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Old 26th May 2011, 06:10 AM   #1
tomchr is offline tomchr  United States
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Default Universal filament regulator

Folks,

I've been building this 300B amp for the past year or so. I want to use regulated DC for the filaments. I have done the math on linear regulators and found that I will end up dissipating more power in the filament regulators under worst case conditions than the amp will deliver to the speakers. I find that ridiculous... Needless to say, I started dabbling in switchmode regulators. They actually work really well...

So I was thinking. Wouldn't it be nice with a universal switchmode filament regulator? Universal, as in programmable by a resistor change to different output voltages. I'm thinking to support 2.5 V for 2A3 and the like, 4.1 V for the Russian tubes, 5.0 V for 300B, 6.3 V for just about everything, and 12.6 V for the tubes with series filaments.

Are there any other common filament voltages that should be supported?

I'm thinking to use an LM3102 as it allows me to hit the above voltages without too much fuzz. Its max output current is 2.5 A (just enough for a 2A3 I think). It's also in a package that's hand-solderable by most humans.

I'm throwing this out there to get the process started and to gauge the level of interest in such a project.

~Tom
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Old 26th May 2011, 09:26 AM   #2
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Tom, were you going to start with a smps plugpak with regulated 12V output, and use the buck for lower voltage options?
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Old 26th May 2011, 07:00 PM   #3
tomchr is offline tomchr  United States
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Originally Posted by trobbins View Post
Tom, were you going to start with a smps plugpak with regulated 12V output, and use the buck for lower voltage options?
Nah... I'm not a fan of the plugpak/wall-warts. I figured I'd use a small-ish 12 V transformer -- or two 6.3 V heater windings in series to power the filament regulator.

In the ideal world, the regulator would run off of one 6.3 V winding. However, if you do the math, you'll figure out that by the time you account for voltage ripple, regulator drop-opt, etc. under worst case conditions it just is not possible to make 6.3 V regulated DC from a 6.3 V AC winding. Unless.... Unless you go into a buck/boost converter topology. But even then switchers tend to not operate at their optimum point when Vin is close to Vout. I considered using a regular voltage doubler to get 12 V from a 6.3 V winding, but with the currents involved, I'm not sure it's such a hot idea.

~Tom
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Old 26th May 2011, 07:30 PM   #4
TheGimp is offline TheGimp  United States
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Start out with a 12.6V source, FWB gives you around 18V which is plenty of headroom to get the efficency up above 90%.
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Old 26th May 2011, 07:43 PM   #5
tomchr is offline tomchr  United States
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Originally Posted by TheGimp View Post
Start out with a 12.6V source, FWB gives you around 18V which is plenty of headroom to get the efficency up above 90%.
Yep. That's the plan.

I just looked at buck/boost converters. Between National, TI, ADI, and LTI, only LTI has a buck/boost converter that can deliver more than 2.5 A. It's $9/each at Digikey and requires a bunch of external components. With the LM3102 I think the total parts budget will come in under $9 (plus the PCB of course).

So sticking with the plan. 12 V AC in, step-down to the filament voltage.

~Tom
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Old 26th May 2011, 07:52 PM   #6
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Tom, to get 4.2V for that Russian tubes I used the same 6.3V stabilizer and pair of 3 Ohm resistors, to each side of the filament of each output tube.
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Old 26th May 2011, 08:02 PM   #7
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Don't forget 1.5v for the 26. It's the input tube that more and more of us are using - has a particular magic.

Andy
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Old 26th May 2011, 09:01 PM   #8
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I had a similar plan as well. Except mine had modules using a flyback so the output is isolated from the source. Have them all be auto ranging 85-265 vac input and dip switch selectable for the output. Build an all DHT amp with these and only need a high voltage winding...
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Old 26th May 2011, 09:57 PM   #9
tomchr is offline tomchr  United States
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Originally Posted by andyjevans View Post
Don't forget 1.5v for the 26. It's the input tube that more and more of us are using - has a particular magic.
1.5 V @ 1.05 A. You got it. Thanks!

~Tom
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Old 26th May 2011, 10:01 PM   #10
tomchr is offline tomchr  United States
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Originally Posted by astouffer View Post
I had a similar plan as well. Except mine had modules using a flyback so the output is isolated from the source. Have them all be auto ranging 85-265 vac input and dip switch selectable for the output. Build an all DHT amp with these and only need a high voltage winding...
I built a project like that for my final project in college. Except, I wanted +/-45 and +/-60 V out for a class AB sand power amp. It worked pretty well.

But the more I look at mains supplied switchmode, the more I realize that sourcing the magnetic components is a royal pain. Single SMD inductors used in the low voltage regulators are plentiful - and fairly inexpensive, but transformer cores and bobbins needed for the high-voltage supplies are hard to come by. They were hard to come by in the 1990'ies, but now even more so.

This filament regulator will not use custom parts. Just off-the-shelf stuff.

~Tom
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