This is for geeks like me. Its misunderstood, there's too much misinformation out there, and my best customers are the ones know something, so here goes. I'll keep it simple, and cover some stuff you wont find elsewhere!

Preamp tubes are in a self adjusting circuit, so they never need biasing. We are referring now to output tubes.

Bias is an adjustment that determines how hot a tube runs. It also determines what the duty cycle is of a tube in a push-pull circuit. The amplifier design determines what class of operation the tube will run in, and so in turn determines what the correct bias point is. Regardless of class, there is a range of proper adjustment!

You need to know what you are doing to make this adjustment or you can seriously damage your
amp, sometimes irreversibly.

Three 'Classes';

Class A: The tube or tubes in the amp each have a 100% duty cycle. That means that the tube is always doing its job of pushing and pulling the speaker. Any amp with only one (output) tube has to be Class A. If there is more than one tube, it can be wired in parallel or in push-pull. Push pull amps can be any class. The parallel configuration is 'single ended', Class A. Most guitar amps 15 Watts and under are Class A. These amps tend to sound very natural, and sustain well.

Class AB: Here amps use tubes in multiples of two. They are arranged 'push-pull'. The duty cycle is 100% at low volumes, and at some point the tubes pass the baton, allowing for operation of each tube more than 50% of the time, and less than 100% of the time. By switching out a tube and allowing it to cool, it is possible to have an output that exceeds the wattage rating of all the tubes added up, i.e.; two 25 Watt EL34's in a 60 Watt amp. Most guitar amps are class AB.

Class B: Also arranged in multiples of two and in push-pull, the duty cycle for each tube is just over 50%, and is never 100% if a signal is applied. These amps use very little current without a signal applied (idle current), and so the tubes may feel relatively cold if on and not being played. 
These amps also employ extremely high voltages, like 700 Volts in a Musicman HD-120. Musicman are the only guitar amps that I'm aware of that use Class B operation. Class B amps tend to sound aggressive.

Since the Audio transformer design dictates Class of operation, how much voltage verses current at idle is also a function of design. Class A runs a lot of current at idle, Class B barely a trickle. Watts equal Amps times Volts, so depending on the design, you may be setting the bias either too hot or too cold when you set the bias to say, minus 52 Volts.

Tubes also vary, and picking an arbitrary bias Voltage can only be useful if all your tubes run at the same potential. Its not the same thing but if it helps, think of it in terms of efficiency. Under any given set of conditions different tubes will perform more or less work. Tubes within a manufactured lot vary. From one model of tube to another of the same type, there is variation. There is an even greater variation from manufacturer to manufacturer.

To build a 6L6 you need to meet the specifications for tube type (beam power, pin basing), for the maximum Wattage rating (ability to deal with heat), maximum plate voltage, maximum control grid to cathode voltage, some inter-electrode capacitance stuff. The point being that there is a lot that's not said about the design. They vary by design and by the fact that they are manufactured (manufacturing tolerances).

I'll bet this is more concise and complete than anything you've seen. I hope you enjoyed it. Feel free to print it for you own personal use but do not duplicate it, as it is copyright protected material.

At this time, Groove Tubes actually does make tubes. They have purchased the original tooling to produce US made 6L6s' and are manufacturing these in the USA. They also make an import KT66 and one particular 12AX7, useing gold in the construction of the control grid. That's all they make at this time. The rest of their tube line is, as it always has been, a representation of all the worthwhile tubes made, mostly currently. They, like Fender, Marshall, Mesa Boogie, Ruby, TNT, do a very good job of screening out the bad tubes and putting their own name on them. To the best of my knowledge, Groove Tubes is the only one on that list who manufactures any tubes, and that represents only a small portion of what they offer. Fender is now shipping "Fender Groove Tubes'" in their amps. All the ones I've seen still carry a third name, the name of the actual manufacturer: Sovtek.

Marshall has dropped Sovtek from certain model amps, and is currently shipping Svetlana tubes in
their amps (pronounced Sevitlana). Prior to that they had used Tesla tubes for a long time, and are currently made also. EI was apparently on the wrong end of Western politics, and the manufacturing plant in Yugoslavia fell down, went boom. Its production of tubes rises and falls to this day. The highly abused Chinese Sino 12AX7 was the tube everyone loved to hate, while at the same time widely used or the production of new amps. The tooling is said to have been put outside, and has gone to rust. I guess another form of friendly fire. 

There is more than one tube manufacturer in China! Lets not lump everyone together unfairly. Just like there is more than one Russian tube company, there are few enough choices in tubes to start getting a National Bias (is this a joking matter?). For anyone who cares, there is a market for American made tubes, as proven by the gentleman who purchased the tooling and licensing of the name from Westinghouse - who is making the 300B in the good old USA. The import 300B's sell for up to $300, so it's a fair guess the American made Westinghouse will cost more. By the way, the 300B is IT for true hi-fi. If you want to buy a guitar amp that uses this 42 watt triode please tell me.



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