Improved SCA-35/ST-35 Performance

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

Postby wicked1 » Mon Jan 23, 2012 2:08 pm

dcgillespie wrote: In actual fact, the regulator will regulate the output voltage to 1.25 vdc greater than the absolute value of that at the ADJ terminal -- relative to the regulator supply source's "circuit ground" -- NO MATTER WHERE THE ADJ TERMINAL VOLTAGE IS SUPPLIED FROM.

Dave


Thank you for all of that! What I've quoted above is what was really giving me trouble. I spent hours looking at lm337 and lm317 schematics, and basically all of them I found had OUT and ADJ connected through a resistor. I just couldn't figure out how those devices worked.
Now I get it!!

I really appreciate what you, and really all of the online EE's who are in to this stuff have taught me. You're all a great crowd!!
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Improved ST-35/SCA-35 Performance

Postby BruceAnderson » Wed Jan 25, 2012 7:03 pm

I'm a little unclear as to the application of the EFB mod to other than stereo 35 amps. I have a very nice 'pull" from a Magnavox console. It's PP 6bq5 with a bias scheme almost exactly the same as the 35 amps but with pentode output. The output transformer seems quite high quality and produces strong. clean bass. Whether it is the equal of those used in the 35 amp I can't really say but I don't feel that it is lacking in my current setup.
So...is the EFB an effective mod to a pentode output mod, with an unknown output impedance but that is otherwise reasonably high quality (certainly quite a bit more than I expected).
I would think locking the bias voltage to a specific percentage of B+ would reduce distortion for any amp, no? Certainly being able to bias to a lower value for long life ought to be a plus. Why the reference to the 35 transformers being perfect for this mod while others may not be? In case you couldn't guess, I already ordered part for this mod but am now second guessing myself.
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Re: Improved ST-35/SCA-35 Performance

Postby jgf » Thu Jan 26, 2012 10:02 am

BruceAnderson wrote:I'm a little unclear as to the application of the EFB mod to other than stereo 35 amps. I have a very nice 'pull" from a Magnavox console. It's PP 6bq5 with a bias scheme almost exactly the same as the 35 amps but with pentode output. The output transformer seems quite high quality and produces strong. clean bass. Whether it is the equal of those used in the 35 amp I can't really say but I don't feel that it is lacking in my current setup.
So...is the EFB an effective mod to a pentode output mod, with an unknown output impedance but that is otherwise reasonably high quality (certainly quite a bit more than I expected).
I would think locking the bias voltage to a specific percentage of B+ would reduce distortion for any amp, no? Certainly being able to bias to a lower value for long life ought to be a plus. Why the reference to the 35 transformers being perfect for this mod while others may not be? In case you couldn't guess, I already ordered part for this mod but am now second guessing myself.


Maybe Dave G. will jump in here with a detailed response, but my understanding is that the EFB mod is so effective with dynaco Z-565 transformers because a) it converts the circuit to fixed bias operation, which their primary impedance is more suited to and b) it acts like a regulator for the power stage. So, if your transformers are better suited to cathode bias, it might not be an improvement. But if your transformers are better suited to fixed bias, it could well be an improvement.
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Postby BruceAnderson » Thu Jan 26, 2012 10:44 am

Thanks for the reply. Are you saying that an 8K transformer is held to a constant bias/B+ relationship by the EFB but a 10K transformer is not or are you saying the 10K transformer is not degraded when bias as a percentage of B+ varies? As you can see, I have more curiosity than technical knowledge. Any enlightenment will be appreciated.
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Postby jgf » Thu Jan 26, 2012 4:23 pm

BruceAnderson wrote:Thanks for the reply. Are you saying that an 8K transformer is held to a constant bias/B+ relationship by the EFB but a 10K transformer is not or are you saying the 10K transformer is not degraded when bias as a percentage of B+ varies? As you can see, I have more curiosity than technical knowledge. Any enlightenment will be appreciated.


Well, my knowledge on this is limited too, read the EFB mod paper
(linked somewhere on this site.) what it says is that the optimal impedance load for the tube in fixed bias is not the same as what's optimal for cathode bias. And also, Dave G. measured the dynaco Z-565 and found it better suited to fixed bias, and so the EFB mod, as a form of fixed bias provided a big improvement over Dynaco's original cathode bias.
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Postby dcgillespie » Thu Jan 26, 2012 11:11 pm

Hi Bruce -- First, thanks to jgf for his helpful responses -- they are very helpful, and right on target.

