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Old 9th May 2007, 10:52 AM   #1
engels is offline engels  Israel
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Default Parallel vs. series filament supply - or another filament idea

I've got a little problem so I wanted to hear some advice.
I'm making a bass preamp with two 5670 tubes. Everything is finished except for the filament.

I've got a separate filament transformer which is a multiple secondary 1A 0-3-6-9-12V trannie. I've mesaured 7.3V on the 6V wire so I've desided to make a DC power supply. I've etched a small board for that but all of sudden there were no 3A/6V positive voltage regulators at electronic shops, only 12V/3A. I've bought 7806 regulator but it's rated 1A and I'm not sure it will handle 0.7A curent without overheating and shutting down. I've got 78T12 12V regulator rated 3A and I've used them already, they can handle up to 1.2A current.

I've got a couple of choises:

1. 12V DC filament with two tubes wired in series (78T12 regulator). The datasheet is not speaking about series filament supply so the question is - if there any possible bugs with this type of solution?

2. 6V DC parallel filament supply with two 7806 regulators in parallel. Sounds bad.

3. AC filament supply. No idea.

the datasheets:

5670 tube
7806 voltage regulator 78T12 voltage regulator

And of course, if you have other ideas about how to run the tubes without filament supply and such, I'd be more than happy to read it.
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Old 9th May 2007, 11:38 AM   #2
SY is offline SY  United States
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Can you get 317-type three terminal adjustable regulators?

Series heaters will work fine as long as you're using tubes with the same heater rating and similar warmup characteristics- if it's two of the same tube, that's pretty much a given. AC heaters will also work fine if the wiring/layout is done carefully (e.g., tight twisting of wires, dress along chassis edge, etc.) and the heaters are properly bypassed.

One nice trick for increased current is to bypass the regulator with a resistor that carries part of the current. For example, let's say that you have 10V raw supply, want 6V out, and your load will draw 1 amp. You can run (say) half that current through a power resistor connected between input and output of the regulator. For this example, using Ohm's Law, R = V/I = (10-6)/(0.5) = 8 ohm. Power dissipated in the resistor is P = VI = (10-6)(0.5) = 2W. A 4 or 5 watt resistor will work. This will take half the load off the regulator while not significantly degrading regulation.
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Old 9th May 2007, 02:34 PM   #3
garbage is offline garbage  Singapore
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Default Re: Parallel vs. series filament supply - or another filament idea

Quote:
Originally posted by engels

I've got a separate filament transformer which is a multiple secondary 1A 0-3-6-9-12V trannie. I've mesaured 7.3V on the 6V wire so I've desided to make a DC power supply.
you did measure with the tube filaments connected to the 6vac taps right?
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Old 9th May 2007, 11:07 PM   #4
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You could try the 1083, 1084, 1085 regulators from someone like Linear Technology. Their power ratings are 7.5A, 5A, 3A respectively

Charlie
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Old 9th May 2007, 11:28 PM   #5
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Engels,

Watch your step! When you rectify and cap. I/P filter a filament winding, only 0.5X of the VA rating is available.
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Old 9th May 2007, 11:44 PM   #6
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Assuming your transformer is up to the task,

Choice number 4:

Transformer, rectifier, filter caps, two 7806's - one for each tube.

A 7806 will need 8 to 9 volts input to work correctly depending on the chip, and the ripple voltage. Do not connect the 7806's directly in parallel, they don't like it. Each chip puts out a slightly different voltage. The chip that puts out the most voltage will shut down first, followed by the other one.
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Old 9th May 2007, 11:49 PM   #7
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Quote:
Originally posted by tubelab.com
Assuming your transformer is up to the task,

Choice number 4:

Transformer, rectifier, filter caps, two 7806's - one for each tube.

A 7806 will need 8 to 9 volts input to work correctly depending on the chip, and the ripple voltage. Do not connect the 7806's directly in parallel, they don't like it. Each chip puts out a slightly different voltage. The chip that puts out the most voltage will shut down first, followed by the other one.

If 7806s are problematic to source, use 7805s and connect the grounding terminals to ground via 1N4007 diodes. The 1.2 V. forward drop across the junctions tweaks the 7805 to produce 6.2 V.
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Old 10th May 2007, 09:10 AM   #8
engels is offline engels  Israel
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Default thanks

Thank you for that many good ideas!

For the moment seems like two 7806 - one for each tube - is the option I'm going to try. Hope 7806 will handle .35A!

I've finally found 317, the datasheet says it handles around 2.2A in my 6V range, but it requires two resistors with one being adjustable. I'm not sure a little trimmer will not overheat, so it probably needs some PHAT pot - but I'd like just to put a resistor.
I was trying to find a scheme with some simple PS based on a 317 to see the resistors' values, but haven't found one yet. Could someone point me to the right direction?

btw, the 317 datasheet:

317 fairchild

1083, 1084 and 1085 are not for sale in our part of the planet.

2 Eli Duttman:
you say that "When you rectify and cap. I/P filter a filament winding, only 0.5X of the VA rating is available."
I'm afraid I'm too dumb to understand your warning. Could you please translate it into simple English (or Hebrew)?
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Old 10th May 2007, 12:24 PM   #9
SY is offline SY  United States
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Quote:
Could you please translate it into simple English (or Hebrew)?
S'licha, ani lo m'daber Ivrit. But what Eli was saying in English was that if you use a capacitive input filter following the rectifier, the winding's volt-amp rating should be halved.

The National Semiconductor application notes on the 317 are very comprehensive, so I think you'd find what you're looking for there.
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Old 10th May 2007, 12:52 PM   #10
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Default Re: thanks

Quote:
Originally posted by engels
Thank you for that many good ideas!

For the moment seems like two 7806 - one for each tube - is the option I'm going to try. Hope 7806 will handle .35A!

I've finally found 317, the datasheet says it handles around 2.2A in my 6V range, but it requires two resistors with one being adjustable. I'm not sure a little trimmer will not overheat, so it probably needs some PHAT pot - but I'd like just to put a resistor.
I was trying to find a scheme with some simple PS based on a 317 to see the resistors' values, but haven't found one yet. Could someone point me to the right direction?
A couple things here...

1. A 7806 will handle .35 amps just fine - as long as it is well heatsinked. Take the heat sink size you think it will need and double it -then you'll be in good shape.

2. The LM317 only requires the two resistors to program it to a higher voltage. If you ground the adjust terminal without a resistor you have a 1.2 volt regulator. The two resistors are simply there to provide a reference for the adjust terminal. A tip - I've found the regulator works better with a little more current than the data book example circuits use. I use a 120 ohm resistor instead of the 240 ohm the example circuits typically show.

3. You don't have to use a trim pot with LM317s - you can use two fixed resistors. Once you determine the value you need then put a fixed resistor in the circuit. The dissipation on these adjustment resistors is VERY small, .5 watt resistors are plenty stout.

4. As Eli mentioned you can stand these regulators on conventional diodes or zeners to change the output voltage. But whether the regulator is "standing" on a resistor or on diodes I recommend you bypass them with a good low ESR cap. You can use 10 uf or so safely (without having to add protection circuitry) and improve the regulator performance significantly.

Finally, I think Audio Express sells a reprint of the National Semiconductor 1980 edition of the Voltage Regulator Handbook. It's a GREAT book, I highly recommend it!! BTW, there is a nice slow start regulator in the book I've used many times for tube heaters.
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