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Old 1st July 2009, 10:51 PM   #21
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Theyre only half wave and since theyre rectifying in phase they should cancel. At least thats what i suspect he was doing.

Im too poor to pay 500 bux for a matched pair of 300B's to build something like that lol
its one of those things where it seems peter millet got bored and wnated to blow some serious money.
http://www.pmillett.com/uc300b.htm

ive looked over the schematic several times. He has another rectifier or two so im guessing its either for looks or hes generating multiple B+ feeds for the different tubes.
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Old 2nd July 2009, 12:50 AM   #22
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Default Re: 12AX7 heater filament resistance

Quote:
Originally posted by Ipanema
I bought a new reissue tungsol 12AX7. The heater filament pin 4 and 5 measured 40ohms but a RSD 12AT7 only measured 12ohms. When I connect it to 12V DC, it doesn't produce the magic glow. I would like to know whether this tube is malfunction because I got it from ebay. Thanks.

All my 12ax7's can be seen to glow easily.

I must admit different makes can be a bit different.
I have one that glows really bright then dims down after a couple of seconds.
I have others that just gradually glow brighter from switch on.

I now tend to use 12volt DC regulators for my heater circuits to try and get rid of hum.
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Old 2nd July 2009, 03:39 AM   #23
anatech is offline anatech  Canada
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Hi Nigel,
I've had better luck with AC heater power simply biased up +30 ~ +50 VDC.

The problem with an IC regulator is that they are usually not so good at higher frequencies, allowing some supply hash to get through. Are you using an L-C filter before your regulators to get rid of the higher frequency stuff.

-Chris
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Old 2nd July 2009, 09:34 AM   #24
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Quote:
Originally posted by anatech
Hi Nigel,
I've had better luck with AC heater power simply biased up +30 ~ +50 VDC.

The problem with an IC regulator is that they are usually not so good at higher frequencies, allowing some supply hash to get through. Are you using an L-C filter before your regulators to get rid of the higher frequency stuff.

-Chris

I am not using L-C just a large capacitor to filter out the rubbish.
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Old 2nd July 2009, 05:21 PM   #25
Ipanema is offline Ipanema  Malaysia
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What's your favourite heater supply?
1) AC voltage source
2) DC voltage source
3) DC current source

and why it is better than the other IYO?
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Old 2nd July 2009, 06:12 PM   #26
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Thats a matter of opinion.
I prefer DC just because its easier to filter but i dont demand its use unless its a directly heated tube.
I find many old dvd players and such have SMPS with 5.6v and 12v regulated supplies. some with -30v and others. These work just fine for heater circuits. you still need a B+ feed unless you have a low voltage tube or you put the negative 30 to the cathode and 12 to the plate (this rarely meets tube operation specs but you might be lucky)
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