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Thursday, December 19, 2013

High Voltage Rectification



High Voltage Rectification

Tube amplifier uses high voltage rectification. That’s something everybody knows, be it using silicon diode or tube rectifiers, all will need a power transformer with high voltage output. Such power transformers will have a lot of windings due to the step-up needed. It will be pretty difficult to squeeze a lot of wires into the transformer. 

Full wave rectification needs center tap and bridge or half wave rectification will need only 2 wires. So, there comes the consideration of whether to have a center tap or normal 2-wire output. Center tap will have more wires and the wires used in the transformer will have to be thinner compared to the 2-wire version. Things have just become harder for a center-tap type! 

Well, it is not hard to explain this as usually we would use a single rectifier tube to convert AC to DC. Using 2 rectifiers for bridge rectification is possible but is 2x the price of a single tube. Using half way is OK too but the efficiency is just too low. Therefore we resort to full wave and that requires center tap winding. 

It is OK to wind center tap windings. Usually experienced winders will be able to use the right size of wire to use for satisfactory results. The problem is for some lazy, inexperienced or simply ignorant ones will wind from start to end, and draw out a center tap during midway for EI core transformer that most of us use. If they did not calculate and lay properly, the center tap will not be centered, as the diameter for inner windings is smaller than outer windings. 

So, the inner portion will have lower resistance and therefore higher current, and vice versa. This will create imbalance in the current delivered to the filament of the rectifier tube, and cause one side of the filament to age faster and use up faster, where we are not really willing to see. Unless special bobbins are used and windings are balanced, if not, the above case shall happen. 

Well, if the above method is not use, there’s another way of doing it, where both taps of a center tap are wound at the same time, in parallel. Sounds like a brilliant idea, right? Well, wrong again! The voltage of the winding will be out of phase, so, at 1 point, the maximum voltage present between the winding will be the maximum potential difference. This will post as a great risk of breaking down the insulation and cause shorts! This is worse, if the proper steps of making a good transformer are not followed. 

Skimp on quality and you will pay for it eventually.

J&K Audio

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