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Old 24th May 2006, 03:46 AM   #1
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Default 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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Old 24th May 2006, 05:10 AM   #2
rpapps is offline rpapps  Antarctica
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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
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Old 24th May 2006, 05:51 AM   #3
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Have a look at this link and scroll down a bit (2nd figure and text below).

Tom
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Old 24th May 2006, 11:03 AM   #4
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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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Old 24th May 2006, 01:06 PM   #5
lndm is offline lndm  Australia
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If you use the series resistor method, then Zt=SQR(R^2+Zl^2). The total impedance at the point where Zl=R will be 1.414 x R.

Therefore, Vr will equal 0.707 x Vin at that point.
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Old 24th May 2006, 07:06 PM   #6
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Hi Jay,

Quote:
I am ending up with either too much (cap input~500) or to little (choke input=340)
Suggested option to consider:

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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Old 24th May 2006, 09:43 PM   #7
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Quote:
Originally posted by Tubes4e4
Hi Jay,



Suggested option to consider:

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
Great idea, my jazzed up 70 has seperates for each channel. I forgot to mention that I am using a pair (for looks not load) of Jan coke bottle 5R4's. I've been doing some modeling on the bench with a 140mA load. Just last night I got familiar enough with PSUDII to do it that way. This idea will give me some stuff to try. BTW I also found a set of Jan Sylvania 6SL7's. Thanks, Jay
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