Choke tutorial

[phsyconoodler]

What does the 'henries' rating on a choke do?For example,on a Fender amp they often have 4H/90ma chokes and on a Marshall they may have a 40H/50ma choke.
  Does anyone understand this and what the 'Henries' do for tone?
I know it's about inductance,but that's about as far as my knowledge takes me.
  I have a choke that is rated at 10 amps and 1millihenries & 450v.Would it be ok to use in a guitar amp?

[Geezer]

You could start here (whilst waiting for someone "in-the-know" here to answer)

http://www.aikenamps.com/Chokes.html

[Fresh_Start]

Playing with Duncan's PSU, doubling the Henrie value of a choke in a C-L-C filter seems to cut voltage ripple roughly in half.  Based on that, my guess is that inductance in the milliHenrie range won't be useful in a guitar amp.

Chokes also have a resistance rating.  In the choke you used for the Tweed Princeton Reverb, the resistance was rated at something like 200 or 300 ohms.  Again a guess, but a 10 amp choke might have a MUCH higher internal resistance.

HTH

Chip

[eleventeen]

I'm holding here one of two fattish NOS never installed GE 10 Henry chokes I got from the garage of doom. I measured DC res and got 187 ohms and 196 ohms (the connection terminals are a tad corroded so the res readings could be a tad flukey)

They are rated 10 henries and 100 mils. However, as the wire coming out of the insides is minimally 18 ga, I hardly suspect 100 mils exceeds any  kind of current carrying capability.

I have seen inexpensive handheld meters that measure henries of an inductor. My Fluke 77 doesn't.

Why could not an otherwise completely useless part, like a tube-TV vertical output transformer, be used as a choke?

I would think that a larger choke, wound with larger gauge wire, would have *lower* DC resistance. But I've never tested that thesis.

[PRR]

> 10 amps and 1millihenries & 450v.Would it be ok to use in a guitar amp?

In audio-tube-world, 1mH is about as good as a scrap of wire, but less convenient.

Choke impedance is (ideally) Henries times Frequency times a 2pi to make Hz into radians (don't ask). 2Pi is equal to 6 for most purposes.

We have wall-hum at 50/60Hz, guitar low-note at 82Hz, wall-buzz at 100/120Hz and higher. Take "100Hz" to make the figuring easy.

1 Henry at 100Hz is 1*100*6 = 600 ohms.

1 milliHenry at 100Hz is 0.001*100*6 = 0.6 ohms.

10 Henry at 100Hz is 10*100*6 = 6,000 ohms.

Tube impedances are generally 1K to 100K. In a 1K-100K world, we expect 600 ohms stuck-in to change the situation slightly, 0.6 Ohms to make hardly any difference at all, 6,000 Ohms to actually Do Something.

Indeed 10H is the inductance of a good SE OT or power fiter choke, and 1mH chokes don't appear around tubes until we get to radio frequencies (0.001H*10MHz*6= 6,000 ohms, and 1mH chokes are used as plate-loads in SW transmitters).

1mH is also useful in the car. Sound system pulls 12V and 8A, a 1.5 ohm load. Alternator whine is up above 200Hz. 0.001*200*6 = 1.2 ohms at 200Hz, 2.4 ohms at 400Hz, 12 ohms at 2KHz, etc. Relative to the nominal 12V 8A 1.5 ohms, 200Hz whine is less and higher whine is much less.

> what the 'Henries' do for tone?

The general idea in power supplies is to filter hum/buzz. More is better but costs more and has more DC stray loss. Fender 4H versus Marshall 40H is probably about what each guy thought best and what price he could get it at. 

The choke, like a cap, is also a storage tank. The combination of choke and cap is a bouncy storage tank. Computations are possible, but you may not be to that stage yet. And knowing numbers does not tell "tone". Cheap enough to try.

[jjasilli]

I have seen inexpensive handheld meters that measure henries of an inductor. My Fluke 77 doesn't.  Some chokes like for RF are tiny, handle little cuerrent, and can be measured on a small meter.  But filter chokes need to be measured under lad, with current flowing.  Such meters are expensive, but there are inexpensive DIY solutions.  Search this Forum or use google.

Why could not an otherwise completely useless part, like a tube-TV vertical output transformer, be used as a choke? People do this.

I would think that a larger choke, wound with larger gauge wire, would have *lower* DC resistance. But I've never tested that thesis.
It's a "dance" with formulas to follow for wire guage, # of windings; and air core, iron core or powdered core.  These build factors contribute to inductance, DC resistance, current and voltage handling.  Us end users don't really need to know these design parameters.  Your choke looks about the right size for 100mA, especially an enlcosed one, maybe potted?  Agreed that the fat leads do seem like overkill.  Seems good PS in a stereo 20W amp!

[jojokeo]

All things being equal more henries is better up to a point, so you can get more/better filtering of the DC. But, I believe that if you go too high it could stifle your tone. It's trying to resist a change in voltage also besides minimizing hum as PRR says. Inductance also changes continually under operation like a speaker too. I tested once in a SE amp playing w/ a power resistor using a test lead, and switching on the fly to the choke - back and forth and you can tell the difference in characteristics between the two, however it was slight. The distiortion seemed a bit smoother is one thing I remember. I would have to search back a year or two to find the thread.

[tubeswell]

 I have a choke that is rated at 10 amps and 1millihenries & 450v.Would it be ok to use in a guitar amp?

From Merlin Blencowe's site here http://www.freewebs.com/valvewizard1/smoothing.html

"A rule of thumb is that the sum of the two capacitors (in micro-Farads), multiplied by the inductance of the choke (in Henrys) should equal 200 or more for excellent performance. For example, using a 22uF reservoir capacitor with a 47uF filter capacitor and a 5H choke:
(22 + 47) * 5 = 345"

Based on this, you would need really huge caps to make it work well enough with a 1mH choke.