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In Reply to: why use a gm70 when 845 is current production (chinese though) and... posted by jarthel on April 3, 2007 at 18:29:34:
the GM70 has double the heater power nd comes in a copper plate version.
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wut TK said.it is also interesting to note that in the case of the GM70 the double cathode power comes from increasing the voltage which also doubles the voltage gradient across the cathode.
dave
That cain't be good!
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Boo!
With the right plate voltage (1,500 V), potential difference across the cathode is less than 1.5%. For 45, it is almost the same (1%), yet nobody sees it as a problem.
but what about the ratio of the filament gradient to the bias voltage? 20V of a 100V bias is a lot different than the 2.5V of a 50V bias for a 45.As ivan mentioned, the tube to test this with would be the 304TL which you can configure as a 5V 25A or a 10V 12.5A tube. It is interesting to note that when wired for 10V operation two of the plate will cheery up before the other two, and when wired for 5V operation they all cherry up at the same time.
I actually think it would be kinda cool if listening experience showed the 10V connection of the eimac to sound better :-)
dave
Potential difference to greed is more important. But still GM70 isn't so bad:
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.
I'm not convinced that it is such a problem. I sure have a hard time faulting the few M-70 amps i have heard.
Hows about a 304TL wired with all four cathodes in series? ;-)
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