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Bas Horneman 30th March 2011 05:27 PM

UL for EL84. 22-24% is better than the dogma....40% is lowest distortion....
In doing some research about output transformers for the EL84 I found this:

"Virtually all transformer manufacturers today think that "UL" operation is defined as 40%, so their entire line is often offered with that tap point, with little understanding as to how the tap %, the plate-to-plate load, and indeed the tube itself are all intertwined. The 40% point got it's start from the original UL experiments that were done on 6L6 class tubes, that respond most to the UL influence at 43.0% of the winding. But Hafler (and Keroes) quickly learned that the optimum tap % was not only tube specific, but also loading specific as well, such that the optimum tap % is hardly 40% for all tubes or conditions. Early on, much of the information was derived empirically, as was certainly the case for Hafler's original transformer, the A-430. This transformer was specifically designed for EL34 tubes, under conditions of maximum power output with minimum distortion. With a 4.3K primary, it employs taps at 33%. But raise the load to 6K plate to plate or greater, and the optimum point becomes 40% for these tubes. The difference is not insignificant. EL84 and 6V6 class tubes work best with a 25% tap. Using a 40% tap produces very compromised performance with these tubes. Overall power output suffers, and particularly so at the HF end of the scale. 6550s generally do like 40%. Real KT88 are good with anything from about 20% to 50%, because this tube was designed specifically with UL operation in mind, and the list goes on." :: View topic - ST-35 with Tango Output Transformers

Unfortunately the author speaks about power and not so much distortion.

Anyone care to comment on this?

SY 30th March 2011 05:46 PM

Mullard gives detailed performance data for both 20% and 43% in the EL84 datasheet. I wouldn't be surprised if they did the same for some of their other output tubes.

There's an excellent analysis of all of this in Menno Vanderveen's new book. Worth reading for sure.

Yvesm 30th March 2011 06:43 PM


Anyone care to comment on this?
UL: Urban Legend . . . beuark !

Compare drive requirement, output power and distortion.

My faith is to regulate the screen voltage.


Eli Duttman 31st March 2011 12:55 AM


My faith is to regulate the screen voltage.

If a tertiary screen grid winding on the O/P trafo is employed, both regulated g2 B+ and ultralinear, local, NFB are available. Vanderveen's writings cover the subject, to some degree.



Bas makes a good point about the UL %. The highly revered "iron" Freed wound for the H/K Cit. 2 is tapped at approx. 20%.

Yvesm 31st March 2011 07:34 AM

Yes ! I fully agree !
That's what H&K did in the very first published paper around an 807 IIRC . . .
But probably the cost of the OPT was too hi !

bigwill 31st March 2011 10:12 AM

I think pentode mode can offer the lowest distortion, provided you give the screens a good stiff supply. UL is good because you can get away with no dedicated screen supply at all.

Ian444 31st March 2011 12:17 PM

Seven working amps in this house, all pentode mode. Three stereo amps, four guitar amps. There is nothing wrong with pentode mode IMO, in fact I like it the best :D Some people like triode mode, or UL, and others must have varying degrees of global NFB, that's all fine, each to their own.

richwalters 31st March 2011 04:23 PM


Originally Posted by bigwill (
I think pentode mode can offer the lowest distortion, provided you give the screens a good stiff supply..

You recon?

Intermodulation thd performance is lowest with 40% UL and highest with true tetrode. SOmewhere there is a KT88 chart which confirms this.

UL in an MI amp is an expensive non-luxury. For HiFi, that's different. With a decent amount of global nfb, should snuff UL 40% operation IM THD to well below 1% and harmonics F2,3 to a tenth below this. Triode operation has a slightly higher IM THD compared to UL 40%.
So UL with 40% tap offers the lowest IM THD so it claims the quality king. Debatable ??


bigwill 31st March 2011 06:32 PM

The way I see it, is that the purest way to linearise the output stage is actually global feedback, and anything that happens before than is convoluted, such as triode or UL mode. It's better to subtract the error at the beginning than try to do it with the screen grids (imo)

jcx 31st March 2011 07:31 PM

pushing the gain to the outer loop can be shown to be optimum for distortion reduction with non-interacting gain stages
many may not appreciate the extra linearizing consequence of added gain at the output stage - the signal level at all previous stages is reduced for the same output so the preceeding stages give lower distortion due to the lower signal/bias ratio

tube amps can be limited by output xfmr bandwidth so you may not be able to stablize the amp with higher global feedback

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