ST-35 with Tango Output Transformers

for the DIY ST35, the Dynakit and every other PP EL84

Postby dcgillespie » Wed Aug 31, 2011 5:35 am

Hi 20 -- Good to hear from you!

I have never had an A-410 to measure for myself, but if I were a betting man, I'd bet the screens were tapped at 23-25% of the winding in that transformer.

Dave Hafler was the real motivation behind the development of UL. While Keroes was certainly involved, his motivation lie primarily in developing TV horizontal output transformers! So it was Hafler that ultimately developed so many of the popular UL operating conditions that came to be known with various output tubes.

Ed teamed up with Dave early on in 1955 to form what became Dynaco. His contribution was the development of the pentode-driving-a-triode-phase-splitter-all-in-one-tube circuit, which landed him the position of chief designer at Dynaco. Combining his input circuitry and Dave's output circuitry was a natural, and resulted in the original Dynaco MK II. As they say, the rest is history. How many times has that basic circuit arrangement been copied!!

In any event, by 1961, optimum operating points for UL were well known, with Hafler's name and UL basically being one and the same at that point. I have an original copy of the article you refer to, and since he did not represent himself as being part of Dynaco in it, it is unclear what his intentions were. He was walking a fine line though because while he mentions "Tests showed no advantage" (in using the A-410's UL taps), there are in fact clear advantages to using a tapped screen connection that he does not mention -- primarily that being the reduced output impedance and improved damping that connection provides. This is very significant when driving a loudspeaker load.

We'll never know why Ed "deviated" from the Dynaco formula in that article, but it could be anything from trying to show that 6BQ5s can produce good performance in pentode mode (which they can), to simply trying to establish his own name out from under the Dynaco spotlight. I doubt it was an effort to troll for interest in such a design, as by that point, 6BQ5s, UL, and Dynaco products were well established. It may be he was tipping his hat to his boss, since at that point, there were virtually no UL 6BQ5 designs in production (in retrospect, the Dynaco and Heath versions represented the lion's share of such designs), and the Dynaco UL 6BQ5 products were just getting set for release.

In any event, 6BQ5s and 7189s are one and the same when it comes to operating characteristics (not limitations) so that likely had little to do with the matter. As for the transformer though, it would be great if someone who had an A-410 could measure and report the tap placement on it, so the question could ultimately be answered!

Dave
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Postby 20to20 » Wed Aug 31, 2011 8:09 am

dcgillespie wrote:Hi 20 -- Good to hear from you!

In any event, 6BQ5s and 7189s are one and the same when it comes to operating characteristics (not limitations) so that likely had little to do with the matter. As for the transformer though, it would be great if someone who had an A-410 could measure and report the tap placement on it, so the question could ultimately be answered!

Dave


Dave,

Great reply, as always! Insights into those days are fascinating.

I've had to put my hobbies on the back burner for the last few months, because of a few twists and turns of life. Nothing truly serious unless you call wiping out a hard drive, for starters, serious. That took me off-line for a while as I rebuilt my machine. I had swapped HD's on my cable one too many times trying to load different Linux distros and wiped my primary drive by mistake....GRRRRRRR! What a horror!! Then "we" made a decision to relocate puting our place up for sale "In this market?, you ask.."and all that entails with repairs and "staging..." meaning all my stuff goes into the garage... You get the picture... Thanks for the welcome back. Things have simmered down and I can put some relaxation time back near the top of the list. Including the extra time it takes now to "find that %$#%$# thing out in the garage!"

Getting back to the subject, since Laurent decided to use the screen tap for feedback... "never let a perfectly good screen tap go to waste..." it made me wonder about the benefit of going UL with a 6BQ5, or using his cathode feedback, if I were to try building that amp today but substituting the Z-565, with the known good 25% tap.

Knowing what the A-410 had for a tap would put put a fine point it all.

Thought I'd try building one with 6GH8A's driving since they are $3 apiece.

Thanks again!

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Postby 20to20 » Thu Sep 01, 2011 1:53 pm

dcgillespie wrote:
I have never had an A-410 to measure for myself, but if I were a betting man, I'd bet the screens were tapped at 23-25% of the winding in that transformer.


Dave


Dave,

After a bit of digging back through some earlier RE's I found a circuit for 6V6/EL-84 described in the August '58 issue that used an Acro TO-310 that specifies that tranny as having 25% taps made for those tubes in UL mode. So I believe you'd win the bet about the A-410 being tapped likewise.

Still a curious claim by Laurent about the 7189, though. Got to dig into those tube specs ...

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Postby dcgillespie » Thu Sep 01, 2011 3:04 pm

20 -- Great job digging that one out! I've got that article as well, but forgot about it as it relates to the Acro product. That transformer (TO-310) was developed while Hafler was still at Acro, and no doubt he applied the knowledge he developed there to his Dynaco line of transformers as well.

