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43% UL connections

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43% UL connectons

Hey

Now I have studied some schematics showing SE amps in UL coupling.

I do not understand why the UL connection transformer is almost always 43%.
I have some transformers with 25% UL connections.
What has to change at the schematic to use 25% instead of 43% and what is the theory behind it.

Regards

Benny
 
Re: 43% UL connectons

Hi Benny

Now I have studied some schematics showing SE amps in UL coupling.
I do not understand why the UL connection transformer is almost always 43%.
I have some transformers with 25% UL connections.

Don´t get fooled by seemingly "magic" numbers. For instance, if an interleaved transformer is wound, those 43% just come out easily by leading out a tap at the end of the 3rd out of 7 primary winding packets. For 25%, the tap is just at the end of the 1st out of 4 primary winding packets - always counting from the B+ connection to the anode connection.

What has to change at the schematic to use 25% instead of 43% and what is the theory behind it.

I´ll start the other way round. For a pentode or BPT in UL connection, upping the winding percentage for the UL tap means (everything else unchanged) more triode like behaviour (100% UL means just a triode-strapped pentode or BPT),
--> less sensitivity,
--> less Po,
--> less THD,
--> less Zo (hence higher DF)
--> higher input capacitance

So, in your case going from 43% to 25% essentially means:

--> power stage is easier to drive due to higher sensitivity and less CMiller,
--> higher Po and THD
--> higher Zout, hence lower DF

Depending on the actual topology, you can derive actual measures to be taken - according to your preferences and goals) to deal with those changes listed above.

If it is a 3-staged design using considerable ammounts of global loop FB, quite likely you will have to fiddle with the time constants in the amplifier path as well as the feedback path to maintain unconditional stability at complex loads like realworld speakers.

Regards,

Tom Schlangen
 
Think of it this way: triode would be 100%, pentode would be 0%. UL is somewhere between. (Oversimplification alert) The closer to triode, the lower the power, distortion, and output impedance. The closer to pentode, the higher the power, but with higher distortion and output impedance. So a 25% tapping will give more power, but a 43% tapping will give lower distortion.

It's a bit more complicated than this, but this is close.
 
25% tap runs the screen grid closer to tetrode configuration i.e less AC anode to screen feeedback, so a higher output power is obtainable at higher thd. The 43% tap is closer to running the screen grid to the anode i.e triode performances so less distortion is produced for a lower power. The compromise performance is intermodulation thd being highest in true tetrode configuration 20%, and optimum min at 43%. The output impedance changes with the amount of UL (screen grid/anode)negative feedback.

It is entirely the users choice. I tried 20% taps with ECL82 and found thd too high, so I settled for 40%.
Want reckless power i.e don't care with thd then either choose true pentode or 20% UL. Most tube data sheets do specify impedances for proper output tranny impedance matching. The Radiotron hbk goes into more detail.
Study GEC KT88 data sheets at http:/frank.pocnet.net/

The only change at the schematic is addition of screen grid resistor and perhaps global nfb value.

The article you need to read is the GEC application report No3 GEC amplifier circuits for KT88. dated June 1972. Unfortunately I cannot dive further into this topic as I believe is now copyright protected.

richy
 
There has been some confusion over the years between % of turns and % of impedance. They don't match but both ways of talking about UL connections have been used.

I think, from talking to a transformer winder, that 20% turns equals 43% impedance, but I'm a bit hazy on exactly how that went and I'm hungover, so don't quote me.

Have a read of this, for an in depth analysis of why various UL tappings are chosen.
 
Hi TheVoice,

talking about impedance doesn´t make much sense in this case, because we are talking about screen feedback (=UL) ratios, which is defined by the voltage swing ratio (=wire turns ratio) between B+ tap, anode tap and screen tap:

UL% = ((number of turns between B+ tap and screen tap) / (number of turns between B+ tap and anode tap)) * 100

or,

UL% = (Vpp,screen / Vpp,anode) * 100

Regards,

Tom
 
Benny,

The other posters have summed things up well. A smaller UL % means more pentode like behavior.

The H/K Cit. 2 uses 20% UL taps on the O/P trafos. By way of compensation, the "Duece" employs 3 nested NFB loops. The amount of NFB in the outer, global, loop is surprisingly small. The design is (sic) unconditionally stable. I suggest you follow a similar path, with only a few dB. of NFB in an outer, global, loop and some sort of inner, partial, NFB setup.
 
Dav,

I recall seeing a plot of distortion vs UL output tap percentage. I think it was in:

Vacuum Tube Amplifiers; Valley & Wallman; McGraw-Hill (MIT Radiation Labs. Series); 1948

but it could have been in RDH4... I'll see if I can dig it up. Basically, with a screen tap with 40-50 % the number of turns of the anode tap, you get the best power/distortion trade-off.

~Tom
 
Ex-Moderator
Joined 2003
Here's a plot for KT66 taken from page 52 of "High Fidelity Sound Reproduction," E Molloy (ed.), Newnes, 1958. Beware that although it shows minimum distortion at about 20%, that just means it gave a minimum reading on an (unspecified) total harmonic distortion meter and says nothing about the relative proportions of 2nd versus 3rd harmonic. And don't forget that very few loudspeakers present a resistive load, so many of these "optimum" configurations (which usually rely on cancellation) are pretty tenuous anyway. Realistically, the generalities offered by SY earlier are as good as it gets.
 

