I recently sent my Lampizator DAC in to be upgraded. As part of the upgrade the tube heater circuit for the DHT power tubes remains energized when the unit is in standby. My question is this, does leaving the the DAC in standby with the power tube heater circuits energized shorten the life of the tube? I also verified that no high voltage is applied to the power tubes when in standby. Thanks.
Short answer: It's likely; I would not leave filaments on in standby.
Long answer: There are several mechanisms that affect tube lifespan: plate dissipation/heat; plate voltage and/or current; filament use; filament voltage accuracy; filament warm-up; on/off duty cycle; B+ application relative to filament voltage application; mechanical failure... It is hard to say whether leaving the filaments on continuously will be a contributing, or sole limiting, factor in tube life in your case.
Also, Directly Heated Triodes don't have "heaters". DHT cathodes (filaments) are directly connected to the power-supply, causing the cathode to heat up and release electrons. In contrast, In
directly Heated Triodes (IDHTs) have a heater connected to the power-supply - the heater causes the cathode to heat up and release electrons.