Of course one of the things he could not copy from his days at Acro was the term "Ultra Linear". In all of his days after he partnered with Keroes to found Acrosound, he never -- ever -- referred to his Dynaco transformers as being Ultra Linear transformers, since Acrosound owned the rights to that term. David always referred to his transformers as providing "tapped screen" operation -- but everybody knew what "that" meant. In later years after Acro shut down, Heath obtained the rights to that term since they were so heavily invested in the Ultra-Linear concept from the start, in almost every vacuum tube amplifier they offered.

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Postby 20to20 » Thu Sep 01, 2011 9:28 pm

dcgillespie wrote: That transformer (TO-310) was developed while Hafler was still at Acro, and no doubt he applied the knowledge he developed there to his Dynaco line of transformers as well.


Dave


Here's another interesting tidbit about their UL development from '51 to '54.

I came across an article by Hafler in '51 detailing the new wonders of UL and a circuit he used in the piece with a TO-300 and 6L6 tubes. His "experimentation" revealed the best tap point was 18.5%. Obviously not far from 23%-25% claimed to be optimum for 6V6/EL-84.

In '54 he produced another article, this time about UL with 6V6 tubes where he made a point to differentiate between the UL characteristics of 6L6's and 6V6's.

He says, "The characteristics of the 6V6 are not at all similar to the 6L6 family, and the connection arrangement which is optimum for 6V6's is quite different from that which can be used with the large tube types."

So I wonder if he had a research enlightenment in between and found a new higher "optimum" UL tap point for 6L6? Not sure if that is the common 43%.

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Postby dcgillespie » Fri Sep 02, 2011 6:12 am

Hi 20 --

The '51 article and the amplifier design it presented is in fact the introduction of the Ultra-Linear concept by the Acrosound duo to the world, and their TO-300 transformer that it used was the first Ultra-Linear output transformer they produced. All the rest of their UL offerings (rather quickly) followed later.

However, be careful as to what the percent of screen tap being discussed actually represents. In the early articles, it was sometimes referred to as the percent of the primary winding, and sometimes as the percent of the primary impedance. While there is a direct relationship between these numbers, it can certainly be confusing to those unaware, as casual reading may not pick up, or understand the difference.

Today, in discussions relating to UL operation, the screen's relationship is almost universally referred to as a percent of the winding, as the concept is easier to deal with. For the purposes of your observations from your articles however, be aware that the best tapping point for UL operation of 6L6 tubes is -- and always has been -- at 43% of the winding, which is also the equivalent of saying that the taps represent 18.5% of the primary impedance. I have no doubt that with careful re-reading, you find that this is the case for the apparent "discrepancies" you found.

Relating this then to the 6V6 family of tubes, the best tapping point was determined to be -- and always has been -- at 23% of the winding (or 5.3% of the primary impedance) for these tubes. In comparing this information then to the best tapping point found for 6L6 class tubes, you can now see that the best tapping points between these two family of tubes is quite different indeed.

Now you can also apply this information back to my original post that you referenced, and have a better understanding as to why this is all so important relative to today's transformer manufacturers, who think that a 40% UL tap (by winding percent) is the end all-be all for all tubes. It is simply not the case.

If you could provide me with a copy (or point me to where I can obtain a copy) of Hafler's '54 article on UL operation of 6V6 tubes, I would be most grateful. I am very aware of the article, and most of what it has to say, but obtaining a copy of it has still eluded me over the years. I would love to have a copy for my files! If you can help it would be most appreciated!

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Postby 20to20 » Fri Sep 02, 2011 6:26 am

dcgillespie wrote:Hi 20 --


If you could provide me with a copy (or point me to where I can obtain a copy) of Hafler's '54 article on UL operation of 6V6 tubes, I would be most grateful. I am very aware of the article, and most of what it has to say, but obtaining a copy of it has still eluded me over the years. I would love to have a copy for my files! If you can help it would be most appreciated!

Dave


Done and done!

Well... Today I had trouble opening the PDF links directly so I had to right-click and tell it to save the file first, then I could open it... dumbputers...

http://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_01/Sec_2/0150_UL_6V6s.pdf

http://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_01_Sec_2.html
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Postby dcgillespie » Fri Sep 02, 2011 7:07 am

Hi 20 -- Thanks so much! Finally got it!

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Postby 20to20 » Fri Sep 02, 2011 7:13 am

Today, in discussions relating to UL operation, the screen's relationship is almost universally referred to as a percent of the winding, as the concept is easier to deal with. For the purposes of your observations from your articles however, be aware that the best tapping point for UL operation of 6L6 tubes is -- and always has been -- at 43% of the winding, which is also the equivalent of saying that the taps represent 18.5% of the primary impedance. I have no doubt that with careful re-reading, you find that this is the case for the apparent "discrepancies" you found.



