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12AX7 heater filament resistance
12AX7 heater filament resistance
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Old 1st July 2009, 10:51 PM   #21
bacon665 is offline bacon665  United States
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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
nigelwright7557 is offline nigelwright7557  United Kingdom
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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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12AX7 heater filament resistance
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
nigelwright7557 is offline nigelwright7557  United Kingdom
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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
bacon665 is offline bacon665  United States
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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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Old 24th August 2019, 11:42 AM   #27
breakfastbuddy is offline breakfastbuddy
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forget the B+ this is about the heater filament, and its 6.3 volt ,
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Old 24th August 2019, 04:26 PM   #28
RJM1 is offline RJM1  United States
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You do realize this thread has been inactive for over 10 years?
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Old 24th August 2019, 06:26 PM   #29
6A3sUMMER is offline 6A3sUMMER  United States
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The 12AX7, 12AU7, and 12AT7 all have 12.6V 150mA filaments (84 Ohms HOT). If you wire pins 4 and 5 together, and apply 6.3V from Pins 4+5, to pin 9, you will get 300mA filament current (21 Ohms HOT). As mentioned earlier the resistances (COLD) are just about anything (from manufacturer to manufacturer). Some will glow very brightly at power-up, and then grow dim. Others will slowly go to very bright. Others go to medium bright.

The 12AX7, 12AU7, and 12AT7 have exactly the same pinout, pins 1 to pin 9.

Filament and Pinout. But the similarities end there.

If you look at common operating conditions for each tube (there are 2 for each tube (2 x 3 tubes = 6 conditions), you will find:

The transconductances varies from 1250 to 5500 micro mhos The plate resistances varies from 6,500 80,000 Ohms The u varies from 17 to 100. The plate maximum dissipation varies from 1.2 to 2.75 Watts.

Just plugging in one tube type and then tube-rolling with another tube type . . . The results can vary widely, according to the circuit topology and circuit parameters. For example, you may destroy a fragile 12AX7 in some 12AT7 or 12AU7 spots. Or, you may be safe, but distorted with the wrong tube in there. etc.

Yeah, I noticed too late.
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Old 24th August 2019, 06:29 PM   #30
anatech is offline anatech  Canada
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12AX7 heater filament resistance
It's alive!

A necrothread!

Hi 6A3,
Yes, exactly. I've seen toasted plates on 12AX7 tubes before in repairs from "Tuberollers"

-Chris
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