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Old 2nd January 2004, 03:27 PM   #1
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Default High Freq. AC Power for Filaments of 300B SE Power Amp?


I am using DIY 6SN7 - 6V6 - Interstage X'former - 300B - Output X'former power amplifier to drive my 97 db eff. horn speaker system. I use AC power supply to light the filaments of 300B and the hum is small but audible.

Instead of using DC, can I use a very high frequency which is beyond audible frequency (say 100 Khz) power supply to light up the filaments of the 300B? Are there any technical difficulties? Where can I find the schematics?

Any advice is greatly appreciated!

Best Regards,

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Old 2nd January 2004, 04:04 PM   #2
dhaen is offline dhaen  Europe
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This has been dicussed on the forum before.
Unfortunately, I cannot find the correct search terms to find it.
Perhaps someone else can remember.

There are varying opinions....
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Old 2nd January 2004, 04:44 PM   #3
316a is offline 316a  England
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A few folks on the Audioasylum have tried this , notably Kurt Strain . There was another recent thread there which also discussed the use of modified low voltage lighting 'transformers' which incorporate a small 40kHz switched mode supply .

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Old 2nd January 2004, 11:31 PM   #4
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Basically for high freq AC, the incoming mains are rectified without a transformer and smoothen with caps. Then this DC is switched via a switch mode configuration like push-pull, or half bridge configuration. This is coupled to a smps trans of ferrite core material. The trans has much higher mu hence must smaller in size then a mains trans. This allows savings of space too and weight. The frequency is recommended should be around 200khz so that trans can be smaller and less amount of switching can reach the audio band. Output of trans, is the correct amount of secondary winding compared to primary. It is a lot of work to get it to work if one has to be build it from scratch. Suggest using it with transmitter tubes like 845,211,gm70 and above as their current draw is much higher and worth the effort.
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Old 3rd January 2004, 11:48 AM   #5
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Just a gut feeling.... Use an LC (NOT CLC) filter to round off the edges of the high frequency square waveform. Something approaching a sinewave would be nicest but not absolutely necesary. Absolutely have the HF wiring twisted, maybe even twin shielded like microphone wire. Have the PSU *and* filter located maybe several feet or more away from the amp. It will be tricky to measure the voltage fed to the filament but as long as the filament is the same colour/temperature then all should be well. Perhaps you could also run a small light globe that shines onto an LDR or phototransistor as part of a feedback loop that regulates the HF supply.
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Old 3rd January 2004, 01:28 PM   #6
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i taught of that before. I did something like that to smoothen a square wave into sine wave by using a 12order low pass filter usign op-amps. The sine wave almost perfect when only 12 order is used. For a LC it would be a 12db low pass filter. Hence from my exprience i should then used at least LCLC. A LCLC waveform looks a cap charging and discharging wave hence might used more order then. Since the low pass filter is set much higher than the square wave freq, around 100-200khz, the parts size will be smaller since they operated at such high freq. Also probably would used a torodial wound like a common mode choke to increase inductance per wire size.
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Old 3rd January 2004, 06:21 PM   #7
dhaen is offline dhaen  Europe
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Default I remember..

This is the thread where it was discussed before:
EC8010's comment is pertinent.
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