ST-35 with Tango Output Transformers

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

Postby 20to20 » Fri Sep 02, 2011 5:10 pm

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.


From my simple point of view, I imagine the ratio of tube output Z to tranny load Z and try to imagine what happens to a tube that is loaded with too low of speaker Z, which when reflected into the tranny lowers the load Z and pulls more "power" from the tube until it melts or blows the VC of the speaker. So I further extrapolate from that to say if a 10K tube is loaded with a 5K tranny, or a 2.5K tranny, the power through the tube rises... above design. I may be completely upside-down in my logic about that. Perhaps I should see it as if the tranny primary were a straight wire, there would be no power transfer at all, but what about current? Not sure what the result of a primary shorted to itself would be on tube current, other than the obvious reduction of DCR.

I was doing my comparison of the 7189 characteristics with EL-84 characteristics with data sheets that specified K biasing and with the EL-84 the Za-a was 8K regardless of the tap point but the 7189 rose to 11K Za-a when in 43% UL mode. That sheet didn't show a Za-a when connected to a 23% tap, unfortunately... or this question wouldn't have come up.

I really appreciate the time you take to post such thorough replies. I think it will all gel someday.

Headed for Shelby HF tomorrow.

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Postby dcgillespie » Fri Sep 02, 2011 6:13 pm

20 -- Your analysis of current going up through the tube as the load numerically goes down is correct. In that scenario, the turns ratio within the transformer is decreasing, so that its output voltage on the secondary goes up, and therefore, so does (or can) power output. Of course, limitations of other circuits and components come into play as to how far this exercise can play out (power supply limitations, tube current flow limits, etc.).

In my response, I was addressing basically the opposite scenario, as taken from your comment:

"Since the A-410/TO-310 is 8K, what effect would puting an 11K load on it have? I'm assuming more power out......."

My response was to show that in fact as the load is numerically increased, power is reduced -- ALL ELSE BEING EQUAL.

The bottom line is that you will find that from a point of pure efficiency, maximum power output is always transferred to the load when the impedance of the generator (power supply and output tubes) is made exactly equal to that of the load (speaker). So if the load current needs goes up, so does the need for the power supply and tubes to be able to supply that current -- and visa versa. Changing just the transformer impedance -- either way -- without appropriately changing the other elements it interacts with then will compromise maximum power transferred to the load.

BTW -- I went to Shelby about 4 years ago, and felt it has slipped significantly from what I had recalled it to be many years ago (say in the early 90s). Have you been going to Shelby long enough to know otherwise?

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Postby 20to20 » Fri Sep 02, 2011 6:44 pm

dcgillespie wrote:
The bottom line is that you will find that from a point of pure efficiency, maximum power output is always transferred to the load when the impedance of the generator (power supply and output tubes) is made exactly equal to that of the load (speaker).


BTW -- I went to Shelby about 4 years ago, and felt it has slipped significantly from what I had recalled it to be many years ago (say in the early 90s). Have you been going to Shelby long enough to know otherwise?

Dave


1:1 ratio = good... yep. (lol)


Never been to Shelby. Didn't know it existed until just before last year's meet and I decided not to go then, but I gotta do it this time because it might be my last chance while we are still close. If relocating happens I'll have to drive for 6 hrs. to get there. It's probably 90 min. from here.
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Postby Ty_Bower » Fri Sep 02, 2011 7:31 pm

dcgillespie wrote: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).


Ouch. My brain hurts. For the record, be aware that my degree is in Chemical engineering, with nearly 17 years experience in process automation. I wish I had paid more attention in those classes of signal theory - oh wait, that was for the EEs. I never took those classes.

Is that a constant voltage, or a constant current power supply?
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Postby dcgillespie » Fri Sep 02, 2011 8:07 pm

A perfect power supply for our needs (of this conversation anyway) would supply a perfectly constant voltage under all conditions. That means that the voltage would be supplied from a source of zero impedance and capable of infinite current. In the real world, this can effectively be achieved (for all practical purposes) by using a raw supply of adequate current capability (for the needs), supplying a regulator capable of an output impedance down in the milliohm region.

Of course the other approach is to say to heck with regulating the power source, and build an amplifier that has a perfectly constant current draw from it, regardless of the power output it is developing. That way, no voltage regulation would be necessary. It's called a Class A amplifier.

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Postby 20to20 » Tue Oct 18, 2011 2:28 pm

dcgillespie wrote: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.

Dave


Dave,

I've been doing some patent forensics on Acro/Dynaco to try and understand to chronology of their products and came across a KE pamphlet that Herb put out with a table for his UL licensing qualifications that lists the tap of 40% for the EL34/6CA7 and with that number combined with the TO-330 tranny matched for those at 3800R p-p leads me to believe that Hafler was trying to skirt the patent Acro held when the MKII was designed by moving his tap to 33%, which would still be a great improvement but not strictly "UL," and hence Hafler calling all the Dynaco products Super Fidelity. That 33% tap carried to the A-470.

Between the 3800R p-p for the Acro tranny and the 6K p-p tranny you cite with the optimum still at 40% it makes me even more certain that Hafler was just skirting the Acro patent.

20

EDIT: Or perhaps a gentleman's agreement as part of the IP divisions when they parted company for Hafler not to use the 40% for EL34 amps along with not buiding "UL" "compatible" trannies and compete with Acro's licensing plans.
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Postby 20to20 » Sun Oct 23, 2011 1:34 pm

A little more digging revealed the difference between the 3.8K load Z and 6K load Z specs are based on Fixed biasing (3.8K) vs. K biasing (6K). The Mullard EL34 sheet shows "43% taps for minimum distortion", "20% taps for maximum power," with K biasing. Since the UL specification is determined by the base of the distortion curve it seems Mullard is also citing @ 40% tap to coincide with the Acro UL specification.

