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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: Too old, not relevant... posted by Alex Kitic on September 5, 2014 at 06:15:34:
Glad to hear you've had good luck with these. All the modules I've tested so far have oscillated at 20 kHz to 25 kHz. Perhaps some of the newer models oscillate at a higher frequency, but I'm not comfortable even with 30 kHz. At one time during the early stages of my work, Lynn Olsen cautioned about the possibility of IM in the audio band. If I remember correctly, he referenced the fact that some devices, such as CD players, generate digital noise above 20 kHz. This low-level noise is usually benign, but mixed with the square wave on the filament, it might generate audible artifacts. That was enough to convince me the frequency should be as high as is practical. It's only a little work to modify the first transformer, and that raises the frequency above 60 kHz.
The earlier post that I linked to from 2003 noted the value of the capacitor I used at the output of the rectifier. With that cap, ripple is almost nonexistent on the square wave, and there is no audible filament hum at the output of the amplifier. I think the value was similar to what you're using.
The output transformers in the supplies I bought were not toroidal, and that was the main reason I didn't attempt to modify them. As for regulation, I haven't found it to be a problem. These units are no worse than a conventional filament transformer, so this is an area that I don't worry about. Regarding the measurement, the output of the supplies is a reasonably good square wave. I simply used a calibrated 'scope to set it for +/- 10V (20V p-p) across the 211 filament.
Yes, it would be interesting to fine tune this approach, and maybe make something available to the DIY community. I'm currently in the process of rebuilding my amplifiers, so I might revisit all this in the near future. :)
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