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Old 21st December 2012, 10:56 AM   #11
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Elvee: I have described one or two such solutions in the PSU section.
Can you give the title of that thread ? I did look but you have a a lot of posts !

I have to admit to not making a valve amp since 1962 (mullard 5/10) so I found the concept interesting. I don't remember the idea of dc heaters being discussed much then. Where could we discuss it anyway it in those pre internet days

If only the first amplifier stage heaters were heated by dc, the problems of rectifying and stabilising would be a lot less I guess. (when starting with only 6.3v ac) And I always thought that was where most hum problems began.
Supplying all the filaments and using a very large reservoir cap perhaps could be source of 100hz buzz from the small angle charging spikes ?.Just thinking aloud..

Last edited by hugo_zair; 21st December 2012 at 11:03 AM.
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Old 21st December 2012, 10:56 AM   #12
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Originally Posted by Vinylsavor View Post
Hi!
I never had a heater failure on an indirectly heated tube due to inrush current.

Different story for directly heated tubes, especially those with thoriated tungsten filaments there a soft watrt will help to avoid filament damage

Thomas
Hi Thomas,

Indeed, filament failure is not very frequent, but there are some small tubes that make a flash at power on. I hate to see them flashing, especially expensive NOS.

So, if someone uses a linear regulator for the heater, it only costs a transistor+R+C to make the thing turn on slowly, protecting the filament... It's certainly better than a brutal turn on.
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Old 21st December 2012, 12:21 PM   #13
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by hugo_zair View Post
Can you give the title of that thread ? I did look but you have a a lot of posts !
Here is one:
6.3 filament from 5vac winding

Another one:
6.3vdc form 6.3v ac PSU
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Last edited by Elvee; 21st December 2012 at 12:24 PM.
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Old 21st December 2012, 03:25 PM   #14
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Quote:
Originally Posted by Vincent77 View Post
Hi Thomas,

Indeed, filament failure is not very frequent, but there are some small tubes that make a flash at power on. I hate to see them flashing, especially expensive NOS.

So, if someone uses a linear regulator for the heater, it only costs a transistor+R+C to make the thing turn on slowly, protecting the filament... It's certainly better than a brutal turn on.
.............................
I often choose a linear regulator with a foldback current limit that is a little higher than the hot heater demand current..The tungsten cold filament resistance....will force it into foldback current limiting...and will rise slowly. The most simplest foldback i.e transistor across current lim resistor works by altering the low resistance series. See pic.

This simple circuit has an in/out drop out equivalent to many commercial 3/5 pin packages. Ignore the common mode inductor at Vin. I use it right through my power amps and has an excellent CM rejection ratio.

Worth experimenting.

richy
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Old 21st December 2012, 04:53 PM   #15
Magz is offline Magz  United States
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Originally Posted by hugo_zair View Post
I ask this as a question..is it possible to get 6.3 vdc regulated from 6.3v AC after factoring in 1.4v drop in the bridge rectifier, perhaps 1.5v in the low drop out regulator and 10% droop on the 100hz raw input ? Wouldn't you need to start with approx 8v ac ?
It's doable with a 1084 regulator as stated above and these 2A 40V Schottky diode bridges from Digikey with only a 0.5V forward voltage: part# 641-1213-1-ND. Surface mount, but I was able to solder them into a point-to-point circuit with a little ingenuity.

http://www.digikey.com/product-detai...3-2-ND/1678583

Last edited by Magz; 21st December 2012 at 04:59 PM. Reason: added link to Digikey
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Old 21st December 2012, 05:06 PM   #16
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Hi!

Quote:
Originally Posted by Vincent77 View Post
Indeed, filament failure is not very frequent, but there are some small tubes that make a flash at power on. I hate to see them flashing, especially expensive NOS. .
Yes I also have some tubes which do that. But it's normal and doesn't harm them. realy no need to worry about

Thomas
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Old 21st December 2012, 07:29 PM   #17
chip647 is offline chip647  United States
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Another great way to drop a few volts with very little effort is to put a 1n5401 in line. Each one drops about 1.2 volts. An oversized inrush current resistor like a CL-90 will soft start and provide some drop as well.

http://www.mouser.com/images/gesensi...L%20Series.JPG
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Old 22nd December 2012, 02:18 AM   #18
cotdt is offline cotdt  United States
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With 6.3VAC (that measures 6.6VAC at lighter loads) I was able to get up to 7.0VDC with LT1084 and 4.9VDC with LM338 with a pair of 300B as loads.
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Old 23rd December 2012, 07:58 AM   #19
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How about a different approach ?
DC Boost Buck Converter Step Down Step Up Voltage Regulator Power Supply Circuit | eBay
If not that one , search ebay for dc-dc converters. There are plenty to choose from.
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Old 23rd December 2012, 03:16 PM   #20
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by Vinylsavor View Post
Quote:
Originally Posted by Vincent77 View Post
Indeed, filament failure is not very frequent, but there are some small tubes that make a flash at power on. I hate to see them flashing, especially expensive NOS.
Yes I also have some tubes which do that. But it's normal and doesn't harm them. realy no need to worry about
I have a Mullard ECC83 that has been doing this every time I switch the amp on for the past 20 years. I gave up worrying about it 19 years ago...
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