Power Supply Inductor per plate, stupid?

[Ripthorn]

Hi guys, I am working on a submini tube amp and am working on the power supply. I downloaded Duncan's power supply design program and have been toying with it. I found that a pi network with a 68mH inductor and 100uF caps. I found some 68mH inductors for cheap but they only handle about 20mA. This would be enough for about two preamp plates per inductor, but I was wondering if it is a valid idea or if it is just stupid. Also, I was wondering if it would be more worth it to get a single large choke. I just am not a huge fan of having a whole ton of things mounted on the chassis, but I also feel that performance outweighs everything except for extreme cost. Any thoughts or ideas?

[xk49w]

I found that a pi network with a 68mH inductor and 100uF caps.

Will do what? (And what are you trying to do?)

[Ripthorn]

Sorry about that incomplete sentence, my one and a half year old required attention and when I came back, I forgot to finish. The pi network will reduce the ripple to a low level. I want to get a very smooth plate voltage because I am doing some very high gain stuff and my breadboarded circuit is louder than all getout (I understand some is the breadboard, but I want to make sure that my power supply is very clean). If anyone has a proven power supply filter design for high gain, I would appreciate it. Thanks.

[mojo2001]

A 68 mH inductor is a very low value of inductance. This is more of an RF choke to filter out high frequency garbage.

You need to be looking in the H range...5-10 Henries is more like it.

Hammond makes attractively priced filter inductors that will get you where you want to be.

[Ripthorn]

Then I guess I will be looking for something bigger, I was just going off of what I saw in the Duncan power supply program. Thanks for the input.

[mojo2001]

Think of it this way: A larger coupling cap passes lower frequencies, right? Well, a larger inductor BLOCKS lower frequencies.

Something in the 68mh range will not even be in the way at 120hz, which is what we are seeking to block in a power supply filter.

[Ripthorn]

Well, I found a 10H inductor for a good price, but they have it available with 90mA max current or 50mA max current. The tubes that I will be using list a "typical operation plate current" of about 6.5mA. I assume that is per plate, so for my 5 gain stages that would be just over 30mA. Do I need to worry about inrush current or other high current demand situations? I see a lot of amps that have chokes of about 50mA rating, but they aren't high gain like this. I guess if all else fails I can just put the inductor on the first three or four gain stages to get rid of the mass amplification of the 120Hz stuff, no? Anyway, I would appreciate an opinion as to the current rating I should get (I want to place the order here as soon as I can). Thanks.

[mojo2001]

Sounds like plenty (50mA rating) if you're only running 35mA. Inrush current is not an issue. In fact, the resistance of the choke will limit the voltage on inrush.

The other consideration in an inductor spec is DC resistance, lower is better, particularly if you don't want to throw away volts across the choke.

I'd say a typical DCR for a 10H 50-75Ma choke would be on the order of 150-200 ohms.

Higher current chokes will usually have lower DCR because they use heavier wire. Low DC resistance chokes can get big and heavy for the same reason (big core, thick wire).

Typically, you will get more inductance out of a choke with less current flowing through it, so a 10H@50 Ma choke might have, say, 13 H@35mA.

Basically filtering is determined by LC product (L x C). A microfarad is a lot cheaper than a Henry, so if you are using big caps, a small value choke will multiply that C likely enough to make you happy.

A 10H filter choke is a fairly sizable inductance, by the way. That is, you are not skimping on the inductance!

There are some musings on filter chokes. Hope it helps.

mojo2001

[Ripthorn]

Well, then I think I will get the 50mA one and after doing some more with the power supply designer, I have think I have found my winner. Though I do have one question, is the plate resistance what I would use to calculate out the power supply load resistance or is it plate resistance in series with the plate resistor or what? Luckily I have a few volts that I can spare for a larger DCR with my choke.

[mojo2001]

Power supply load resistance is calculated by Ohms Law.

E= IR

R=E/I

You know the voltage and the current, so solve for R.

Here's calculator if you're lazy:
http://ourworld.compuserve.com/homepage ... hmslaw.htm

It is like determining what value resistor will provide a given current at a given voltage... nothing to do with plate resistance or the DC resistance of power supply components.