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Friday, December 20, 2013

Plate choke

Plate Choke

If you look at my 2A3 / 6B4G design (provide link), it is using parafeed configuration with a plate choke. For those that are not aware, parafeed configuration will split the load and the impedance change to 2 components. The parafeed choke will act as a load for the tube, and the output transformer will act as the impedance changer from high to low. There are pros and cons on such configuration but I will not go deeper there since it is already mentioned in many websites. 

A lot of people asked me, what value of choke should I use on my design? Well, let’s try to solve it here in a simple way. I saw that there are complicated mathematical model out there too that simulate the interaction with the parafeed choke, parafeed capacitor and parafeed output transformer. By all means, you can use that to you advantage also. 

 Let’s take the 2A3 / 6B4G single ended triode design I had as an example and see if that make sense to you on how I choose the choke.

I used BCP-15 from Magnequest. It is 50mA, 40H and 550 Ohm DCR. 

For triode, the load can vary from 2 to 6 or 7x. The lower the load is, the higher the output power or distortion, and vice versa. 

Z (impedance) = 2 * pi * F * L 

Let’s take Z = 4x the tube impedance = 3200 Ohm.

L = Z / (2 * pi * F) = 3200 / (2 * 3.142 * 20) = 25H

Therefore, the BCP-15 is more than enough for the job.  Let’s see how much inductance we need to extend the low frequency response to 10 Hz. 

L = 3200 / (2 * 3.142 * 10) = 50H 

So, if you like more bass extension, get a bigger choke! BCP-15 is barely enough! If you choke is too low in value, the bass will be greatly impacted. 

If the plate choke is used in the driver/preamp stage, then it is advisable to go for higher impedance and therefore the inductance will be higher too for better linearity.
One of the shortcomings I see with plate choke is the matching to have identical left and right channel sound. It is not as easy as matching a load resistor. So, since we match tubes (where some feels that it is bogus), we must match chokes too and therefore, selecting a good vendor for the choke will be very important.

Another thing I see is that the impedance change of the choke over the audio spectrum will be critical too. We need the choke impedance to be constant, just like how we design the output transformer. If you can match your choke with respect to the audio spectrum using a LCR analyzer or audio analyzer, that will be the ideal case. I’m still not seeing it from the vendors that I know at the time of writing this article.

For the pros, you can read up yourself. LOL. 

J&K Audio Design

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