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Old 9th March 2005, 01:35 PM   #1
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Default 6Vdc from 6,3Vac

The main question is:
How can I have 6Vdc from 6,3Vac?
Iíd like to feed the filaments of my 12B4A preamp with DC. And power transformer has only 6,3Vac CT winding, 5Vac CT for the rectifier and the HT.
I donít like to add a separate transformer (the best solution) because thereís no space in my chassis.
So, I was trying to find other solutions for that. But Iím not sure if they are correct.
Over here I need your help.
First thought is to use a voltage doubler and a 6V regulator. Sth like the attached schem.
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Old 9th March 2005, 01:36 PM   #2
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Second thought is to use a FWB and with two regulators have +/-3Vdc. And feed this to the filament. Sth like the attached schem.
Will these circuits work? And which is the best?
If you have sth better than these, please ďsaveĒ me.
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Old 9th March 2005, 01:56 PM   #3
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Hi, I'm really glad you posted this question because I have been thinking about converting my own amps to DC heaters.

The +/- 3v looks good!

The advantage is obviously lower hum, but are there any disadvantages?
I'm using ecc33, ecc35, KT66

regards, jeff
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Old 9th March 2005, 02:24 PM   #4
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+/- supply is useless. Ground in the circuit is floating for one, putting the regulator grounds in series. Probably a good way to make smoke.

6.3VAC > FWB (schottky if necessary) > resistor > heaters will work just fine. About 2000uF/ampere before and after the resistor.

A regulator is not necessary but if you must, there is enough overhead to use a 7806 (or any lower voltage type with the proper ground elevation).

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). 80% of the time, any hum in the circuit is from insufficient PSU filtering, ground loop or power transformer > heater-cathode pickup (which this cures). The remaining 20% (or maybe less, whichever) are highly sensitive circuits (either small signal or high precision) which DO actually need DC or regulated heater supply.

Tim
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Old 9th March 2005, 02:32 PM   #5
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Hi there......the power tubes wouldn't know AC from DC....unless a real htr-cath short existed.

I regulary DC'er my front ends....to advantage. Circuit enclos....a scrounge low dissipation circuit using SB series schottky diodes and very low dropout (0.3V in/out diff) which has been around for eons. The 6.3V AC rects to around 8-9V which is more than enough.

richj
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Old 9th March 2005, 03:10 PM   #6
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tim is correct, and you can also use a low dropout voltage (ldo) regulator to give yourself a little breathing room. a 7806 usually wants about 8V, a good ldo may need less than 7v
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Old 9th March 2005, 04:21 PM   #7
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Are there any pros/cons to using either a resistor or a capacitor to decouple the heater DC regulator ground from the amp ground?
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Old 9th March 2005, 04:48 PM   #8
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I have always had good luck with a simple RC filtered FWB with a 10,000 uF cap and schottky (1N5818) diodes.

Tim
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Old 9th March 2005, 07:07 PM   #9
Sherman is offline Sherman  United States
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Default Re: 6Vdc from 6,3Vac

Quote:
Originally posted by resident
The main question is:
How can I have 6Vdc from 6,3Vac?
Iíd like to feed the filaments of my 12B4A preamp with DC. And power transformer has only 6,3Vac CT winding, 5Vac CT for the rectifier and the HT.
I donít like to add a separate transformer (the best solution) because thereís no space in my chassis.
So, I was trying to find other solutions for that. But Iím not sure if they are correct.
Over here I need your help.
First thought is to use a voltage doubler and a 6V regulator. Sth like the attached schem.

I just converted my preamp heaters from AC to DC. All I did was run the 6.3V into a bridge I had and then into a CRC filter using a 1.8 ohm resistor to bring the voltage back down to 6.3. For the caps I used a couple of 1000uF 50V electrolytics I had in the bin. That's it. It got rid of the hum I had previously and everything seems to be functioning flawlessly.
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Old 9th March 2005, 08:16 PM   #10
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Default Re: Re: 6Vdc from 6,3Vac

Quote:
Originally posted by Sherman



All I did was run the 6.3V into a bridge I had and then into a CRC filter using a 1.8 ohm resistor to bring the voltage back down to 6.3.
Yup..... thats the other method I used. Except where I live the AC mains varies +/-10% nom 230V.....with direct AC the lower limit would underun the heaters.

richj
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