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Old 10th March 2005, 12:33 AM   #11
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You were on the path to a "simple" solution, when you mentioned voltage doubler. If the RMS current capability of the 6.3 VAC filament winding is 4X the draw of the 12B4's heater when wired for "12" V., you are home nearly free. 6.3 V. RMS is 8.9 V. peak. Doubling that gets you to nearly 18 V. That's more than enough headroom for a modest cost 7812 3 terminal regulator. Put a 10 muF. 'lytic in close proximity to the regulator across its O/P. Put a 2nd 10 muF. 'lytic paralleled by a 10 nF. ceramic part across the heater connections at the socket.
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Old 10th March 2005, 02:16 AM   #12
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If you feel that you must use DC (as Schematic pointed out, it is rarely necessary), use a simple schottky full wave rectifier, a CRC filter, and put a 6.2V zener diode across the heater terminals.

That's a simple way to get the desired regulation, and well within the voltage tolerance of most valves.
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Old 10th March 2005, 06:58 AM   #13
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Thanks everybody for your help!
So thereís solution.I like very much Timís idea.
Quote:
FWIW, before you try DC heaters, try grounding a resistor divider, or better still than grounding, elevate it a few volts above the highest cathode voltage (if possible).
As I know AC sounds better than DC!
As I understand, use a resistor divider from HT to ground and from the joint of the two resistors feed the heaters. About how many volts?
I will not ground the CT of the heaters winding,right?

If this doesnít work,Iíll go dc just like that.
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6.3VAC > FWB (schottky if necessary) > resistor > heaters will work just fine. About 2000uF/ampere before and after the resistor.
or
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use a simple schottky full wave rectifier, a CRC filter, and put a 6.2V zener diode across the heater terminals.
........I think I'm thinking too complex ..........
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Old 10th March 2005, 09:42 AM   #14
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Grounded means two things, AC or DC. Just remember that you want the center of the heaters grounded, if just AC-wise. So a bypass cap, say 10uF tacked on that voltage divider will do well. If you don't have a hum balance pot or CT on the winding, use two 100 ohm resistors to obtain a center tap.

I wouldn't recommend a 6.2V zener because it could draw a *lot* of current if voltage rises too much. Things could get very messy, very fast. Not to mention it likewise has to be rated a good bit of current... 1W need not apply! A 7806 is perfect, though cold heater surge current might upset it; a resistor, however, is current limited and adds filtering (though no regulation, +/-10% is good enough for tubes).

Or if that 5VAC winding is going unused and is rated for the current, a FWB on that might work right off the bat, no resistor or regulator needed!

Tim
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Old 10th March 2005, 06:12 PM   #15
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I have attached a pic.Hope it's something like this.
But,how many volts must I apply to filaments?
Ht is around 250V.
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Or if that 5VAC winding is going unused and is rated for the current, a FWB on that might work right off the bat, no resistor or regulator needed!
I have a tube rectifier.So I need this winding.
Smart thinking though.I'll know it for future projects.
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Old 10th March 2005, 07:02 PM   #16
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Quote:
Originally posted by resident
I have attached a pic.Hope it's something like this.
But,how many volts must I apply to filaments?
Correct.

What are the cathode voltages in your amp? You want heater at least a few volts above the highest, but only if it doesn't exceed any H-K ratings. If that becomes an issue, then somewhere between will suffice.

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Old 10th March 2005, 07:46 PM   #17
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It's a line preamp.One 12B4A at each channel.
250V HT with 6K resistor for load.Cathode resistor is 390R(unbypassed) and cathode voltage is around 12V.
So I need sth higher than 12V?
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Old 11th March 2005, 12:47 PM   #18
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Yeah, +30V will do it.

Tim (likes cathode followers better)
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Old 16th March 2005, 09:25 PM   #19
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Tim,
If I'd like to try CF...just take the output from the cathode,right?
I'd like to give it a try!
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Old 4th April 2005, 01:06 PM   #20
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If I'd like to try CF...just take the output from the cathode,right?
OK....it's not so simple.

Quote:
Grounded means two things, AC or DC. Just remember that you want the center of the heaters grounded, if just AC-wise. So a bypass cap, say 10uF tacked on that voltage divider will do well. If you don't have a hum balance pot or CT on the winding, use two 100 ohm resistors to obtain a center tap.
Tim,
Suppose R1 is the upper resistor and R2 the other (I'm reffering to the voltage divider in my pic).
How much must be the total resistance of the voltage divider?
R1+R2= ?
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