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KOTriode wrote:

It's 60Hz hum, caused by both mechanical and magnetic field coupling to the output transformer. I used a toroidal transformer and thought it would not caused this problem but it did. When using an external power supply for both B+ and heater (AC), the noise went down to around -74db, another 12db improvement. The output transformer is 1:1 and does not have magnetic shield, it picked hum easily. So, for now I just use the preamp the way it is, but in the future, if I build another 6C33C preamp, which I certainly will, I will have the power supply on separate chassis.

Well, the sexy configuration of your chassis does certainly have toll. Whatever I even built, no matter how little it was used separate chassis for transformers and power chokes with the last cap on the control chassis. Well, I did built one headphone amp that had all together but that project went to nowhere as it sounded like crap.

I think with use good cans it is still possible to mount a good toroidal in soft wax and to hide it mechanically and magnetically from the rest of the circuit. It for sure shall not be sitting in the middle of chassis…

BTW, one very important think, did you check your PS for incoming DC? Toroidals are notoriously horrible if they see any DC on primary. Most of the RMS meter will not measure DC properly with 120AC, at least my Flukes do not do it. so, you might run juts for teat the am from some kind of 1:1 isolation transformer and to see it comfort your in chassis toroids. I have a good success to place the transformer on the fat sheets of sorbotan and isolated them with MuShield sheets. Still, nothing can beat the transformers sitting in … basement and rung a long cable to control unit. The long cable in this case acts as a wonderful decupler between the filtering stages….

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