Changing the frequency of AC from mains to lower, out of audible range??? - diyAudio
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Old 9th February 2010, 08:42 PM   #1
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Default Changing the frequency of AC from mains to lower, out of audible range???

Hello,
I have heard of the merits of using AC on the filaments of the big triode power tubes. I am not sure I understand why this is to sound better but I will take it at face value at this point. The problem is the frequency of the AC is within the audible range at 60hz.....why not lower the frequency to something like 15hz? I am not sure how this could be done. I was wondering if anybody has ever toyed with this idea or ever achieved it. Would like to hear others thoughts or experience with this.
Jeff
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Old 9th February 2010, 10:21 PM   #2
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There are issues I have read regarding too low of a frequency causing BTU heating fluctuations, but I'll leave that up to the experts. In my mind, producing 15 Hz is no small task; iron must be large if you want to transform it. PWM would be an option, but that gets quite complex and then you have to deal with high frequency hash.

If you're unhappy with 60Hz heating, I would suggest either DC constant current or high frequency quasi resonant type supplies at >40kHz.

FWIW, my AC heated projects have such minimal hum (1mV or less in push pull designs - SET is in the works) that I don't see any big issue with it. My latest build was a 6n6p PP into a 46 DHT PP into a 300B DHT PP. No hum pots, just one 'null' pot in a key location, and I get inaudibility.
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Old 9th February 2010, 10:57 PM   #3
kmtang is offline kmtang  Canada
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Hello Jeff,

I believe it just cost too much to generate a pure sine wave 15Hz power supply for the filament.


Johnny
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Old 9th February 2010, 11:11 PM   #4
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Forget about cost, several problems emerge.

First the size of the iron goes up substantially.
Second, there has to be some means of generating the lower frequency.
The phone company used to do this by "passive" means, this may be worthy of some research and consideration. They used the lower frequency AC for "ring"!

One of the issues with DC apparently, or so I have been told is that one side of the filament can be hot and the other cool... I dunno.

Making a simple power oscillator that runs at 15hz surely can't be too difficult, but maybe a little lower is better if you have really good subwoofers? The good part is that frequency stability is not a big factor in this application, so if it drifts, no one cares!

:-)

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Old 9th February 2010, 11:16 PM   #5
rknize is offline rknize  United States
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I think the issue with DC filaments on a DHT is that the cathode varies in voltage from one end to the other over whatever the DC supply is. Some tubes filaments are "center tapped" for this reason. I'm not sure how much a few volts really matters compared to the 100s of volts potential between the cathode and the plate. Maybe some interaction with the proximity of the grid.

As far as running at 15Hz...I don't think it will help all that much because it is often the intermodulation products that you end up fighting.
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Old 9th February 2010, 11:46 PM   #6
tomchr is offline tomchr  United States
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Lower frequency --> higher transformer core area needed to prevent transformer saturation --> higher cost. You'd be better off going with a higher (super-sonic) filament frequency, though, it'll couple more strongly into your circuit through the Cgk and Cak.

I'm mulling over a 300B design that I've been toying with in my head for a while. Theoretically, the electron emission will be slightly higher in the end of the filament/cathode where the |Vgk| is lowest. As others have said already, I doubt it makes a huge difference, but I have considered using a +/-2.5 V DC supply rather than a single +5 V. I haven't made any conclusions yet. The 300B does not have a center tapped filament, so it's probably a moot point anyway. But that's a thought.

~Tom
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Old 9th February 2010, 11:54 PM   #7
rknize is offline rknize  United States
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Quote:
Originally Posted by tomchr View Post
The 300B does not have a center tapped filament, so it's probably a moot point anyway. But that's a thought.
I thought one or more of the Shuguang 300Bs did. HF is another option, but you still have to deal with IMD products.
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Old 10th February 2010, 12:07 AM   #8
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maybe I don't understand something that is obvious to me. When I look at an old style light bulb the filament in that light bulb is hot all the way across. The voltage at one end is 117volts and progressively gets to 0 at the other end. Yet the lightbulb glows evenly across the whole filament. Is the same not true for tubes? Why would one end of the filament be colder just because the current is AC or DC. Common now. Reverse the dc current and voila the tube still glows. If one end of the tube is hot now the other end should be hot if you reverse the DC. Yeah right! The heat in the cathode filament would only vary if the device was poorly designed and not if the current was AC or DC. Early tubes were designed to run on Batteries for the simple reason most people did not have any electricty in their homes in the early days. When houses were wired up, AC became the common method to heat up tubes. less parts needed = cheaper costs to build. Early DC rectifiers were large and not reliable. There were millions of table radios sold that were called AC / DC They could run on 110 AC or 110 DC. The heaters were all wired in series and added up to 110 volts.
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Old 10th February 2010, 12:16 AM   #9
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The temperature of the filament is constant because the current is constant and passing through the same cross-sectional area as it goes. However the voltage at any point along filament is different. If that were not true, rheostats and other potentiometers wouldn't work!
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Old 10th February 2010, 12:17 AM   #10
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I'd think pushing the freq. above the audible range would be better/easier. Transformers can be (much) smaller,along with smaller filtering chokes and caps.
15hz is gonna require some mondo magnetics,and giant caps for smoothing.
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