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In Reply to: Re: It is not so bad. posted by dave slagle on April 4, 2007 at 16:07:08:
Potential difference to greed is more important. But still GM70 isn't so bad:
Follow Ups:
DC filaments suck the life out of DHTs. I've never heard a DHT with DC that didn't sound better with AC. Ultrasonic filament supplies are the ultimate solution.
GM70 was mostly used with AC heating in PP amps where filament hum cancels out.DC filaments sound wrong not because of principle, but because of implementation. Diode bridge plus capacitor, or voltage regulator are not the right way to do it. The right way is dedicated filament transformer, choke input filter, and CM choke to block line contamination by audio signal. It could be 40 pounds of iron per one GM70 filament, but it sounds good.
I just like to write something down. Are we looking at something like this:transformer
bridge rectifier
10mh
10,000uf
10mh
10,000uf
5mh CMC
hum pot
filaments
The critical value for the input choke is about 6 mH, but it should be rated 9 A. The second choke has to be rated only 3 A, so its value can be higher than 10 mH. 50 mH 3 A choke with 1-2 Ohm DCR has a reasonable size. CMC is easy as it does not need a gap. Cap values are right, but they may be higher than 10,000 uF. A tuned snubber circuit for bridge rectifier would be also helpful to keep the line clean.
I researched further into the subject matter, and found an article by Lynn Olson. It appears that I should use 4 chokes and a CMC.I quote"
If you were really hard-core, there would be four chokes: a pair on the high and low-side between the bridge and first cap, and a second pair on the high and low-side between the first and second cap. The first pair, since it emits magnetic noise, would be physically isolated from the audio circuit (on the far side of the chassis, close to the dedicated filament transformer), and the second pair would be close to the DHT, since it is part of a LC filter circuit. Another improvement would be to insert an RF common-mode choke of the type seen in the input of computer power supplies. This would reduce the common-mode RF noise that can sneak through the large power-frequency chokes."
However, I'd rather have one choke of higher inductance than two smaller chokes. Or even better, a choke with two windings: one in the positive and one in the negative rail (diff. mode choke). This would allow to reduce the amount of iron without compromizing the performance. Too much weight is a problem of GM70 amps.With GM70, power supply on a separate chassis is the way to go. Therefore, it is logical to keep CMC and the last filter capacitor on the amp chassis, and everything else on the PSU chassis.
I do not think that small computer supply RF CM choke with ferrite core is appropriate. Such small choke (microhenries) would do nothing to prevent audio signal from leaking into the power line. Proper CMC should have inductance of at least 100 mH. It can be wound on a silicon steel toroid. A great source for toroid cores are current transformers. They can be bought cheap from surplus dealers.
i have built with all of the above mentioned tubes and like the 45 and the copper plate GM70. I also love the 2A3. Boy it really sucks when you cannot find science to justify your opinion.
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