Dave,

I owe you about a hundred beers...

I wonder how the construction quality of new trannys made by other winders would affect the performance vis-a-vis the originals? Have you ever Z'd-out any new ones to see if they are accurate for the tap point?

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Postby dcgillespie » Fri Sep 02, 2011 8:36 am

20 -- Glad I can help shed some light on this for you.

To be honest, I've never tested any of the newer transformers, because I've always tried to only deal with the best of the vintage transformers -- Acro, Peerless, Dynaco, Chicago, etc. Most of the typical transformers manufactured today (as shown by their own specifications) could not hold a candle to these older transformers in terms of frequency response and power bandwidth response. That becomes hugely important when the transformer is included in the feedback loop if any significant amount of feedback is going to be employed. Also, most manufacturers today only offer a 40% UL tap on their transformers, which as you can now see, is hardly the best point for all tubes. That was the whole point I was trying to make in my post you originally referred to.

The one exception (I believe, and that I know of) is the Triode clone of the Dynaco output transformers. Again, while I have not personally had any of these transformers to measure, all indications are that they are every bit as good as the originals, since it is purported that these transformers are manufactured exactly as the originals were.

With today's group of audiophiles, "UL operation" as degraded into simply hooking the screen grids up to screen taps if they happen to be offered on the particular transformer they are using. But without clear knowledge of what the optimum requirements are of the tubes they are using, and what the transformer itself is presenting, then performance can easily suffer versus not using the taps at all.

Since you led me to the 6V6 article I was missing, we'll call the beer count even!

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Postby joeriz » Fri Sep 02, 2011 9:18 am

dcgillespie wrote:Hi 20 -- Good to hear from you!

I have never had an A-410 to measure for myself, but if I were a betting man, I'd bet the screens were tapped at 23-25% of the winding in that transformer.

Dave Hafler was the real motivation behind the development of UL. While Keroes was certainly involved, his motivation lie primarily in developing TV horizontal output transformers! So it was Hafler that ultimately developed so many of the popular UL operating conditions that came to be known with various output tubes.

Ed teamed up with Dave early on in 1955 to form what became Dynaco. His contribution was the development of the pentode-driving-a-triode-phase-splitter-all-in-one-tube circuit, which landed him the position of chief designer at Dynaco. Combining his input circuitry and Dave's output circuitry was a natural, and resulted in the original Dynaco MK II. As they say, the rest is history. How many times has that basic circuit arrangement been copied!!

In any event, by 1961, optimum operating points for UL were well known, with Hafler's name and UL basically being one and the same at that point. I have an original copy of the article you refer to, and since he did not represent himself as being part of Dynaco in it, it is unclear what his intentions were. He was walking a fine line though because while he mentions "Tests showed no advantage" (in using the A-410's UL taps), there are in fact clear advantages to using a tapped screen connection that he does not mention -- primarily that being the reduced output impedance and improved damping that connection provides. This is very significant when driving a loudspeaker load.

We'll never know why Ed "deviated" from the Dynaco formula in that article, but it could be anything from trying to show that 6BQ5s can produce good performance in pentode mode (which they can), to simply trying to establish his own name out from under the Dynaco spotlight. I doubt it was an effort to troll for interest in such a design, as by that point, 6BQ5s, UL, and Dynaco products were well established. It may be he was tipping his hat to his boss, since at that point, there were virtually no UL 6BQ5 designs in production (in retrospect, the Dynaco and Heath versions represented the lion's share of such designs), and the Dynaco UL 6BQ5 products were just getting set for release.

In any event, 6BQ5s and 7189s are one and the same when it comes to operating characteristics (not limitations) so that likely had little to do with the matter. As for the transformer though, it would be great if someone who had an A-410 could measure and report the tap placement on it, so the question could ultimately be answered!

Dave


Just have to say - what a great post. Dave, I always read your posts and you are unfailingly polite, well-spoken (written??), and informative. Thank you!

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Postby 20to20 » Fri Sep 02, 2011 9:25 am

dcgillespie wrote:20 -- Glad I can help shed some light on this for you.


With today's group of audiophiles, "UL operation" as degraded into simply hooking the screen grids up to screen taps if they happen to be offered on the particular transformer they are using. But without clear knowledge of what the optimum requirements are of the tubes they are using, and what the transformer itself is presenting, then performance can easily suffer versus not using the taps at all.

Dave


My take from Hafler's '51 article is that using a lower tap point than the optimum would be "safer" , accepting some higher D for more power as you move back toward full pentode. He says it all starts going to H pretty quickly when you go above the optimum, so 40% is a no-no for sure for 6V6 family but 40% wouldn't be huge trade for 6L6 and larger...

