Improved SCA-35/ST-35 Performance

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

Postby Marshman » Mon Dec 06, 2010 6:53 am

Can you post some pictures please? Angel


I can! Please do not get too critical of my implementation. As I said, there is probably a better way, but this is how I chose to do it. Ugly, but it works.

As an aside, I take no responsibility for anyone trying this on their board!!

The 100ohm resistors are in the yellow and green heat shrink on the transformer leads. I know they probably should be closer to the tube, but could find no consistent way to get them closer.

Image

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Thanks again Dave, we have about 10 hours of listening in and it just keeps getting better!!

mm
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Postby dcgillespie » Mon Dec 06, 2010 7:42 am

Thanks Marshal! The pics were very helpful to me as well, as when your original post simply said "stereo 35" I just assumed that you were working with the Dynaco product. Your pics help me to better understand the effort involved with the diy board now. Thanks again!

Dave
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Postby Ty_Bower » Mon Dec 06, 2010 12:21 pm

Maybe Shannon could build this feature into Rev E...
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby dcgillespie » Mon Dec 06, 2010 9:00 pm

Ty -- The article on the performance of the Heath 51-29 transformer is now posted in my section of the Tronola.com website. At the site, click on my section (Dave's Lab), and then scroll down to the bottom of the page, where a new section has been added for Short Subjects. The followup piece is located there.

In searching various sites, there is always so much discussion as to which transformer may or may not work in these amplifiers by way of manufacturer or general impedance offered, but little to no discussion as to how the type of bias used impacts performance for a particular load impedance used. By comparing the performance of the Heath transformer to the data I had already developed for the Dynaco transformer relative to bias, it becomes easy to see where the load impedance needs to be for the type of bias used. Hopefully, it becomes one more tool to help folks get the best out of what they've got!

Dave
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Postby GreatTone » Fri Dec 17, 2010 12:02 pm

Hey folks!

I'm new here. I just got an SCA-35, and am reading about this mod with interest. Upon plugging in my new amp, it promptly blew the bypass cap section of the filter can. I tacked in a 100uF cap for now, but I'm thinking of getting one of the power supply boards that are available. These boards have separate cathode resistors for each tube pair, so I realize I would have to add the additional pair of trim pots to do the mod. Or I may just replace the filter caps and keep the single cathode resistor and use a matched quad.

The new power supply boards increase the filter capacitance a good bit -- does this affect this mod at all? Also, if I just want to replace the filter caps myself, should I increase the capacitance? If so, what are the upper limits for the SCA-35 and/or this mod?

Thanks for the great ideas -- I think I'm going to like this amp.
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Postby dcgillespie » Fri Dec 17, 2010 2:15 pm

Hi GreatTone -- welcome to the group and thanks for your interest in the EFB mod. I saw your question posted over on the Tronola website, but thought it would be easier to answer your question over here.

The replacement cap boards do usually include individual cathode resistors and bypass caps, but you would not use any of these components with the EFB modification. However, with slight modification to these boards, you could still use their provision for these components to mount the new 5 ohm cathode resistors the EFB modification requires. Note however that with the modification, these resistors do not connect to ground as the original components of the replacement board do. That is why some modification to the replacement board is required when used with the EFB modification. Also note that the modification still requires the use of the original Dynaco cathode bypass cap. Therefore if you are using a cap replacement board, you will still need to provide a similar single cap, connected as shown on the schematic. No doubt, the replacement cap board would be an excellent location for this component as well. So, while you would not use any of the cathode components the replacement cap board was originally designed for, it could still serve as an excellent platform for the new EFB components with slight modification.

The original article was written for modification of a stock amplifier, so obviously, non-stock situations will require careful thought for correct implementation. Others have been able to perform the modification successfully in non-stock situations as this thread has shown. Hopefully, they can chime in with their experience to help you out as well.

I hope this helps. Good luck, and keep us posted on your progress!

Dave
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Postby GreatTone » Fri Dec 17, 2010 2:58 pm

Thanks for the reply. If I'm not going to use the cathode sections of the replacement board, I need to decide if I shouldn't just skip the board and replace the filters. There are some cans available that are close to the original values.

One thing though -- on at least one replacement board, some of the cap values have been increased to as much as 1800uF. How high is it wise to go, or should I just stick to the stock values?
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Postby dcgillespie » Fri Dec 17, 2010 3:20 pm

The principle job of the "cathode cap" of the EFB mod is to provide regulator stability. Therefore, the stock Dynaco value is more than capable of the job it needs to do. The impedance of the 3TR is so low (typically .01 ohm), that the value of this cap provides little benefit to the actual audio signal. It can of course be further bypassed with smaller caps to assist with the supersonic characteristics of the cap, but it is not there to provide bypassing in the traditional cathode bias sense. While the bias is still "injected" at the cathodes, the modification causes fixed bias operation to occur in the output stage, every bit as much as if negative bias were injected into the output tube grids in traditional fixed bias fashion.

