When I disassembled my Dyna Stereo 70, I converted it to have one EL34 per channel in Triode wired single ended mode.
I took the A470 output transformers apart, and re-stacked the laminations. All E's on one side of the coil, then put Kapton tape across the end of the E's, and then put all the I's on the Kapton tape, at the other side of the coil.
They were now Air-Gapped output transformers, suitable for Single Ended Service.
When I turned the re-wired amplifier on, Immediately I heard Hum coming from the loudspeakers. Hum from a cold amplifier.
The rectifier was cold, the output tubes were cold, but Hum was emanating from the speakers in the room.
The culprit was the air-gapped output transformers which were on either side of the power transformer, and they all were mounted on a Magnetic Steel Chassis.
Yes, when the rectifier, and the triode wired EL34s were warmed up, the lower rp of the plate reduced the hum, but it did not make the hum go away completely.
Your Mileage May Vary.
My tube amplifiers that are built on aluminum chassis, have less than 100uV of hum.
I took the A470 output transformers apart, and re-stacked the laminations. All E's on one side of the coil, then put Kapton tape across the end of the E's, and then put all the I's on the Kapton tape, at the other side of the coil.
They were now Air-Gapped output transformers, suitable for Single Ended Service.
When I turned the re-wired amplifier on, Immediately I heard Hum coming from the loudspeakers. Hum from a cold amplifier.
The rectifier was cold, the output tubes were cold, but Hum was emanating from the speakers in the room.
The culprit was the air-gapped output transformers which were on either side of the power transformer, and they all were mounted on a Magnetic Steel Chassis.
Yes, when the rectifier, and the triode wired EL34s were warmed up, the lower rp of the plate reduced the hum, but it did not make the hum go away completely.
Your Mileage May Vary.
My tube amplifiers that are built on aluminum chassis, have less than 100uV of hum.
The example you gave is significant, i.e. the amount of hum depending on the material of the chassis used.
Then there is also the question of the positioning of the components and a certain overall geometry.
I noticed that keeping the elements symmetrical and spaced out in a proper way makes things better. I remember as an example the first amplifier made with the 2A3 I made. It played well, right from the start. Surprisingly no noise at all.
I also used a symmetrical and central, Y-shaped ground bus together with a complex humdinger for the 2A3 cathode (a fine-tuned resistor network project I found on the web).
Then there is also the question of the positioning of the components and a certain overall geometry.
I noticed that keeping the elements symmetrical and spaced out in a proper way makes things better. I remember as an example the first amplifier made with the 2A3 I made. It played well, right from the start. Surprisingly no noise at all.
I also used a symmetrical and central, Y-shaped ground bus together with a complex humdinger for the 2A3 cathode (a fine-tuned resistor network project I found on the web).