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Need advice on proper use of anode chokes.
Need advice on proper use of anode chokes.
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Old 4th September 2008, 10:59 AM   #1
pietjers is offline pietjers  Netherlands
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Default Need advice on proper use of anode chokes.

Could anyone give some additional information regarding proper use of anode chokes. Or is there an article on the net with more in depth information on the use of these chokes.

Yesterday I received a pair Lundahl LL1668 chokes I ordered (the 25mA 200H version, Rdc = 330 Ohm). Did some quick testing in a 5965 grounded cathode stage and found out that it is difficult to obtain a reasonably flat frequency response. Also noticed a dip of around 2.5 dB at 10.5 kHz. Square wave reproduction is really horrible, with clearly visible resonances in the response. Could it be that (miller) capacitances play a big role in getting a flat frequency response?

(The 5965 tubes I use have a mu of about 60, Rp is about 4k, used Rc = 120 Ohm bypassed with 470uF , Vb = 120V)

Peter
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Old 4th September 2008, 02:19 PM   #2
Shoog is offline Shoog
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Have you tried reversing the connections to the choke. You may be experiencing a resonant tank between leakage capacitance to the core and the inductance. This parasitic is generally worse in one direction rather than the other.

Shoog
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Old 5th September 2008, 07:50 AM   #3
pietjers is offline pietjers  Netherlands
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Shoog,

I connected the choke as described in the Lundahl datasheet. But I will reverse the connections and see what happens. Didnít ground the chassis of the choke, that could be important too.

My intention is to use the choke in a mu follower stage. Tried it first in a common cathode amplifier more out of curiosity and because it is so easy to set up.

Because I was very curious if an anode choke would better the sound of such a circuit I gave it a try in my power amplifier yesterday. This power amplifier has a 6N30 mu follower gain stage that runs at 30 mA. Unfortunately above the maximum 25mA Lundahl states for the LL1668 in the 2 x 100H version. Tried it nevertheless. I connected the chokes (one LL1668, one coil for each channel) in series with the resistors between the upper and lower triodes. This is an interesting position because the coils reactance is amplified roughly by a factor mu of the triode. Tried it back and forth several times with various CDís.
The first thing I noticed with the choke in place is that the soundstage is more open and less between the loudspeakers. Everything sounds much more relaxed, with a real black background and a very sweet and natural reproduction of instruments and voices. Recommended!

Peter
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Old 5th September 2008, 08:33 AM   #4
Shoog is offline Shoog
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My experience has been that adding any form of inductor to the circuit brings magic to the sound - just as you have described.
Unfortunately these chokes are complex things and interact with circuit and parasitic capacitances. It is essential to use a scope to debug them and sometimes it takes a lot of trial and error to fault find ringing and the resultant harmonic and intermodulation distortions. If you get it wrong then the sound is inevitably edgy and fatiguing.

Shoog
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