Improved SCA-35/ST-35 Performance

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

Postby dcgillespie » Tue May 17, 2011 2:53 pm

For those of you who may not feel comfortable installing the point-to-point version of the EFB(tm) modification in your amplifier, I have made arrangements with Pacific Audio Regenesis to include the modification on a new power supply board they offer to replace/upgrade the SCA-35's original power supply twist-lock can caps. This significantly simplifies the installation of the modification, making it a much easier task to perform now. You can contact their website for more information.

Dave
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St35 and preamps (or not).

Postby Geezer » Fri May 20, 2011 8:34 pm

Thanks much to Dave for the detailed info and the time he's devoted to addressing my bonehead questions and concerns.

For Dave and anyone else out there who's reading this, I have all the parts on order and have been doing advance planning for doing the EFB upgrade on three amps--I picked up a pile of original Dynaco gear at an estate sale several years ago and much of it has been gathering dust--an ST35, an ST70, four Mark 4's, two Mark 3's, an SCA35 and a pair of PAS preamps. I got suckered into shelling out $450 for the trunk-load of gear. I've picked up two more SCA35's since then and have completed three diytube ST35's with the pirated parts.

My bench is stacked deep with works in progress. I've finished a stock rebuild of the ST35, a VTA-80 ST-70 upgrade, a pair of Mark 4's with Shannon's Poseidon driver and a plain-vanilla diytube ST35 using the stock BOM. I rarely finish a project then leave it alone; I've played around with voltage regulated power supplies, CCDA designs and other gizmos trying to learn more about what's audible and what ain't in amp design.

I stripped all three of my SCA-35's to pirate the iron and chassis', thinking that the original circuit was so dated and compromised that it wasn't worth rebuilding. However after reading Dave Gillespie's comments and corresponding with George Ronnenkamp at Pacific Audio Regenesis I plan to order a set of SCA35 replacement boards and rebuild one of the SCA35's to spec (with EFB, of course). George took the time to write me at length and I was surprised to learn that the PECs in these half-century-old amps survive the test of time. I had taken one look at these stone-age spider-like proto-integrated-circuits and decided they were trash but George says that's not the case.

Those projects are in the future: My major project at hand is a diytube ST35 with a two-channel EFB module. Before learning about Dave's mod I had already started on a pimped-out version--tube rectifier, beefed-up PT, PRP/Takman/Mills resistors, Auricaps, a stepped-ladder attenuator and a matched quad of near-new Mullard EL84's I discovered in one of my garage-sale amps. With EFB I've gotten more excited about the possibilities of a maxxed-out ST35.

I know that the ST35 has adequate gain to function with a 100K volume pot and line level inputs, but I'd also like a balance control due to a crappy listening room. I thought I could simply include a balance control or separate volume pots for each channel, but after reading Dave's articles and learning that the original circuit design causes one channel to leech gain from the other, I would probably have gotten some weird and frustrating results.

I apologize for the lengthy lead-in, but from the beginning I intended to build my upscale diytube ST35 as an integrated amp...for starters I'll just be including line inputs and a 3-way source selector switch but I still listen to vinyl and will add a phono section if the amp meets my expectations.

I'd appreciate any opinions on how to handle the input--Maybe I could probably get by with a passive preamp, or no preamp at all. Since I have other Dyna gear sitting around I considered using the original SCA35 or PAS3 preamp sections in conjunction with the diy ST35, but that presents some problems.

I've built a couple of items from Glass Audio, checked out their site the other day and spied their ACF-2 (Aikido Cathode Follower), a highly customizable, simple and inexpensive line preamp ($35 for the PCB). It's a unity-gain buffer for folks who don't need any voltage gain, but do need more current than a passive line stage provides. It has a 4"x6" footprint, modest power requirements and accommodates a variety of tubes--I'd probably keep it simple and go with a pair of 12AU7's. You can see it at: http://tubecad.com/2011/01/blog0198.htm

I'm a believer in KISS and I don't want to clutter up the signal path any more than I need to. I'd greatly appreciate feedback from builders who've tried different methods for preamplification and input selection with the ST35. In a sense the diy ST35 has a built-in line preamp already but if using more than one input (or considering a phono input) a unity gain buffer seems like a clean, simple and inexpensive solution for matching inputs and eliminating noise.

