I had acquired a SE 6AQ5 amp from an electronics warehouse some years ago, tearing out the existing circuit and rebuilding the amp with a parallel 5751 RC coupled driver stage.
All the heaters for rectifier, driver and output stages were run off a single 6VAC secondary winding. Not the best arrangement for utilizing the amp on a high efficiency Altec horn system.
I wanted to experiment with a SMPS (switch mode power supply) to create a DC heater supply for the driver and output tubes. I used a 19V 65W HIPRO computer power brick from an old ACER laptop that I no longer have in service. The DC output switches at a frequency of 60KHz.
Common practice suggests using an low pass LC filter at the output of the power brick. Typical specs for such power bricks suggest 80 - 120mVp-p AC ripple at 47 -63KHz.
Note: I have read that some SMPS will have a difficult time starting/warming up cold tube heaters or filaments due to their low resistance when cold. I didn't experience that problem and the output voltage seems relatively stable. My advice is to spec the power brick for 3-5 times the rated heater wattage in your circuit and add a bit of resistance to the filaments.
I measured 0mVAC ripple at the output of the amp under an 8 ohm resistor load. Subjectively, the amp is now dead silent at idle and sounds fine during playback of random music tracks.
The drawbacks would be the bulk of the computer power brick , the LC filter parts and the heat generated by the dropping resistor.
See schematic posted below.
DT 667 12/25/2019