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2 hours ago
Hi Guys
I forgot to mention, that in the case of adding an aux PT for some of the heater load, the new PT can be used for the preamp tubes if that works out better. The preamp tube heater load is usually quite small, but removing it from the stock-PT may allow you to use the power tubes you wish to without having to rewire the octal sockets. As an example, many of the later Fender "reverb" amps have six 12A_7s, totaling 1800mA - enough to support a second pair of 6L6s, or switch the stock octals to any other pair. Supporting those six little tubes requires a 2A transformer which should be small and easy to find a place for.
Using the example from post-2 with 2x 6L6 and 3x 12A_7, the heater capacity is known-good at 2700mA. A pair of EL34s over-loads it 500mA. This represents 18% based on the 2700mA reference.
For reference, TUT3 has a list of power tubes and the heater current they require. Some tubes (especially the KT variants past 88) have vague current requirements ranging from 1800mA to 2500mA per tube. Luckily, the more common tubes used in guitar amps are less than or equal to the load of the EL-34.
Have fun
I forgot to mention, that in the case of adding an aux PT for some of the heater load, the new PT can be used for the preamp tubes if that works out better. The preamp tube heater load is usually quite small, but removing it from the stock-PT may allow you to use the power tubes you wish to without having to rewire the octal sockets. As an example, many of the later Fender "reverb" amps have six 12A_7s, totaling 1800mA - enough to support a second pair of 6L6s, or switch the stock octals to any other pair. Supporting those six little tubes requires a 2A transformer which should be small and easy to find a place for.
Using the example from post-2 with 2x 6L6 and 3x 12A_7, the heater capacity is known-good at 2700mA. A pair of EL34s over-loads it 500mA. This represents 18% based on the 2700mA reference.
For reference, TUT3 has a list of power tubes and the heater current they require. Some tubes (especially the KT variants past 88) have vague current requirements ranging from 1800mA to 2500mA per tube. Luckily, the more common tubes used in guitar amps are less than or equal to the load of the EL-34.
Have fun
