Question about inductors in power supply

[John Fisher]

I was just wondering. Why is it necessary to have the big heavy inductor as a filter in the tube amp power supply when you can just use capacitors, (It seems?).
Is there some real advantage to it. I have seen some designs without it.
Thanks!

[defaced]

This is called a Pi filter, but here's Wiki entry on it. Basically, an inductor filters AC current because it resists the change in current, while the capacitors filter AC voltage. The combination makes an efficient filter. Capacitor-input filter - Wikipedia, the free encyclopedia

Edit: more to read; http://www.aikenamps.com/Chokes.html

[John Fisher]

Thanks for your input. I do understand the principals involved but I guess my question was more like that now a days it seems they make good capacitor smaller then the old ones and isn't it more practical with weight and space and cost to just use caps to get the same results?

[defaced]

There are MFGs out there who do this. Peavey is one I know of that has been doing it since before I was born. It's done by using a resistor instead of an inductor.
Does it work? Yep.
Does it work as well...? Probably not.
But does it work well enough? Seems to for what we're doing. I never heard a difference, but I also didn't sit there with a scope and have my golden ears tuned to the phase of the moon and do a proper AB test.

[Enzo]

I find the term "necesary" interesting in your question. Frankly I think inductors are not all that common. Not rare certainly, but many many amps do not bother with them.

[octal]

Well, all things being equal, an inductor based power supply will have less screen sag than one with resistive filtering. (I'm talking about the traditional inductor between the first and second nodes of the power supply.)

Nathan

[Steve Conner]

Thanks for your input. I do understand the principals involved but I guess my question was more like that now a days it seems they make good capacitor smaller then the old ones and isn't it more practical with weight and space and cost to just use caps to get the same results?

Yes. It's also more practical to use transistors to get the same result as tubes. The inductor is a throwback to the old days when large value e-caps weren't available.

Geek corner: The CLC filter actually smooths better than a C or CRC. It attenuates the ripple at 12dB/octave instead of 6. So, you can have low ripple with a small first filter capacitor, and that lets you use a smaller mains transformer because the RMS-to-average ratio is more favourable.

But then, you had to use extra iron and copper to make the inductor, so that outweighs the smaller mains transformer.

[Wilder Amplification]

Yes. It's also more practical to use transistors to get the same result as tubes. The inductor is a throwback to the old days when large value e-caps weren't available.

Geek corner: The CLC filter actually smooths better than a C or CRC. It attenuates the ripple at 12dB/octave instead of 6. So, you can have low ripple with a small first filter capacitor, and that lets you use a smaller mains transformer because the RMS-to-average ratio is more favourable.

But then, you had to use extra iron and copper to make the inductor, so that outweighs the smaller mains transformer.

Yeah but Steve..come on now...what kind of filtering were all the rock greats using all those years?

[Steve Conner]

Sure, but only on the screens

[imaradiostar]

Let's also be clear- in modern designs with inexpensive capacitors there is no reason why you can't increase the value of the second cap in a CRC filter and achieve largely the same result. If it's good enough for Hiwatt it's good enough for me!

Also- this allows you to use the resistor between the screen and plate nodes as a tuning element for the amp. I've enjoyed switching this value around for different sounds.

jamie

[Gtr_tech]

Don't forget about a tube regulated screen supply. They work well and there's even a circuit in the last RCA tube manual in the applications section that uses such an arrangement.

[Steve Conner]

Good call, except these days you can just substitute a MOSFET for the tube.

You can also use a tube or MOSFET as a "capacitance multiplier", which makes a small cap look like a gigantic one as far as ripple reduction is concerned. This would be the way to go if you don't like regulation. Some people prefer the amp to sag and squish when overdriven. I once built an amp with fully regulated plate and screen supplies, out of curiosity to see what it would sound like, and I guess I can see where they're coming from.

And once you start playing with active devices in your screen supply, extra bonus, you can add current limiting to save your tubes from an open load! Maybe with the tube option, a simple way to do that would be to use an undersized tube: a 12AU7, 6AQ5 or whatever.