For a better understanding, I too would encourage you to read my article (see my initial post of this thread for the site it is posted on) wherein I introduce the EFB modification as a very real and results oriented solution to the underlying performance concerns, as produced by the combination of bias system and transformer specifications used in the small Dynaco amplifiers. It is an easy read, designed for new comer and experienced folks alike, and explains what the problem is, why it occurs, and how the EFB modification is such a simple, effective solution.

To thoroughly understand the reasoning behind why the EFB modification is so effective with specifically the Dynaco amplifiers first requires an understanding of the use of load lines as drawn on a "family" of plate curves, and what the resulting performance is that a given load line represents. It also requires a little understanding of David Hafler, and what his design decisions were in the manufacturing of his output transformers. Finally, it also requires an understanding of the design compromises made in the small Dynaco amplifiers, and what the results of those compromises are.

As to your specific questions, let me give you some basic generalities that you might find helpful. First, let us assume a perfect power supply is in place, that provides a perfect required B+ voltage, of infinite current capability -- in other words, it is perfectly regulated. In it's most basic form then, the output stage represents the output transformer and the output tubes as two components wired electrically in series. This series combination of tube and transformer is then wired in parallel across the output of the power supply. From this it can then be seen, that the output tubes are the active elements that are transferring power from the power supply, through the primary winding of the output transformer, and on to the load connected to the secondary of the transformer.

With this connection, it can be shown that the maximum power transfer into the load will be had when the impedance of the load is made exactly equal to the impedance of the the generator -- or in this case, that of the tubes. That is why that the best performance is had when the load of the output transformer matches the optimum load the tubes require.

The optimum load that a tube requires varies with a whole host of issues. Factors such as operating mode (triode, pentode, or UL operation), bias scheme used (fixed or resistive cathode bias), and the voltage the screen grid operates at all have a huge impact in determining what is the optimum load is for a given set of tubes in a given setting.

For the case at hand, when traditional resistive cathode bias is used, because it is an additional resistive element in series with the tubes, it necessarily raises the impedance of the generator, and so a numerically higher load impedance is then required to maintain maximum power transfer. With traditional cathode bias, the ultimate power will always be lower because of the additional resistive element it represents, so it will always require a higher load impedance to produce the maximum power it is capable of.

On top of this factor, because of the significant AC influence on the screen grid with UL operation, it to reduces the maximum current flow that a tube is capable of flowing, so just as with the use of cathode bias, the load impedance must be raised when UL operation is used.

These effects are cumulative. The use of cathode bias raises the required load somewhat. UL operation raises the required load somewhat. UL operation with cathode bias raises the required load a lot.

The crux of the matter is that the Dynaco transformer was wound to produce maximum performance out of 6BQ5 tubes when operated in UL mode with fixed bias operation -- but their amplifiers were designed with cathode bias as a matter of economy. Therein lies the problem -- and the opportunity as spelled out in my article.

As a final comment to your question, while the use of the EFB modification produces a significant performance improvement in the Dyanco amplifiers, it's use in other designs that employ a more appropriate load for the operating conditions used (including bias system employed) will obviously not produce the size of performance increase as shown in the Dynaco amplifiers for the reasons explained. On the other hand, it can hardly hurt either. You may also be interested in a short followup article I wrote regarding the potential use of the EFB modification with Heath's #51-29 OPTs, as used in most of their 6BQ5 offerings. In that setting, it would produce a slight improvement in power output, but the impact on distortion is muted as those transformers are well designed for used with resistive cathode bias operation from the get-go. You can find that article (as well as the original one) under the "Dave's Lab" section of the Tronola website.

I think if you read both of these articles, it will help you gain a better understanding of what the modification does, and what it is capable of doing in other designs.

I hope this has helped!

Dave
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Postby wicked1 » Fri Jan 27, 2012 4:02 pm

Since Bruce mentioned pentode mode, I was just thinking about pentodes w/ regulated screens and un-regulated B+.
In this case, would I want to connect the EFB circuit to the regulated screen voltage, rather than B+? I wouldn't want the bias to follow fluctuations in B+ in this case.
But then actually, maybe a different fixed bias scheme would be better, as there's no need for the "enhanced" part.

(I realize we're straying far from the st/sca-35 path now, so no need to go into details..)
Last edited by wicked1 on Fri Jan 27, 2012 4:14 pm, edited 1 time in total.
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Postby BruceAnderson » Fri Jan 27, 2012 4:08 pm

Thanks Dave
I believe its at least a little more clear to me now.
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Postby dcgillespie » Fri Jan 27, 2012 10:08 pm

Wicked -- You are absolutely correct. When using EFB in a strict pentode design, the B+ reference for the EFB regulator should be connected to the screen B+ supply, since this element ultimately determines the amount of current that a pentode tube can pass.

