Measuring Choke Henries

[plexi50]

Is there a way to measure the henries of a choke? I think i may be using a choke that is the wrong value. I need one between 8 and 10 henries
Im reading 41.1 Ohms if that helps....

[FYL]

Is there a way to measure the henries of a choke?

Get a small dedicated LCR meter. You can find entry level models for around USD 25.

[plexi50]

Will do. Thanks. FYL

[Bill McKenna]

FYL,
I'm not seeing any LCR meters online for less than about $200 BK Precision makes a nice one. Once you have something like this what is the process to find Henries for an Inductor or choke? Its not really clear from the manual I read.

Something like this seems like a good unit
http://www.bkprecision.com/www/np_specs.asp?m=875B

I did find this explanation but it makes my head hurt
http://engr.nmsu.edu/~etti/fall96/electronics/induct/induct.html

Thanks
Bill

[texasrockguy]

http://cgi.ebay.com/Digital-LCR-Capacitance-Resistance-Meter-Tester-NEW-AR_W0QQitemZ260205860420QQihZ016QQcategoryZ25421QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
these work. i own one and its not bad.

[plexi50]

I bought this today. It's a cheapy but will give me a clue at least to some range even if not perfectly accurate. Plus it measures caps. Cant beat that price with a dead EH tube......

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=160201259603

[texasrockguy]

dont forget to turn it off! you will have a dead 9v in the morning for sure. no auto shutoff is really my only complaint with those things but for 27 bucks i guess you dont get the world. :)

[FYL]

I bought this today. It's a cheapy but will give me a clue at least to some range even if not perfectly accurate. Plus it measures caps.

Don't worry: this kind of meter is përfectly adequate for most bench applications and accurate enough for most jobs. You don't really care about 5-digit multi frequency measurements et al.

[FYL]

dont forget to turn it off!

Yup, excellent suggestion.

[FYL]

BK Precision makes a nice one.

Their 875 is quite nice, but at app. USD200 a Tecpel LCR-612 'sold under various brands) is IMO a much better choice.

http://www.tecpel.net/LCR-612.html

Once you have something like this what is the process to find Henries for an Inductor or choke? Its not really clear from the manual I read.

You basically measure them just like resistors... Just pick 120 Hz if they are swinging/smoothing chokes, 1 KHz if AF filtering.

[Bill McKenna]

THanks FYL
I will have to get one of these I'm sure if I ever expect to put the Standel together. I have to find 2 chokes
1.    1.5h 120Ma 90 ohms
2.    15h   70Ma  90 ohms

May end up with a few spares at the rate I'm bidding on Junk HiFi gear on Ebay :)

Thanks
Bill

[FYL]

I have to find 2 chokes

If your SCH schemo is correct the amp uses a swinging choke just as Williamson did. A swinging choke always shows a low R but a high L at low currents and a comparatively lower L at high currents thus offering a degree of regulation.

Standard smoothing chokes shouldn't be used there: they'll vibrate, hum and overheat in high current situations.

[PRR]

> I think i may be using a choke that is the wrong value.

Why do you think that?

If the rectifier feeds directly to the first cap, the main function of the choke is to reduce ripple for the stages which are fed through it. If buzz is low, and voltage drop is not excessive, the choke is fine.

The $29 meter will read the zero-DC small-AC inductance. Or you could series the choke with a resistor across a 6VAC 60Hz winding, and find the resistor which gives 3VAC. Inductance at operating DC current and ripple voltage may be more or less.

If you have a 'scope, and know how to use it around large DC without killing the input, you can measure ripple before and after the choke, check the capacitor after it, and compute the inductance which must be happening in actual operation.

If you are super-geek, you apply a switched transient load and watch the recovery bobble. This bobble may affect guitar tone. Much testing is needed to see it it really matters or if "choke sound" is really something else. The get-er-done approach would be to tack many different chokes in and try.


If the choke is between the rectifier and the first cap, this is a VERY different thing. There is a minimum inductance for low current, else the output soars. Less inductance is needed to stay choked for high current, though large inductance is needed for low ripple at high current. If ripple is allowed to be high, such as speech-only systems, the "swinging choke" is applicable. 99% of guitar amps don't do it this way. The choke-input Hammond chassis we saw chopped recently might have used a swinger, but probably didn't (by the time the rectifier warmed, total current draw was so nearly constant that there was no swinging going on). Class cool-AB/B speech amps with choke-input rectification are the primary use for swinging chokes. Very important to radio-hams, not to people who carry gear from bar to bar. Hi-Fi amps sometimes ran semi-swinging chokes for low hum at idle with minimum cost, accepting that high ripple on rare peaks may go un-noticed. But guitar-amp weight+cost issues have generally (post 1935) favored large caps over heavy chokes.

[Bill McKenna]

Thanks FYL and PRR

Interesting but confusing discussion to a Newbee,

In the case of the Standel project I'm struggling with, I'm clearly very
far from Williamson at this point if I believe the schematics.
I wonder how much stock I can even put on the statements we have read about Williamson being the basis for Standel

I have the first Choke driving right off the 5U4 with a 300 ohm and a 20uf cap tied to ground on the input leg and the output going right to the center tap of the output transformer center tap.
This looks similar to a Fender style choke to center tap of the OT. Except they have 2 70uf caps with some 220 ohm resistors too.  
This is the 1.5H 120ma 90 ohm choke in the .sch.

The other choke is feeding the screen of the 807 and I haven't seen anything similar in another sch. at 15H 70ma 90 ohms I assume this will be a big chunk of iron, that I won't be finding in the usual places.
but I need to look before I say that for sure :)

I still don't quite understand all the behavior and choke lingo. Need to continue to read up and learn I guess.

Thanks again for the education
Bill

[FYL]

I have the first Choke driving right off the 5U4 with a 300 ohm and a 20uf cap tied to ground on the input leg and the output going right to the center tap of the output transformer center tap.
This looks similar to a Fender style choke to center tap of the OT. Except they have 2 70uf caps with some 220 ohm resistors too.  

Not at all: the Standel has the choke and R in series, with the R having a major impact on this stage voltage and current-wise. Minimum DCR of this stage is 300+ ohms. The choke sees app. 100V variations and should be a swinging choke for minimum noise.

A typical Fender PS uses two caps in series for higher voltage handling (two 70µF/350V in series behave like a 35µF/700V cap), with a couple of 220K R's in parallel in order to balance currents and bleed the caps when the amp is off. Minimum DCR is app. 1 ohm, 300 times less than the Strandel PS...

The other choke is feeding the screen of the 807 and I haven't seen anything similar in another sch. at 15H 70ma 90 ohms I assume this will be a big chunk of iron, that I won't be finding in the usual places.
but I need to look before I say that for sure :)

Heyboer and other manufacturers can wind custom chokes for your project. All off the shelf 15H models I'm aware of show 3x to 4x the spec'ed series R.

I still don't quite understand all the behavior and choke lingo. Need to continue to read up and learn I guess.

May I suggest that you download a copy of Duncan's PSUII and run simulations with it? Things will get much clearer after a while.

http://www.duncanamps.com/psud2/index.html

[Bill McKenna]

Thanks FYL,
I will play around with the simulator and see if I can figure out what you
are trying to tell me. I don't know much and that last post was mostly greek :)

THanks
Bill