It is proabably easiest to understand the merits of Parallel-Feed (PF) by understanding Direct Feed (DF). Direct Feed is when the DC current that biases the output tube runs through the output transformer. This is true for both Single-Ended and Push-Pull. When the designer of the output transformer designs a DF output this DC current is a hamering variable that makes the design complex, especially for Single-Ended designs.
To get good response from the output transformer, the designed of DF transformers must make acceptable inductance in the output transformer to load the the output tube. This makes the design complex because the amount of inductance required is a squared property of the amount of DC current, meaning that the transfomer for a power of transformer X and one that has twice as much power could be 4 times larger. This inductance also works against the designer for top end preformance. The more inductance the more capacitence which kills the top end. The designer must make choices as to which variables they choose to get the best sound possible.
What Parallel-Feed does is seperate the output transformer up into it's basic pieces. The choke can be designed seperately from the output transformer. Coupling the two via a the PF Capacitor. The designer can now make each piece individually and optimize each part. The benefits from the PF design are more power and lower noise as well as better frequency response. The drawbacks are more parts, higher expense and that coupling capacitor.
So why is it that I do not use PF on higher output amplifiers. Well... diminishing returns, see this technology works great on amplifiers up too 8W and above 16W's but between that it is easier to design using Direct Feed. The capacitor required get's larger in the area between 8-16W and the choke does as well as the output. Requiring a much larger heavier and not so much better amplifier. For amplifiers above 16W's PF is better because the designer can make a better fullrange output, choke and capacitor combination.
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