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10-09-2024, 12:45 PM
Hi Guys
Two important details regarding tube voltage ratings should be mentioned here.
The first is that with all power tubes that exhibit gain (triodes, tetrodes, pentodes) it is assumed that the plate load will be inductive, i.e., a transformer primary to couple the plate signal to the load. Normal transformer-action involves fly-back voltages that equal the amplitude of the plate drive voltage, and at full output this can be nearly twice the DC supply voltage. This high voltage is only present across the tube when it is not conducting, so the tube simply has to be able to sustain this voltage pressure without shorting out.
The voltage at which the plate shorts to the cathode is called the "arc voltage". This spec is not generally listed in the tube data sheet, but it is typically a bit more than twice the rated plate voltage. For example, the EL-34 is rated at 800Va and its arc voltage is 2kV. Mullard provided this information in articles presented through Wireless World magazine back when the tube was first introduced. 2kV is 25% higher than twice the rated 800V, but this cannot be taken as typical although we would hope that it is.
The lowly 6V6 has "secret" capabilities inasmuch as it can withstand 1500V pulses on its plate and therefore must have an arc rating reliably higher than that. This tells you that operating it with 500Va is no big deal and that the 6V6 can be used in nearly any amp Fender made that came equipped with 6L6s, just as we reported in TUT in 1995.
The second important detail regarding tube voltage ratings is that the tube never loses this capability to withstand high voltage. The cathode will lose emission capabilities with use, but the geometry of the tube does not change and thus neither does the voltage rating. The tube will last longer operating at high-voltage and low-current than at low-voltage and high-current.
Two important details regarding tube voltage ratings should be mentioned here.
The first is that with all power tubes that exhibit gain (triodes, tetrodes, pentodes) it is assumed that the plate load will be inductive, i.e., a transformer primary to couple the plate signal to the load. Normal transformer-action involves fly-back voltages that equal the amplitude of the plate drive voltage, and at full output this can be nearly twice the DC supply voltage. This high voltage is only present across the tube when it is not conducting, so the tube simply has to be able to sustain this voltage pressure without shorting out.
The voltage at which the plate shorts to the cathode is called the "arc voltage". This spec is not generally listed in the tube data sheet, but it is typically a bit more than twice the rated plate voltage. For example, the EL-34 is rated at 800Va and its arc voltage is 2kV. Mullard provided this information in articles presented through Wireless World magazine back when the tube was first introduced. 2kV is 25% higher than twice the rated 800V, but this cannot be taken as typical although we would hope that it is.
The lowly 6V6 has "secret" capabilities inasmuch as it can withstand 1500V pulses on its plate and therefore must have an arc rating reliably higher than that. This tells you that operating it with 500Va is no big deal and that the 6V6 can be used in nearly any amp Fender made that came equipped with 6L6s, just as we reported in TUT in 1995.
The second important detail regarding tube voltage ratings is that the tube never loses this capability to withstand high voltage. The cathode will lose emission capabilities with use, but the geometry of the tube does not change and thus neither does the voltage rating. The tube will last longer operating at high-voltage and low-current than at low-voltage and high-current.
