Tube biasing

[dewey finn]

I have a 50 Watt Fargen Mighty Plex with Fixed bias. I have always operated under the impression that I should get middle numbered tubes when i replace them. I am not sure that I am correct in that assumption. Should I get Lower numbers? Higher? Any advice is greatly appreciated.

[crunchee]

I think it depends on what the amp had originally. If all else fails, ask Fargen!

Theory is, is that IF the power tubes are a certain number in a rated system, and the amp came out of the factory with the bias 'fixed' for those exact brand tubes at that exact same number, then you should be able to drop in new power tubes as long as they are the same number in the same system...and the system is probably only followed or featured by ONE manufacturer, there's no industry standard that I know of. I don't know if Mesa number ratings are the same as Groove Tube number ratings for power tubes, but unless someone can confirm that they are, then I doubt that they are. That means, if you want to stick with this system, then you're gonna be limited to only those tubes in the system, again probably only offered by ONE manufacturer. The numbers aren't arbitrary, presumably they mean something about how the tube is rated regards bias, but I doubt that the manufacturer(s) go into much detail about what the numbers mean unless you ask directly.

Fender had amps in the 50's and early 60's that used fixed bias, before they started using bias/bias 'balance' pots in the circuit. A qualified tech that is familiar with how this is done should be able to go into any amp with a fixed bias circuit and be able to change the resistor or resistors in the bias circuit, if needed, so that 'unnumbered' power tubes will work properly. That is, if you want to go to the trouble to install different branded or even NOS tubes.

[Bobby Marshall]

Mesa has a color system. Grove tubes has numbers. There is a post somewhere on forum.grailtone.com (the Boogie Board) with the conversions.

Boogie red and yellow are go to distortion sooner followed by white, and down the line until you get tubes that run very clean.

Wish I could help directly. Find Fargen's forum maybe?

[FrankieIII]

Have a bias pot installed...

[seeker]

Fixed Bias is a bit of a misnomer. What it means is that the bias voltage is a fixed value, determined by a set resistive value -in combination with the characteristics of the tube(s)-. That resistive value can either be a single value resistor, or a variable resistor (potentiometer). Mesa, et al, sell numbered tubes screened to match a set resistive value as installed.

However, if you change that resistive value, the bias voltage will change.

I've had a Mighty Plex myself, and would be pretty surprised if Fargen installed a single value resistor; should be a pot. In other words, as others have said, get the tubes you want and have a tech bias them. Or better yet, get a bias meter and do it yourself. Cost only a little more.