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In Reply to: RE: Too old, not relevant... posted by Triode_Kingdom on September 4, 2014 at 09:56:55:
[QUOTE]
Well, on a better note, I see from your blog that we're using essentially the same method of ultrasonic heating. I still worry about the possible consequences of squarewaves, but so far, I don't hear a reason to change. My modified units are going on more than ten years now. One died from excess circulating currents when I tried to filter it, but other than that, they've been reliable. :)
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It was interesting to read about your experience and method. I was surprised that you did tamper with the "splitter" (first transformer) but not with the output transformer.
1) For European market units voltage is obvioulsy doubled, while current is halved... (I suggest the addition of a 200-330uF/400V cap).
2) Modern units usually have frequencies in excess of 40kHz (good quality modern units operate at 60kHz). The unit pictured on my blog (black) operates at 30kHz. Even with that unit, there are no audible artefacts, and I am satisfied.
3) The audibility of hum/noise is related only to the "modulating" 100Hz (or 120Hz), if that is solved there is not hum/noise to be found whatsoever. I am considering a "regulated" approach whereby the input DC after the bridge would not only be additionally filtered with the cap, but actually regulated, eliminating the 100Hz (hum) modulation.
4) Not all units will encounter the problem you mention (fusible resistor) and besides the resistor it depends obviously on the diodes. I usually upgrade the diodes to 1N5408 or BY255. The fusible resistor can be replaced with a fuse (3A slow blow) or even better, with an NTC thermistor of adequate current capability (current rating for thermistors should be chosen based on "constant draw", not initial peak expected). Some modern units might even already include the NTC for slow turn-on.
5) If the output transformer is toroid, it is rather easy to adjust the output voltage by changing the turns ratio. In particular, several secondaries can be used simultaneously - which might be particularly advantageous when the technique is applied to low wattage heaters (the 300B or 2A3 consume about 6W, while the 813 requires 50W...) since most units actually have a "lowest current limit" under which they will not operate or operate intermittently.
I am aware of the theoretical problems (audible?) that this system might have, but not due to the (imperfect) square wave output, rather because of the possible intermodulation, the frequency mixing, etc. Still, I cannot hear anything, and none of my friends and family was ever able to hear anything (less hum and/or noise than with DC, definitely).
My opinion regarding why the frequency is not that critical, and why the expected effects cannot be heard is that the application of a humdinger pot (resistors arrangement) effectively cancels the first harmonic of the HF applied (let's say 30kHz) leaving us with much attenuated second harmonic content (for 30kHz that would be 60kHz). The effect thus starts from a rather high frequency and is already attentuated to the point where the effects are not audible.
In reality, once we get rid of the initial rectification hum (let's call it modulation), the frequency mixing is the only phenomenon that might be a cause of audible artefacts. Even at 30kHz, the "hum" is already attenuated in respect to the music signal, and very few OPTs can pass 30kHz without appreciable attenuation... not to mention tweeters... and finally our ears. I do not know anyone who hears 30kHz tones :D
I know the approach is not new, but seldom applied, and little documented. I would like to introduce at least some predictability in the process for fellow DIYers (make repeatability of results possible).
The only practical problem I see is how to measure the output and how to fine tune it... but that would be another topic. I prefer not using resistors to burn out voltage for the heaters (in the HF AC approach), thus I modify the output transformer. One of the problems here is that most transformers originally do not give a 12V output (rather 11.2-11.6 depending on the load) and that after the addition of the cap after the rectifier bridge the ouput voltage will be higher in RMS terms. How can you be sure that without modifying your output transformer you are getting a correct output voltage for your 211 tubes? Do you measure it, and how?
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Follow Ups
- RE: Too old, not relevant... - Alex Kitic 06:15:34 09/05/14 (1)
- Ultrasonic Heating - Triode_Kingdom 09:05:27 09/05/14 (0)
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