As a result, EFB could find very good use in the never ending supply of (for example) 7591 integrated amplifiers that use a simple resistor to drop the B+ to the screens of these tubes when they are operated in pentode mode -- as they almost always are. As power output is increased in both channels, the screen voltage drops significantly in these amplifiers (in an Eico ST-70 for example, the quiescent screen voltage is ~ 425 vdc, while at full power output with both channels driven, it typically falls to under 350 vdc), which disturbs the initial bias setting significantly. If EFB were used to provide the bias for the output tubes rather than the traditional method used, it would significantly reduce high power distortion levels by way correcting the quiescent bias to the new operating conditions present at elevated output levels. This action is at the heart of EFB operation, allowing it to produce significantly improved performance in such a setting.

Dave
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Postby BruceAnderson » Tue Jan 31, 2012 7:24 pm

Thanx to Wicked and Dave for broadening the discussion to include pentode EL84 PP amps and how they might be effected by the EFB bias supply. That's the direction I was trying to go with my questions. I wasn't really concerned with greater power output potential for my Magnavox amp. It has more than enough power to fill my rather spacious listening room when driving my Lowther based speakers. What I was looking for was a way to bias my tubes to a somewhat less stressful level and if I could reduce distortion to an even lower level at the same time, so much the better. I just completed installation of an EFB built point to point on a perfboard and playing the amp for a couple hours concentrating on dynamic, bass heavy music played louder than normal (love Jimmy McGriff, for instance). I wanted to make sure it wasn't going to "let the smoke out" and I wantd to know if I could hear any difference (I don't have the equipment to measure distortion.) I have to say I'm very happy. We all know it's easy to be convinced that the mods you build make the system sound better but...I hear a little tighter bass and more snap to such things as sticks striking a drum. Highly recommended.
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New diytube ST-35 Board with EFB Status

Postby rhing » Thu Feb 02, 2012 8:17 am

Just wondering if Shannon has an ETA on the new ST-35 boards with EFB.
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Postby Stunch » Thu Feb 09, 2012 2:11 pm

When I hook up a continuity tester to the LM337 (clipped on at the metal end), I get continuity at the center pin (input) and also at the output pin, but not quite as bright. I get nothing on the adjustment pin.

With a multimeter at diode test (2000R) I get 1R at the center pin, 872R at the output and nothing at the adjustment.

Sound right? This is actually done on a LT337A
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Re: New diytube ST-35 Board with EFB Status

Postby Shannon Parks » Thu Feb 09, 2012 7:09 pm

rhing wrote:Just wondering if Shannon has an ETA on the new ST-35 boards with EFB.


Alas, I am *still* working full throttle on my commercial Budgie and have done precious little on the ST35 Rev EFB. I'll see what I can do this weekend on it, and then give an ETA. I apologize for the delay.

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Re: New diytube ST-35 Board with EFB Status

Postby rhing » Thu Feb 09, 2012 8:03 pm

separks wrote:Alas, I am *still* working full throttle on my commercial Budgie and have done precious little on the ST35 Rev EFB. I'll see what I can do this weekend on it, and then give an ETA. I apologize for the delay.

Shannon


No apologies needed. I will patiently wait. Thanks.
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Postby paart » Fri Feb 10, 2012 2:53 pm

In his original article, Dave stated that the EFB bias concept is applicable to other amplifiers utilizing low bias output tubes such as 6V6, 7591, etc. While he acknowledges that his mod was developed for the Dynaco EL-84 amplifiers, it also seems to me, that from a strictly practical point of view, one would have very little to lose by at least trying this technique with similar circuits. When one looks at the potential for improvement, vs. cost and labor involved, how could one go wrong? The parts cost of the EFB bias mod is about $10, using top quality new parts, and probably half that, if one has a “well stocked junkbox.” The amount of labor involved, for the average constructor shouldn’t exceed much more than 2-3 hours. Comparing these costs and potential for improvement with nearly any other popular upgrades, puts things in perspective. The cost and effort required for the relatively modest undertaking, of say, a simple, quick power supply upgrade with only better rectifiers and added bypass capacitors, or even the popular “tube rolling” experiments, put things in perspective. A changeover to “boutique” capacitors and resistors will impact your financial condition far more than implementing this mod on any ten amplifiers! The best part is that the improvement with EFB bias mods is measurable, documented and confirmed. It’s “no contest” time or moneywise. If you have a power amp using a quad of KT-88s, then this isn’t for you, but if you are using one of the dozens of low power tube amps readily available, I can’t imagine many applications where the EFB bias mod wouldn’t make at least some worthwhile improvement.

Try it! It’s quick and easy and cheap! Report your experience! Your results will be of interest!
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