Ultra Linear OT

[C Moore]

My friend has a 135 watt Twin. I was looking at the back panel and under the speaker jacks it says "Minimum Load 4 Ohms".
Does that imply it would be OK to use a 8 or 16 ohm load? It almost sounds like the wording for a SS amp. Are UL transformers different than a "normal" output transformer in that regard?
Thank You

[David Root]

UL operation is effectively between pentode and triode modes. The knee in a pentode plate curve is kind of smeared out a bit in the UL plate curve because UL operation results in some internal plate-screen feedback that moves it closer to triode mode. Triode curves don't have knees....

Generally UL operates a bit differently on the OT primary impedance than pentode operation, but because the knee is elongated you can move the operating point around a bit more. I may be wrong but I think this allows a similarly less critical operation on the OT secondary side.

Anyone who knows the math involved please jump in.

[C Moore]

Hey David -
I meant to say thanks for the info.....
Anybody else have an opinion.?
Thanks