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In Reply to: RE: Too old, not relevant... posted by Triode_Kingdom on September 03, 2014 at 23:07:31
With all frankness, I don't understand where is this leading?
It seems that you are not familiar with me or my work. You assumptions are totally incorrect...
Besides, it seems to me that you are intent on discrediting me in order to try proving your point. I don't think that is going to work.
http://rh-amps.blogspot.com/
Follow Ups:
Dear Sir,
I did visit your blogspot and for demanding such exacting scientific proof in this thread I sure read a lot of probably, shoulds and oughtos in the 813 se build text. Just saying....JH:-)
n
[QUOTE]
I sure read a lot of probably, shoulds and oughtos in the 813 se build text
[/QUOTE]There are exactly 12 "should" in the RH813 text, and none of those are given to show uncertainty or inexactitude.
After all, this is a DIY text proposing an (excellent, if I am free to say so)amplifier to fellow DIYers who might wish to build one, or are looking for ideas and inspiration.
I really do not know what else can I comment about this... probably even this comment was not necessary.
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http://rh-amps.blogspot.com/
Edits: 09/05/14
Alex,
I've been looking at your site for about a year. Lot's of interesting information. It seems that the technical specifications for your amplifier designs are from spice simulations. Have you done any actual measurements to compare with the simulation results? Just curious.
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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Buy Chinese. Bury freedom.
[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. :)
[/QUOTE]
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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http://rh-amps.blogspot.com/
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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Buy Chinese. Bury freedom.
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