choke help

[ernie_jr]

Need some help finding a value on a choke I found. only number left on it is 4509C.
Any ideas would be great.
Thanks,
Ernie

[kagliostro]

Hi Ernie_jr

also I have some unknown choke

I've a cap meter with build in inductance meter but I can't use it because is only rated for low inductances

much less than those are of our interest

so I've find this method

http://et.nmsu.edu/~etti/fall96/electronics/induct/induct.html

not yet tested the method but I think it will be feasible

Kagliostro

[HotBluePlates]

What Kagliostro posted is the general idea.

Use a generator to apply a sine to the choke in series with a variable resistor. Pick pot/rheostat value and signal frequency to make sense together; if you expect ~2H, and have a 10k pot, a 2H choke would have a reactance of ~5k at ~400Hz.

Likely power supply choke values are 1-30H.

You'll have to sensibly guess the current rating. If it's a fairly small choke, and has a high inductance, the current rating is likely small. High-current chokes tend to be large (need a big core) and also lower inductance. For example, I have some chokes rated for 400-500mA at ~5H, and they're as big as a 60-80w output transformer.

Apply your signal to the rig described above. Adjust the rheostat until you get half the applied voltage across the rheostat. Turn the generator off and measure the resistance of the rheostat; the choke reactance is ~same. Use the equation L = 2*pi*f*R (L in henries, f in Hertz, R in ohms).

[sluckey]

Use the equation L = 2*pi*f*R (L in henries, f in Hertz, R in ohms).

I think the equation should be L = R/(2*pi*f).

[HotBluePlates]

Good catch!

I dunno what it is lately, I'm getting everything mixed up... 

[sluckey]

For the past month or so I've had keyboard dyslexia, switching letters around as I type common words. And it was repeatable! I don't mean just finger farts and oops. I mean "this and that" would become "htis and htat". And some of it would slip through proofreading too. I think I'm coming out of it now but I let a lot of that stuff get out in the wild! Google spell check has saved me a lot here at home, but I can't use it at work. Can't wait 'til I start drooling!   

[jeff]

That's great guys. I've a couple of chokes I was wondering if I could use.

My question is if you underestimate the mA the choke can handle and to blows will you damage the amp?

Basically that would kill the B+ to everything BUT the plates. What happens then?
Will you cause damage to something else: tubes, OT, etc.?
What happens to a power tube when you lose B+ on the screen?

[HotBluePlates]

It doesn't work that way, unless you put amperes through a choke with very fine wire rated for a couple-mA's.

Instead, what happens is the too-big current saturates the choke's core, and inductance drops to near-nothing. So, you'd likely get big hum.

Want a second-opinion? Use a shipping scale to weigh the choke. Compare to known ratings and weights for other chokes (for example, from Hammond's line). For example, Hammond's 158L, 158M and 158Q are all 1.25 lbs. However, the inductance ratings are 15H, 10H and 5H, respectively. Our rule was bigger current = bigger weight, for a given inductance. We have same weight, different inductance, so we could figure that the highest-inductance unit is the lowest current capacity. In fact, the chokes are rated for 75mA, 100mA and 150mA, respectively.

If I had all my stuff at-hand (instead of in-storage), I shoot a pic of a 50-75mA choke next to a 200-500mA choke. VERY big difference, and you're not likely to mistake them. Seriously, a 5H 500mA choke is a hefty hunk of iron.