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Old 13th November 2006, 08:42 PM   #1
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Default UL Feedback Tap Calculation

What's the design criteria for determining the feedback percentage on transformers with a UL tap?
For example, most transformers use 33% or 40% which is easy for the winder, but what's preventing someone from winding a 20% or 50% tap? How would the sound be affected?

Feedback welcome.

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Old 13th November 2006, 09:27 PM   #2
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Some OPTs do have a 20% tap for more power. Operation here is more pentode like, higher output impedance as there is less feedback.

50% is more triode like, but your entering an area of diminishing returns. Z-out doesnt improve much until your getting near full triode. The EL34 datasheet offers a comparison for this tube for distributed load operation at 20% and at 40% tappings.

Ultra-linear Circuit Characteristics:

The following graph, courtesy of GEC and AWV Radiotronics Magazine (May 1959), shows the comparative characteristics of the KT88 in triode, pentode and ultra-linear connections. These comparative relationships between the different connection configurations should be typical for most (but not all) tetrodes, pentodes and beam power tubes.

Click the image to open in full size.
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