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Eli Duttman said:
Benny,

By way of compensation, the "Duece" employs 3 nested NFB loops. The amount of NFB in the outer, global, loop is surprisingly small. The design is (sic) unconditionally stable. I suggest you follow a similar path, with only a few dB. of NFB in an outer, global, loop and some sort of inner, partial, NFB setup.


Eli,: I would go further. UL stuff ain't magic and it ain't easy. Til one fully understands the interaction of all three feedback loops with one another, and snubbing properties with UL transformers, once all mastered, one is King.

You mention unconditionally stable; I presume that includes s/c and open circuit operation. This is nasty. Was there a US classification on this ? In the UK unconditionally stable was defined under obsolete BS3860.


richy
 
Tubes4e4 said:

talking about impedance doesn´t make much sense in this case, because we are talking about screen feedback (=UL) ratios, which is defined by the voltage swing ratio (=wire turns ratio) between B+ tap, anode tap and screen tap:

Hi Tom,

That's true, but an old winder here that I used a few years ago asked me to clarify which I was talking about when I asked for trannies with 43% UL tappings.

He reckoned that back when he did a lot of winding for tube amps (he has worked from the late 40's onwards, mostly retired now) it paid to ask which ratio was being talked about, as he recalled both impedance and turns being specified.

Anyway, my now somewhat laboured point, is that just because we are talking about the turns ratio doesn't mean the transformer maker wasn't talking about impedance when they stamped the specs on the trannies.

Of course, this could be an old guy who didn't remember properly, or a local thing here.
 
Hi TheVoice,

one can state "UL ratios" by impedance ratios, indeed, but that is uneccessarily complicated because it will require computing the square root to get the winding ratio.

But okay, granted, it will take your old winder maybe only a second or two to compute that using his ancient, but fast and more than sufficiently precise slide rule :)

Likely that man was much more often confronted with impedance figures than voltage ratios from his clients, so it makes full sense he asked to make sure what really was meant.

Tom
 
I believe the '43%' standard is a relic Hafler's famous articles on UL using KT88s, that being the tapping which gave the best performance with that particular valve.

Other valve types will have different 'ideal tapping percentages', depending on the cathode-screen-anode spacing. I think the EL34 is around 20%-ish.

The 43% thing has stuck though, since so many amatuer designers assume one size must surely fit all...
 
43% is commonly available because most tubes generally used in UL operation have a ratio of a max plate voltage to max screen voltage rating.

http://www.webace.com.au/~electron/tubes/ul.html

"[] in November 1951 edition of "Audio Engineering" magazine.

Specifically, the 6L6, 807and KT66 families prefer a Screen-grid load of 18.5% Plate load impedance. (43% turns or 43% Plate Voltage).

Interestingly, they also claim the 6V6 family prefers a Screen-grid load of 5% Plate load impedance. (22.5% turns or 22.5% Plate Voltage)."


"The screen tap may be positioned anywhere from 0% (pentode connection) to 100% (triode connection), however performance and tonal qualities change relative to the ratio of the screen tap []

In practice, research by the inventors of this method of amplifier configuration suggests that the ideal for most tube types is in the range 40 to 50% turns of each half primary, measured from the primary centre tap.

Further research by Mullard UK for the EL34/6CA7 and EL84/6BQ5, GEC/MOV Valve Co. for the KT88, GE USA for the 6550, and RCA for the 6973 (6CZ5 hi-fi), 7027A (6L6GC), 7591, 7868 (6L6 family), also recommends 43% turns (or 43% of plate signal voltage), or 18.5% impedance."


"[] the Screen Grid always being operated within its Rated DC Screen Grid voltage and at a DC voltage less than that of the Plate.

Applied plate (B+) voltage for ultra-linear connection should never exceed Grid 2 rated voltage, so standard ultra-linear configuration is only suited to tubes designed for audio applications having a Grid 2 rating approximating either the plate voltage rating (or actual applied plate voltage if less than rated maximum).

Unfortunately, only a small number of tube types are thereby suitable for ultra-linear operation, because in ultra-linear mode the screen grid is operating at or above the plate voltage - a dangerous operating region for any tube.

Only a few tube types were recommended by their manufacturers as being suitable for ultra-linear connection, the most notable being EL34/6CA7, EL84/6BQ5, KT88, 6550, 7027 and 8417 - see manufacturers' data sheets for typical circuit values.

Unfortunately, most audio tetrodes, pentodes and beam power tubes are designed such that the Screen Grid may be operated only up to a maximum DC voltage that is well below the Plate voltage - typically 150 to 300 volts."
 
I have some transformers with 25% UL connections.


Let's get back on topic, please. The OP has "iron" in his possession and wants to make rational decisions regarding its deployment.

With the UL taps at 25%, behavior will be quite pentode like and considerable help is necessary to get a satisfactory damping factor. I'm VERY leery of a simple global NFB loop. Core saturation and other "nasties" come to mind. Doug's "Exolinear" topology seems like a possibility, perhaps with full pentode operation and regulated g2 B+. One way or another, only a few dB. should be applied via GNFB.
 
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