AC Heater vs DC Heater

In almost every manufacturing made amplifier, heater is DC because DC heater will have little hum. But with acceptabe hum, AC heater is always better than DC heater, especially for directly heated tubes. So what is acceptable hum? For my ear under 1mV, without NFB, is acceptable. With that range of hum, you can use your amp with 100db sensitive speaker without noticing hum even when music is not playing. With audio tubes as 2A3, 45, 71A, VT52 or 300B, using AC heater is always better in any case, if you can reduce the hum under 1mV. But with transmistion tubes like 211, 811A, 808, 100th, 450th, because the heater is working at very high current, high voltage, it is impossible to use AC with SE amplifier. In PP (push pull) tube amp, with the characteristic of PP amp, you can use AC heater with any kind tube. As looking into a PP amplifier, if heater is DC heater, you can think the builder is an amateur guy.

Here is some example of comparing the hum from AC heater and DC heater with audio tube used in a Single Ended amplifier design.

• 300B : AC: 0.6mV, DC: 0.2mV
• PX-25: AC : 0.5mV, DC: 0,2mV
• 2A3, 45, never use DC heater, even with AC heater hum is around 0.2mV-0.3mV only
• VT-52: AC: 0.5mV, DC: 0.1mV

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