Be sure that the peak on the raw 13Vac sine wave voltage does not sag enough so that the dropout voltage of the regulators is approached or exceeded when current is drawn from the supply. Easy way to check that is to put the supply on a variac and slightly raise the AC input voltage on the power transformer's primary. If a little raise in that voltage cures the problem, then you need to go with a transformer that has a little higher output voltage, say 14 or 15Vac or so.
We haven't seen a full schematic of the equipment/system to identify if valve pulling, or some other simple temporary change can better diagnose the origin of the hum, so it seems strange that the majority of posts obsess about changing the heater DC supply. Imho, a targeted set of diagnostic tests should be the first aim, as the dc heater may be a complete red-herring.
The iPhone app is a good first-in indicator. If you have a PC with a soundcard input, and a microphone, then use a free spectrum analyser software (like REW or ARTA or ...), and a quiet ambient environment to get a better appreciation of the relative mains related signal levels. Imho never trust your ears in such hum matters - so many forum posters show that trust to fail.
The iPhone app is a good first-in indicator. If you have a PC with a soundcard input, and a microphone, then use a free spectrum analyser software (like REW or ARTA or ...), and a quiet ambient environment to get a better appreciation of the relative mains related signal levels. Imho never trust your ears in such hum matters - so many forum posters show that trust to fail.
