Hi nephilim,
Here are my experiences with hum in the Elise: As you say, it depends on the tubes. With any ST 6N7G, ECC31 and FDD20 I get hum. To check the hum, I turn off the music source and turn the volume up on the Elise, and then I listen to the noise through my integrated amp (all listening through speakers). At very loud listening levels there is a marked hum. For some strange reason the hum increases as I turn up the volume on the Elise, but near full volume it decreases, but does not go away.
So I leave my Elise on full volume, and adjust the playing volume through my integrated amp. At 50-60% volume I cannot hear any hum; after that the hum gets louder and louder. Luckily, I do not need more than 30-40% volume to get satisfying levels. Even playing a recording recorded at very low sound levels and turning up the volume to 70% does not let the hum through since it is masked by the music (OK- it is irritating to know that it is there but I cannot hear it unless I disconnect the music.)
All this is using speakers. Headphones are a different story. First of all, even with the 6BL7, turning the volume past 12 o'clock, it is too loud (most tubes only require 9-10 o'clock), so I cannot use the Elise volume knob turned up all the way up as I do with speakers. There is a pronounced hum through my headphones (Yamaha MT-220) at any listening level using the above listed ST tubes, and I cannot listen at all through the Elise using them.
Other tubes produce less hum, and the headphones are listenable.
The solution? My amp has a headphone output, and for some reason the hum is gone there - no problem. But the sound is not as satisfying as directly through the Elise.
Tried various grounding ideas - some made things worse and some slightly better. I have a grounded power supply for the FDD20. Connected a wire to the negative 12V output and connected it to the RCA negative side of the plug on the Elise - with a resulting whining sound. Strangely enough, just letting the wire hang there without being connected to something at the other end lowers the hum a little bit (!).
My room is a veritable snake pit of cables, wires and extension chords, usb hubs, portable phones, wi-fi router and fluorescent and LED lights. It is just too much work for me to go through all the stuff and try to sort out the culprit - I am happy enough with what I have.
Tried yesterday to connect the Elise to an unswitched AC output on my integrated amp. It says max 100W - hope the Elise doesn't draw more current. It is not grounded so I had to use a "cheater plug" converting grounded into ungrounded. It did lower the noise slightly, but I am afraid to use it....
In sum, I find that the hum is to a large extent tube dependent, and that I can find workable solutions around it that do not impact on my listening pleasure.