Hafler is splitting the difference with a 33% tap "by preference" as stated in his amp patent.

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Postby dcgillespie » Wed Oct 26, 2011 9:38 pm

20 -- I apologize for my late response. I've been in the middle of a move, and my ability to communicate has been spotty at best.

I'm not so sure that Hafler was looking for a convenient middle ground tap percent to use as he was honestly looking for the best operating parameters performance wise.

The 3.8K TO-330 Acrosound transformer was never designed around the EL34 or the 6550 for that matter either. Rather, it was designed around optimum PPP KT66 operation, since those tubes were available first on this side of the pond, and their use in that configuration then created the big brother of the original PP UL Williamson amplifier.

Later, when the EL34 and 6550 became available, the TO-330 was tried with those tubes in PP fixed bias UL mode, and found to perform very well indeed -- especially so with the 6550.

Later, when Hafler broke away from Acrosound to form Dynaco, he chose the EL34 for his first amplifier, and started with a clean sheet of paper as to what transformer specifications produced optimum performance with that tube. When used with fixed bias operation, the specifications he chose to use did in fact produce optimum performance, and became the basis for so many of the transformers to follow in the Dynaco line.

In his original comments on the subject, Hafler commented on how linear the EL34 tube is, and that the UL tap was not needed with this tube to produce low distortion as was required with the original UL transformer (TO-300) for the 6L6 class family of tubes. Instead, the screen tap in Hafler's transformers was chosen to not upset the already excellent linearity of the tubes, but still provide good regulation of the output stage (damping), that would otherwise not be had without the taps.

So, while it may look like Hafler was trying to get around patent infringements, in reality, I believe he really was looking to achieve optimum performance from EL34 tubes with his brand of specifications. In deed, when all things are considered, when EL34 tubes are operated with fixed bias and tapped screen operation, the Dynaco A-430 family of transformers will very definitely out-perform the Acrosound TO-330 transformer with this tube. The TO-330 is a superb transformer to be sure. But in this case, with this tube, the proof in very definitely in Hafler's pudding!

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Postby 20to20 » Fri Oct 28, 2011 7:14 am

dcgillespie wrote:20 --

In his original comments on the subject, Hafler commented on how linear the EL34 tube is, and that the UL tap was not needed with this tube to produce low distortion as was required with the original UL transformer (TO-300) for the 6L6 class family of tubes. Instead, the screen tap in Hafler's transformers was chosen to not upset the already excellent linearity of the tubes, but still provide good regulation of the output stage (damping), that would otherwise not be had without the taps.

So, while it may look like Hafler was trying to get around patent infringements, in reality, I believe he really was looking to achieve optimum performance from EL34 tubes with his brand of specifications. In deed, when all things are considered, when EL34 tubes are operated with fixed bias and tapped screen operation, the Dynaco A-430 family of transformers will very definitely out-perform the Acrosound TO-330 transformer with this tube. The TO-330 is a superb transformer to be sure. But in this case, with this tube, the proof in very definitely in Hafler's pudding!

Dave


Dave,

No apologies... your replies are always worth the wait, for sure.

On Hafler's comments about the linearity of the 6CA7, I'm assuming you refer to the AudioCraft article. I dug back through my gatherings and found that "Modernize..." piece but the one I have, (probably the same one everyone has) is missing a page or two just as Hafler starts to address the main points. Do you have a complete copy of that article or a link to a full reprint?

We should ask Shannon to move this to the ST-70 thread with some of the back posts for topic continuity...

Thanks,

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Postby dcgillespie » Fri Oct 28, 2011 8:04 am

20 -- I've got it .......somewhere! With the move, everything is still packed up just trying to get to its general location, let alone having its proper place established and being easily available.

I will be happy to look for it as soon as time permits. With everything going on on my end, just don't let me forget!

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Postby 20to20 » Mon Nov 07, 2011 1:35 pm

dcgillespie wrote:20 -- I've got it .......somewhere! With the move, everything is still packed up just trying to get to its general location, let alone having its proper place established and being easily available.

I will be happy to look for it as soon as time permits. With everything going on on my end, just don't let me forget!

Dave


Dave, I found another complete copy online. Your offer was genuinely appreciated. Hope you get settled soon.

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Re: ST-35 with Tango Output Transformers

Postby MLVA123 » Sun Jun 09, 2013 7:19 am

I just came across this thread while doing a bit of research on Z565 OPTs.
What an excellent thread; great historical context, technical details (that are accessible to non EEs like myself), and just so well-written. Thanks to all who contributed - it's a gift of your time that is still giving. (i will admit that I will need to re-read this a few times :$ )

I've read elsewhere that the Z565 was optimized for 6BQ5 tubes, but the rationale for that still eludes me. I think it has to do with the use of fixed bias design. Can you guys help shine a light on this for me? My reason for asking is that I'm wondering what amp design would make the best use of these Z565s that I have.
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Re: ST-35 with Tango Output Transformers

Postby EWBrown » Tue Jun 11, 2013 3:34 pm

If anyone has a good use for them, I have two more original Dynaco Z565s, which have high DC resistance measurements across half of the primary,
probably one of the paralleled windings is open or has a bad internal connection. I hate to just "toss" them. :/ *)
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