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24th May 2006, 04:46 AM  #1 
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Join Date: Jan 2006
Location: Kansas

Henrys
Anybody know a simple way to determine Henry value of a scavenged choke. The resistance is 38 ohms. Thanks, Jay
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24th May 2006, 06:10 AM  #2 
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Join Date: Jul 2004
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Hi Jay
If you have a low voltage AC supply (wall wart?), an AC voltmeter and don't mind doing a little math, try this. Connect a 1K resistor (R) in series with your choke and connect the AC suppply (12 volts should do) across the outer ends. Measure the voltage across the resistor (call it Vr) and across the inductor (call it Vz). The proportion of the two voltages will be the same as the proportion of R & Z (impedance of the choke). So Vr/Vz = R/Z. Transposing give Z = (Vz * R)/Vr Note that Z is a combination of the resistance and inductive reactance (Xl) of the choke but since the resistance is usually small compared to Xl for a choke of this kind, it can be ignored. So in this case we can say Xl = Z. Xl = 2* Pi*f*L where f is 60 Hertz, L is the inductance in Henrys and Pi is 3.14. Transposing gives L = Xl/2*Pi*f L = Xl/376.8 Hope this helps Cheers Rob 
24th May 2006, 12:03 PM  #4 
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Join Date: Jan 2006
Location: Kansas

Thank you both for the excellent replies. Rob, you should be a teacher if you are not. I normally have trouble with mathmatical equations, but you explained that one very well. Tom, great link, good to hear from you. I am still in the PS stage of your SE EL34. I am using iron scavenged from a big old organ amp for the PS. I am ending up with either too much (cap input~500) or to little (choke input=340). I have some other chokes to try as I don't want to deviate from the theory of high voltage and low current. I am expecting to receive the OPT's any day. I had some wound at Heyboer with 10k primary. Thanks Again for the very helpful replies, Jay
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24th May 2006, 02:06 PM  #5 
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Join Date: Mar 2006
Location: nsw

If you use the series resistor method, then Z_{t}=SQR(R^2+Z_{l}^2). The total impedance at the point where Z_{l}=R will be 1.414 x R.
Therefore, V_{r} will equal 0.707 x V_{in} at that point. 
24th May 2006, 08:06 PM  #6  
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Join Date: Jul 2005
Location: Leverkusen

Hi Jay,
Quote:
When using GZ34 with CLC: Use lowish first C (say, 10uF), this will be very gentle to the rectifier tube, then, after the CLC block, you could add another two (one for each channel!) RC combos with appropiate R value to drop the "rest" of excessive voltage. If possible, connect the positive connection of the last C directly at the B+ end tap of the OPT primary, and from there, the other end of the cap to gnd. There is no point of making those additional two Cs larger than, say, 68uF. Generally, this suggestion of a CLCRC filtering split for channels after the 2nd C will further improve on residual hum (not that it would be necessary...) due to the added filter stage(s), and will give better channel separation, too. The later is often overlooked in tubed stereo gear, and certainly is not very important for more or less constant current drawing class A SE stages, but ... why not getting some additional dBs of channel separation for "free" while "burning" excessive voltage ... I am glad you have chosen the high voltage / low current / high reflected load impedance path for trioded EL34, I am sure you won´t regret it Tom
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24th May 2006, 10:43 PM  #7  
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Quote:
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