To get the most life out of your tubes you should warm them up before playing them. Don't leave the amp on forever because the transformers have a 112 degree F ceiling, and no way to cool down unless the amp is off. As far as longevity of the transformers go, 2 hours on, OFF not standby while you take a break, 2 more hours of on-time. Back to tube longevity. Different tubes have different warm up times. Pre-amp tubes like the 12AX7A take 13 seconds, output tubes take about the same time in most cases. They take longer to warm up as they wear out. So giving them 30 seconds should do. If they play loud and clean when you take the amp off of "standby" then they were ready. Other exceptions are the EL84/6BQ5, and my favorite rectifier the GZ34/5AR4. The EL84 takes about 45 seconds to warm up. The GZ34 takes longer to warm up than any of the other tubes do, and that is by design.
The GZ34 is a slow-start rectifier tube. It's job is to convert the high AC voltage into high DC voltage. Most standby switches disconnect the DC high voltage from whatever rectifier there is in the amp from the other tubes. It would be beneficial to any amp that does not have a standby switch to use a GZ34 rectifier tube. It is pin compatible with the most common rectifier tubes. Any issues would arise if the pin basing were different of if the GZ34 drew more heater current than the other rectfier type. In most cases it's close enough and the power transformer can handle it with out over-heating. The advantage of the GZ34 is mainly that it warms up after the other tubes do, eliminating the need to have or use the standby switch completely. It happens to have just enough compression to please me, greatly.
A lot of Fender amps use a GZ34 at one point in time and then a 5U4 at another. If you use the GZ34 in the amp without a standby switch tubes will last longer because they will not suffer from "cathode stripping." The 5Y3 also has the same pin basing.
Now the biggie no one ever told you about. You need to let your amp cool down for 3 minutes before you move it! A normally functioning tube amp will have bulb temperatures over 300 degrees F. It takes 3 minutes once completely powered down to reach 140 degrees F. Scalding hot is 180 degrees F. The tube is like a light bulb (vacuum, filament), and both undergo changes in shape and in flexibility as they cool. As the filament cools it also becomes more brittle, so if you move the tube as it is transitioning from its' orange hot malleable state to its cold brittle sate you stand a good chance of wrecking the tube. And it not only the heater that behaves this way, but the rest of the electrodes too. The screens get hot too, and I've seen plenty of screen wires get loose in the tube and short out to something else.
So tell your tour manager, the tube amps get switched off first and taken off stage last.
Who makes the best GZ34?
Bias;
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.
