UL Feedback Tap Calculation

[korneluk]

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.

-- josé k.

[Tweeker]

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.