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Postby GreatTone » Fri Dec 17, 2010 5:43 pm

Right, I'll keep the 100uF cathode cap. How about the other filters? As I mentioned, some of the others on the replacement board have been increased as high as 1800uF and more. If I just replace the filters, should I keep the stock values or is there value to increasing them?
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Postby dcgillespie » Sat Dec 18, 2010 2:19 pm

There is certainly value in increasing the size of the B+ filter caps, although 1800 mfd is quite a stretch. Generally, going much beyond 100 mfd at any B+ take off point reaches a point of diminishing returns. I would be careful about increasing the size of the cap at the output of the rectifier, unless you plan on adding an inrush limiter to protect the diodes and transformer windings.

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Postby GreatTone » Sun Dec 19, 2010 12:08 pm

Thanks for the filter cap info. Another question, if you don't mind -- are plain IN4007 diodes okay replacements for the originals? I have several sitting around I could use. I'll probably add a CL-90 inrush limiter just to help with a slow start.

I hope to be able to contribute answers instead of just questions at some point.
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Postby dcgillespie » Sun Dec 19, 2010 12:48 pm

That's what these forums are for -- to ask questions, learn, and give feedback. It helps all of us!

The 1N4007 diodes are excellent replacements for the original diodes from a specification standpoint, and will work just fine -- particularly nice since you already have some on hand! My particular SCA-35 in fact still has the original diodes in place.

As you gain experience with your amp and wish to explore other things to enhance it's performance, many feel the use of ultra fast switching diodes (i.e. UF4007 in this case) provides improvement by minimizing the switching noise created in conventional Si diodes. An alternative is to bypass each traditional diode with a .01 mfd cap to minimize the noise, but be aware that these caps need to be rated at 1000 vdc themselves to handle the maximum PIV that appears across these diodes.

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Postby paart » Sun Dec 19, 2010 7:15 pm

The addition of power supply capacitors was addressed in detail by Walt Jung in and article titled: "Build an ENERGY STORAGE BANK"--Audio Magazine, August 1980. He states that increased capacitance improves performance that is noticeable in "bass separation, and reproduction, greater power, and in some cases increased clarity and definition---from top to bottom."
This has always been my experience, also. Anytime that I've added increased capacitance to an amplifier, I've heard a noticeable improvement. His article goes on to explain in detail, the technique of adding 200,000mfd to a solid state amplifier!

Near the beginning of the article, however, he notes in bold print:-this is a project for the advanced experimenter only, and not for the novice! At a minimum, a number of safety features MUST be incorporated, as well a soft start circuit, well thought out bleeder resistors, and careful selection and testing of the components involved. The amount of energy stored in a 200,000mfd, 100volt supply is shocking! Or, as one wag put it, these kinds of circuits literally can, and do "raise people from the dead" (defibrillators)!
The ST-35 that I'm currently building will incorporate power supply caps in the range of 1100-1200 mfd. However, I'm also using an external "ramp-up" device, 3 amp diodes, bleeder resistors, and I'll do lots of careful testing.

In short, one can't just go and add lots more power supply caps and "turn it on". That asking for trouble.
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Postby Geek » Sun Dec 19, 2010 7:26 pm

paart wrote:The ST-35 that I'm currently building will incorporate power supply caps in the range of 1100-1200 mfd. However, I'm also using an external "ramp-up" device, 3 amp diodes, bleeder resistors, and I'll do lots of careful testing.


Which will save your diodes on charge, but not your transformer in the long run.

Dude, seriously, you do not need that much cap.

Cheers!
-= Gregg =-
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Postby EWBrown » Sun Dec 19, 2010 7:27 pm

A few years ago I did some work on a friend's VTL MB450 monoblock amps. These each use eight 6550Cs in a 4X2 PPP circuit, and the amps each have two very large 3200 uF / 450V caps, connected in series, with "dividing" resistors to equalize the load and voltage across each cap.
So we're talking 1600 uF on the main B+ HV supply (about 550-600VDC)
They each are good for 450W in pentode mode and 250W in triode mode.

The paralleled 12AT7 VA stage has its own small PSU, and the 6350 LTPI feeds from the main PSU and a chain of a few more Rs and Cs to decouple it from the OP section.


As fas as the DIY35 boards, I've typically used four 180 uF / 450V caps,
or four 180 uF / 500V caps, which I obtained at a junque shop in Milford NH a few years ago. These are the right size (22mm X 30mm) to fit on te board and under a 2 inch high chassis. This is a LOT more capacitance than with teh original small multisection can cap in the Dynaco ST35, and the differences in sound are quite evident. The Dynaco capacitors are just barely minimum which will do the job acceptably (think of bean counters holding the trump trump card over the design engineers on this one).

At teh last NEAR-fest in NH back in 2009, I got two very low ESR 1,800 uF / 450V caps, for some future heavy duty amp project.


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