Umm maybe.
There is an optimal cathode resistance for each type tube and operating point. An EL34 amp in Class AB P/P with 315VDC on the plates wants to see a 130 ohm resistor shared between the two tubes. A pair of 6L6 @ 360VDC, wants 250 ohms shared.
Check
http://tdsl.duncanamps.com/tubesearch.php for tube sheets.
From Randall Aiken's site:
What about cathode-biased amplifiers?
Do cathode-biased amplifiers need to be biased? The short answer is yes. The cathode biasing method is self-regulating, to an extent, because increases in cathode current create a larger voltage drop across the cathode resistor, which in turn, creates a larger negative grid-to-cathode voltage, which counteracts the increase in current. The tube will reach a stable point of equilibrium and stay there. However, just as different tubes from different manufacturers will draw varying amounts of current in a fixed-bias amplifier, the same is true of a cathode-biased amplifier. For this reason, the bias should always be checked, even with cathode-biased amplifiers.
Checking the bias current in a cathode biased amplifier is easy, just measure the voltage across the cathode resistor and divide by the resistance value to obtain the cathode current. Note that if the output tubes share a common cathode resistor, you must divide the current reading by the number of tubes sharing the resistor. Note also that a common cathode resistor does not allow you to determine the individual currents of each tube, so if one tube is drawing more current than the other, you would not be able to determine which is causing the mismatch, and, in fact, you would not be able to tell there was a mismatch at all. You can add individual 1 ohm resistors from the cathode of each tube to the common bias resistor, but you must then measure across the 1 ohm resistors, not from the cathodes to ground, to determine the voltage drop, and thus the cathode current. You can also use individual cathode bias resistors on each tube. The value of the resistor will be double that of the common resistor if two tubes are used, or four times that of the common resistor if four tubes are used. Each resistor would also have to be bypassed with its own electrolytic bypass cap.
The difficulty with cathode-biased amplifiers is that the cathode resistor must be physically changed for another one of different value in order to change the bias current. Although it can be done, very few guitar amplifiers have adjustable cathode bias.
Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.
If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation.
Do I really need to bias my amplifier every time I change tubes?
You will occasionally hear guitarists say things like "In the old days, we just bought new tubes, stuck 'em in, and started playing", and "We didn't worry about biasing back then, and you don't need to now", or perhaps: "Biasing is a myth". In most cases, you don't have to bias your tubes when you change them. You can just plug a new set in and start playing, especially if you aren't too particular about setting up the amp for the absolute best tone. However, if the new tubes you have plugged in are different enough from the ones that were in there, with respect to current draw for a particular grid voltage, they may end up biased too hot for that particular amplifier. In this case, your new tubes will start to glow cherry red on the plates, either at idle or while playing, and they will soon be destroyed. In addition, the tube may short out and take out the output transformer in the process, leading to costly repairs. Tubes of the same type from different manufacturers will usually vary greatly in current draw at a particularly grid voltage, but even two different tubes of the same type from the same manufacturer can vary widely in their current draw. For these reasons, it is always best to check the bias after installing a set of tubes. If you are in an emergency situation, such as a blown tube in the middle of a gig, you can go ahead and stick in your spare tubes, but you should turn the amplifier on and look at the plates of the tubes (the large dark grey metal element) in the dark, both at idle and while playing, just to make sure they aren't glowing red. Often, when a tube fails, it will take out the screen grid resistor, and any new tube you plug in will glow red, or won't work at all. In this case, you have no choice but to repair the amplifier before using it.
How important are matched tubes?
Most tube resellers will offer "matched" pairs or quartets, usually at a higher price than unmatched tubes. Matching generally refers to tubes that have the same current draw for the same given negative grid voltage and the same transconductance, or amplification factor. However, many vendors don't match for both parameters, they often only match for one or the other. Some vendors claim proprietary matching techniques and won't disclose them. You should be wary of this, as the tubes you receive may not be matched at all, particularly if your amplifier is different from their test circuit! Although no resellers do it, the ideal method of tube matching is to use a curve tracer and match by characteristic curves.
Idle current matching is important for output transformer current balance. In a push-pull amplifier, the output tubes on each side of the output transformer primary draw DC current through the transformer in opposing directions. This causes a net DC current of zero, and thus, a net zero magnetization in the output transformer core. This is important because a push-pull transformer has no air gap to prevent core saturation. If the offset DC current is great enough, the primary inductance will drop, and the amplifier's low frequency response will suffer. It doesn't take much offset DC to produce an unacceptable drop in primary inductance in most output transformers. Tubes must be matched at the idle current range to be used in the amplifier, as they may be matched at one particular grid voltage/plate current, but not at another point. As an alternative to idle current matching, individual bias pots, or a combination bias/balance arrangement, can be used for the output tubes, allowing perfect DC balance even with unmatched tubes.
Transconductance matching is important for AC (signal) balance in the output stage. A push-pull amplifier has inherent power supply hum rejection and common-mode input rejection due to the symmetry of the output stage. If one side has a different amplification factor than the other, this symmetry is lost, and the amplifier won't be able to reject power supply hum and noise as well. However, there is another property of push-pull amplifiers that needs to be considered when discussing transconductance matching, and that is the cancellation of even-order harmonics. In a properly balanced push-pull amplifier, even-order harmonics generated in the output stage will be canceled out. Note that even-order harmonics generated in the preamp stages will not be canceled by the output stage, only those generated within the output stage itself will be canceled, otherwise the amplifier would not sound very good! Some will argue that unmatched tubes actually sound better because of the lack of even-harmonic cancellation. This is a matter subject to personal taste, so there is no one correct answer, but it does mean that of the two parameters, idle current matching, or DC balance achieved through the use of a bias balance arrangement, is the more important of the two. Note also that the transconductance varies with the DC bias point, so it is again important that transconductance matching is done at the DC bias range the tubes will be operated at in the amplifier, if truly matched tubes are desired. Generally, it is not that critical to get such absolute precision, but if the tubes were to be operated at true class A, for instance, at 250V/100mA per tube and they were matched for class AB operation at 450V/30mA per tube, they may not be as well matched as desired.
Note that tubes tend to drift with age, and a matched set of tubes will likely become unmatched after awhile. Typically, the most drift occurs in the first several hours of use. For this reason, you should always purchase tubes that have been burned in for several hours before the matching process. This will minimize the amount of drift later in the tube lifespan. Many resellers match tubes after only a short warm-up time, which means that they will likely not end up being matched after you install them in your amplifier and play for awhile. Be sure to inquire as to the burn-in time when purchasing matched tubes.