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Postby dcgillespie » Fri Sep 02, 2011 9:48 am

20 -- You got it!

Joe -- Thanks for the kind words! I well remember what it was like so many years ago when I had so many questions of my own. I could have only wished for a vehicle such as the Internet and these forums to have been available to me. So, I had to learn it all the old fashioned way -- although we never stop learning, do we! At any rate, I've been where so many folks are today with too many questions and not enough answers, so I try to help whenever I can. Thanks again for the interest in my posts!

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Postby 20to20 » Fri Sep 02, 2011 12:18 pm

Dave,

Going back to Laurent's circuit one more time, and looking over an RCA spec sheet for a 7189 it shows a data table for 43% UL at max Va and gives the Za-a as 11K as opposed to 8K in normal pentode ops, as is the EL-84, in either 23% or 43% UL.

Since the A-410/TO-310 is 8K, what effect would puting an 11K load on it have? I'm assuming more power out and higher distortion. So it might not be a suprise that Laurent wasn't thrilled with UL in that case?

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Postby dcgillespie » Fri Sep 02, 2011 4:25 pm

If I may, let me give you some thoughts to chew on to help answer some of your questions.

First, lets deal with the difference between characteristics and maximum ratings of tubes. For all practical purposes, the 6BQ5, EL84, and 7189 are all the same tube, with the exception ratings. As to characteristics, look at the 6BQ5 in your RCA manual. Under "Typical Operation", note that with 250 volts on the plate and screen, and -7.3 vdc on Grid #1, the tube pulls 48 ma plate current, and 5.5 ma grid #2 current under quiescent conditions. Now look up the 7189. Note under "Characteristics" that with 250 volts on the plate and screen, and -7.3 vdc on grid #1, the current draw at the plate and screen are -- you guessed it -- 48 ma and 5.5 ma respectively. Therefore, from a characteristic standpoint -- that is, how they react to circuit changes, these two tubes are identical. The 7189 is simply rated for higher voltage operation, allowing different scenarios to be shown under "Typical Operation" for that tube, that cannot be shown for the 6BQ5 because of its ratings limitation. However, both tubes react exactly the same way to circuit influences.

Now, a few comments on loading, since you mention that power might go up since the load went up to 11K. With all else being equal, as the load goes up, power goes down. In an effort to simplify the issue, understand what an amplifier is basically doing: You are listening to the output of the power supply, modulated by an AC influence (the signal). The output stage (including output transformer) provides the modulation because it is situated between the power supply, and the load (speaker) . The output transformer matches the impedance difference that exists between the power supply/output stage combination, and the load (the speakers in this case). Since the impedance of the load is basically set (usually 8 ohms), as the impedance of the primary is increased, this has the same effect as providing an even greater step DOWN ratio (i.e. greater turns ratio) within the transformer. Since the step down effect is greater with a numerically higher impedance load, less voltage is delivered to the load, so power output goes down. In fact, note that in the 7189 operating conditions, with nearly the same plate voltage offered between the two scenarios, the 11K load of the UL scenario produces significantly less power output than that of the 8K pentode scenario does. There are other factors at play here, but the basic relationship can be seen.

As for the operating conditions offered by RCA for the 7189, the reason they are calling for an 11K load with 43% screen taps, is because that scenario is based on using cathode bias -- which by the way, is basically the same operating conditions that Heath used in all of its 6BQ5 amplifiers. Hafler's transformer (though he didn't particularly advertise it), with the load and tapping points he chose, operate best under conditions of fixed bias. The type of bias format used has a very direct bearing on the value of load that is considered optimum. If all of this interests you, I humbly suggest that you read my recent article on adding a new type of bias I developed called Enhance Fixed Bias(tm) to a Dynaco SCA35. Of course, that amplifier uses Hafler's transformers and his 8K/23% tap specifications, but uses cathode bias for the sake of economy. By adding my simple EFB circuit to the amplifier, the output stage is converted to fixed bias, with an operating point that is maintained regardless of changes in the power supply due to line voltage, power output level, or a combination of either. The point is, that by applying the appropriate type of bias for the loading conditions offered, power output and distortion levels are both enhanced significantly. In the article, I address and answer many of the questions you are asking. You can fine the article here:

http://www.tronola.com/A_New_Look_At_An_Old_Friend.pdf

In the end, Hafler's loading conditions (can) ultimately supply the greatest amount of power, at the lowest distortion between his conditions, and those of RCA's (Heath's). I also address this in a short article you might find intesting that is also posted on my portion of the Tronola website. You can go to it there at:

http://www.tronola.com/

Go to my portion entitled Dave's Lab, where you will see the short article I reference, that deals with the type of bias, versus the type of loading, as exemplified by the Heath and Dynaco operating conditions.

I hope this helps!

Dave
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