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In order to extend tube life in the MKIIIs using the Posiedon boards, we reduced the heater voltage difference between the heater and cathode. A capacitor and two resistors were used (Diego Nardi is the influence we went with and have successfully used this in several other projects). By applying 6V positive bias with to heater power, so the heater is no longer referenced to ground. Promising results were obtained initially--the MKIIIs absolutely blew away a $4.5K Proseed hooked up to a pair of B&W studio monitors! Problem is that increasingly loud hum with a buzzing sound took hold when hooked up to my Zu Druids, 101 db efficiency. Is there something the Posiedon boards don't like here? The enhanced performance is wonderful and I'd love to extend tube life. (BTW, this mod worked wonderfully well in my RCA orthophonic linestage.) Any thoughts on the matter would be most appreciated!

Heaters not referenced to ground = HUM

I think he's talking about this trick:



Cheers!

According to Geek's schematic, you can think of this mod without considering the DC lift element at all. This mod on the Poseidon effectively AC grounds one of the sides of the heater but not the other. This is a great improvement from no ground reference (see old Eico schematics), but not superior to a center tapped ground. That said, you can simply disconnect the filament center tap and connect it to Vh-lift.

If this what you've already done, then the problem might actually be something else, like an oscillation. Had you used these MKIIIs with your speakers before? Rig up a zobel to run in parallel with your Zu Druids and see if that cures the problem.

Shannon

Thank you gentlemen! Geek's schematic is exactly what was done. However, we previously installed a hum balance, so grounding to it's center tap occurs here as well. Interestingly, the obnoxious hum has spontaneously faded away. We can only surmise that the capacitors needed time to settle down.

But I will definitelly pursue the Zobel network addition. With HF sensitive speakers, this may clean up the remaining artifacts quite nicely. Thanks, Shannon!

BTW, these MKIIIs now have 5H Hammond choke input trannies mounted beneath the bottom chassis plate. They are nicely hidden by custom wood cases. I was looking for a way to reduce the voltage to extend tube life and to lower demand on the power transformer. I still have plenty of power to drive the big old 87 db speakers, but the HF Zu drivers absolutely love the improved smoothness.

Thanks again, all!