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Old 8th April 2004, 05:54 PM   #11
dhaen is offline dhaen  Europe
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My opinion is that unless the mains variations are greater than normal: say >+-8%, it is unnecessary.
Of course there is no reason why you shouldn't, just as an engineering exercise

I'm sure some will disagree with me
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Old 8th April 2004, 06:10 PM   #12
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Hi,

Quote:
Of course there is no reason why you shouldn't, just as an engineering exercise
Regulators have an added advantage; they limit inrush current.

For anything at preamp level I always use DC regulated supplies.
In some situations even the input stages of an amplifier may benefit form regulation as well.

Cheers,
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Old 8th April 2004, 10:09 PM   #13
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...IF you design it with current limiting. A 7806 has no internal current limiting that I am aware of and is easily popped under say, a short circuit. I should know..

Of course, the low cold resistance doesn't affect anything anyway.

Tim
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Old 8th April 2004, 10:42 PM   #14
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Here's my little circuit for dc filament supplies. It's 'low dropout', current limiting, rugged and easily scalable for a wide range of output currents by changing the current sampling resistor value & some of the transistor current ratings, plus puts out 5.8-6.1 VDC into load using a common 1N4735B zener as a reference.
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Old 9th April 2004, 01:49 AM   #15
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Hi,

Quote:
A 7806 has no internal current limiting that I am aware of and is easily popped under say, a short circuit. I should know..
I suppose it all depends on which one you pick but I found the Fairchild range very reliable:

The MC/LM series is good for 1A:

From their datasheet:

"The MC78XX/LM78XX/MC78XXA series of three terminal positive regulators are available in the TO-220/D-PAK package and with several fixed output voltages, making them useful in a wide range of applications.
Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible.
If adequate heat sinking is provided, they can deliver over 1A output current.
Although designed primarily as fixed voltage regulators,these devices can be used with external components to obtain adjustable voltages and currents."

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Old 9th April 2004, 08:13 AM   #16
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Hi,

The 78xx have a wide spread on their current limiting. Found values from 1.5A to 2.7A for the TO220 parts. Thermal protection mechanism is very unreliable so good heat sinking is mandatory. If you want to limit the current more defined you can use the attached circuit provided you have sufficient input voltage. Current limit is app. 0.6/R1

Alternatively use the L200 from ST. You can set current limit and output voltage at wish. I am using this part and it has proven very reliable. I set the current limit to twice the nominal current needed to not slow down the start-up time too much.

Cheers
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Old 9th April 2004, 08:28 AM   #17
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Hi,

Low filament voltage poisoning cathod quikly, 5.7V low limit.
May be better damage tube by high voltage than spoil sound.

DC compromising sound. Filament wire has different potential
along its length. Cathod sense it and we here. Esp. for
double triods as 12ax7. I think there are no problems with
AC suppl. Just any filament coil in PT must have middle ground
point. Each tube must have dedicated fill.coil. In my amps all
input tubes got AC and hum could'nt be hear.

Regs, Igor.
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Old 9th April 2004, 08:39 AM   #18
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Quote:
DC compromising sound. Filament wire has different potential
along its length. Cathod sense it and we here.
Duh ??? Heaters in indirect heated cathodes are double spiralled just to minimise hum when AC powered. So there is virtually no potential across the cathode from the filament voltage. Maybe if you put the cathode at high DC potential (like in SRPP) there can be some leakage if you ground the filament voltage.

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Old 12th April 2004, 07:43 AM   #19
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fdegrove,

How does operating filament below 6V cause cathode poisoning ?

Cathode Poisoning: Keeping cathode at full temperature without drawing any current. Am I right ?

MB
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Old 12th April 2004, 11:14 AM   #20
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>>Heaters in indirect heated cathodes are double spiralled just to minimise hum when AC powered.<<

If only it were so. Most indirectly heated tubes have an Alumina coated FOLDED heater. Some types, like genuine NOS 7025s and Sovtek 12AX7LPSes, have the spiral wound hum bucking construction.

An experiment to test the effectiveness of hum bucking heaters is to build a RCA phono stage with Sovtek 12AX7LPSes and energize the heaters with AC. Will the residual hum level be acceptable?
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