It can make a huge difference, but not in every cicuits or stages. It is mostly used in preamps and phono stages. I don’t know the exact reason, but Small signals in input stages are more sensitive to the AC currents. I guess that the AC current can couple into the cathode from the heater. Preamplifiers take small signals and amplifies them a lot, so small noises presented to the input get amplified too and become more audible than the same noise presented to the output stage of a power amplifier, for example. Especially phono stages, and even more if it is for an MC cartridge.
As in everything else, there are exceptions. DC heaters are also used for directly heated output valves like the 300B or 2A3.
There are many members here that are more knowledgeable than me and they might have a better explanation as to where and how to use DC heaters, but I wanted to contribute !
Do not hesitate if you have more questions. Also, have a look at the technical articles and links in the project section of this website. You’ll need to read a lot to complete your project, as I had to do when I first started
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