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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