I wanted to build amplifiers that were individually hard wired to the correct values and NEVER needed adjustment.

 

[gcapel]

I have two DC-5 Heads. They are fixed bias. I've changed them a dozen times each with either GT, Ruby, and Sovtek Tubes both preamp and 6L6. I do have a bias tester for the 6L6 and to check them before and after I change the tubes to make sure nothing looks wrong. Though I have not made a comparisson between the bias readings on mesa vs other. Intresting. Other tubes may change the bias reading a little higher or lower. It would be cool to know which tubes tend to run a little hotter or colder. That would be key in finding the tubes of your liking. I would not sudgest modding the amp with a pot. I have found its hard to find a mesa repair guy. And when you do find one they are expensive. The main board on my heads are packed tight with components.

 

[Outlaws]

Mesa owners ~ How many of you have had a bias potentiometer installed on your Mesa?

If you have not had a bias pot installed, do you use the tubes supplied/tested by Mesa?

Where else might you have you bought tubes from?

If you go to a place like Eurotubes i belive (i don't, my amps are not fixed bias) you can get tubes that are for your amp.

Basically, tubes are no longer produced with the quality and uniformity of the past, so their 'rated' numbers are much more varied than before. So Mesa picked a number they liked and stuck with it. So if you ask for tubes for your Mesa, they should be able to get you something that will match up well.

 

[savageblues]

I've never quite understood why Mesa opted for this route.

My take on this is that they have set the -ve grid bias constant and relied on testing and selection of tubes so that they meet a certain plate current range during operation (given the preset bias, and plate voltage).

The colour/number scheme is a way of categorizing the plate current the tube will run at after having fixed grid bias and plate voltage. They will run somewhere between the low end of acceptable power dissipation (ie. cold bias or bright clean tone) or the high end of acceptable power dissipation (hot bias, more compressed distorted louder). Each tube with the same marking will run at the same power dissipation when new.

Most amp manufacturers offer a bias adjustment so that most any brand of tube can be used (which has its obvious tonal advantages).

Having said that - you probably dont want to mess around with adjustable bias modifications - so if you want to try out different tubes you should consider buying a bias meter.

Good luck.

 

[gcapel]

I've never quite understood why Mesa opted for this route.

.

I think its cause they want to sell there tubes and get it in your head that "you gotta use mesa tubes man or the world is going to end".

 

[boingoman]

I've never quite understood why Mesa opted for this route.

My take on this is that they have set the -ve grid bias constant and relied on testing and selection of tubes so that they meet a certain plate current range during operation (given the preset bias, and plate voltage).
.

Yup, and like Outlaws said, back when you could pretty much get tubes with the same specs every time. No bias adjustment needed. I mean, you never hear about how people used to have to adjust the bias on their radios or TVs, right? Just pop down to the corner store and buy some new ones, slap 'em in and go.

 

[savageblues]

"I mean, you never hear about how people used to have to adjust the bias on their radios or TVs, right? Just pop down to the corner store and buy some new ones, slap 'em in and go."

Yes - but most of the really old stuff was all cathode (or self) biased, which for the most part doesnt need adjustment (although you can run into cases where you need to increase the cathode resistor to prevent over dissipation).

I dont think fixed bias was too common a design until later, and by then solid state gear started to take over.

 

[dirtythermos]

I've never quite understood why Mesa opted for this route.

In the article I linked to above, Randall Smith himself talks about why he went that route. The way I understand it, an adjustable bias lets the amp compensate for tubes that are out of spec. Mesa's position is that they will fix the bias in the amp, and then only sell you "good" tubes that are in spec.

Either way you do it, you are just balancing an equation. For people who like to tweak that stuff, I can understand how Mesa would be frustrating. I would rather play my guitar than measure voltages and tweak biases, so I pony up for the Mesa tubes, plug 'em in, and go play music.