what is required to convert a 60 watt Mark 2B to use EL34's?
- Thread starter gonzo
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You need to install a bias resister to accommodate the EL34's. I don't have the exact procedure but it is possible. I once had a 60w C+ that was wired for EL34's.
Basically they installed a variable pot to adjust the bias.
Basically they installed a variable pot to adjust the bias.
well, this sounds like a wonderful mod, giving me the option to go back and use the 6l6's at will...
where can i find the proper (by mesa's definition for their circuits) bias setting for both the 6L6's and the EL34's?
i assume, i'll use the pre-set fixed bias settings that mesa uses as a starting point?
where can i find the proper (by mesa's definition for their circuits) bias setting for both the 6L6's and the EL34's?
i assume, i'll use the pre-set fixed bias settings that mesa uses as a starting point?
Mesa uses a fixed bias and recommends Mesa tubes because they choose them to fall into that range.
A general rule of thumb is that class AB amplifiers are usually operated at no more than 70% of the maximum plate dissipation of the tube (to account for the higher dissipation that occurs under signal conditions), while true class A amplifiers generally run right at the maximum plate dissipation (the dissipation at full power is lower than the dissipation at idle in a true class A amplifier).
For example, the aforementioned EL34 tube has a plate dissipation of 25W, so at 400V class AB operation, it should be biased no higher than (0.7 * 25/400) = 44mA. At 500V class AB operation, it should be biased no higher than (0.7 * 25/500) = 35mA.
This doesn't mean you should automatically bias all tubes to 70% of max dissipation!
They can be biased at any lower current if desired, and many people prefer a point of around 50% to 60% of the max plate dissipation, which contributes to longer tube life.
In true class A operation at 250V, it should be biased no higher than (25W/250V) = 100mA. Note that a class A amplifier does not necessarily have to be run at the maximum ratings. You could design a true class A amplifier at lower plate voltages and higher currents, but there is a limit to how high the plate voltage can be without exceeding dissipation ratings, or having to go to class AB. There is also a limit at how high the plate or cathode current can be for a particular tube. A class B amplifier should be biased right at cutoff, or perhaps a few mA standing current, to minimize crossover distortion. Class B amplifiers usually have extremely high plate voltages in order to maximize the output power, so they must be biased right at cutoff to prevent over-dissipation at full power. If in doubt about the actual operating conditions of the circuit, call the manufacturer or refer servicing to a qualified amp technician .
A general rule of thumb is that class AB amplifiers are usually operated at no more than 70% of the maximum plate dissipation of the tube (to account for the higher dissipation that occurs under signal conditions), while true class A amplifiers generally run right at the maximum plate dissipation (the dissipation at full power is lower than the dissipation at idle in a true class A amplifier).
For example, the aforementioned EL34 tube has a plate dissipation of 25W, so at 400V class AB operation, it should be biased no higher than (0.7 * 25/400) = 44mA. At 500V class AB operation, it should be biased no higher than (0.7 * 25/500) = 35mA.
This doesn't mean you should automatically bias all tubes to 70% of max dissipation!
They can be biased at any lower current if desired, and many people prefer a point of around 50% to 60% of the max plate dissipation, which contributes to longer tube life.
In true class A operation at 250V, it should be biased no higher than (25W/250V) = 100mA. Note that a class A amplifier does not necessarily have to be run at the maximum ratings. You could design a true class A amplifier at lower plate voltages and higher currents, but there is a limit to how high the plate voltage can be without exceeding dissipation ratings, or having to go to class AB. There is also a limit at how high the plate or cathode current can be for a particular tube. A class B amplifier should be biased right at cutoff, or perhaps a few mA standing current, to minimize crossover distortion. Class B amplifiers usually have extremely high plate voltages in order to maximize the output power, so they must be biased right at cutoff to prevent over-dissipation at full power. If in doubt about the actual operating conditions of the circuit, call the manufacturer or refer servicing to a qualified amp technician .