To ask a slightly different question, if you choose to use the ST35 with a single dedicated input device--a CD player--is there ANY reason to use a passive or active preamp or will a simple volume control provide clean and balanced sound?

Thanks for any thoughts you may have,
Dave
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Postby dcgillespie » Sat May 21, 2011 12:01 pm

Man, if you picked up all that gear for $450, you got a steal!

Personally, I think the ST35 needs a little bit of gain in front of it. I am most comfortable with a line level sensitivity of about 250 mv. The 35 has an input sensitivity of 1.0 vac, which works well enough for hot CDs and some tuner outputs, but gets iffy on lower output sources. Therefore, something with a gain of about 4 is optimum.

On the other hand, if passive controls work well for your sources, a quality level control in conjunction with a standard PAS or SCA35 balance control will work well. The balance control Dynaco used was rather unique in that when centered, it does not produce any attenuation for either channel. When it is rotated, it then starts to attenuate one channel, based on the direction of rotation, but leaves the opposite channel unaffected.

This is quite unlike the balance control as used with Eico equipment for example. When their balance control is centered, it produces an equal attenuation in both channels, so that when rotated in either direction, it not only attenuates one channel, but accentuates the other, providing more range. This type of control would not be appropriate with a passive control center due to the loss of signal it affords when centered.

Personally, I prefer the Dynaco approach, as it works just fine, and is not as prone to noise. If you use one of these with a level control for a passive approach, the signal should first go to the balance control, and then on to the level control.

Good luck!

Dave
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Thanks again

Postby Geezer » Sat May 21, 2011 8:43 pm

Dave, you are a fountain of knowledge...I would have used a replacement balance pot and never have guessed it could cause problems. And yeah, the Dynaco score had me smiling for some time.

Since you're an original Dynaco guru and speaking of unique balance controls, I'm now restoring a PAS-3X preamp--the one with patented tone pots that Dynaco claimed were transparent at center position, like a bypass switch (I expected a detent at dead center and have seen references to one in forums but mine are the real deal--I checked the part numbers--and there's no notch). The only difference I noticed between the PAS-3 and the 3X on the original PCBs was a diode between the ganged pots and the line amp board.

I'm using a pair of completed Vintage Electron PCBs, which copy the original circuit but use modern components--metal film resistors and film or silver mica caps.

Nearly everyone recommends eliminating the tone controls when rebuilding or modifying PAS preamps. Do the "3X" pots perform as advertised--will I sacrifice fidelity with the tonal circuits in place? I spent a couple of hours cleaning up the pots--I first squirted them with De-Oxit but so much green gunk foamed out I figured they needed a more thorough approach so I crossed my fingers, soaked them in pure isopropyl alcohol for a couple of hours and worked the wipers, then let 'em dry out and hit them again with De-Oxit. My DVM tells me that they're in good shape. If you know of any other chemical cleaning tricks for 50-year-old components I'm open to suggestions (I've got a pair of SCA 35s on a shelf that are intact except for the pulled transformers, plus a third that I've stripped and cleaned).

I had planned on using the three SCA chassis' for non-Dynaco diy projects--as you know, they're cleverly made with shields of a sort isolating the front controls and rear I/O's from the transformers and circuit boards. BUT, now that Audio Regenesis has replacement PCBs I'll be rebuilding at least one...I've begun to share some of your enthusiasm for them (or at least for their potential with some simple mods). I powered one up on a Variac when I first got it and though it survived the smoke test it sounded very muddy due to oxidized controls, ancient caps and random mouse turds after decades in storage.

I figured they were only good for salvaged transformers but there's definitely something cool about the little EL84 integrated amps, which is why I'm building one using a diytube PCB and a simple line stage as I mentioned. But why reinvent the wheel? I wrote off the SCA design because it's old but duh, it's got vacuum tubes. There aren't that many different ways to skin an electron, if you catch my drift.

The only thing I dislike about the SCA35 and the PAS preamps is there's so much crap in the signal path in the form of cheap pots, switches, PEC's, filters etc. That stuff was useful in the days of vinyl records and ceramic phono cartridges but with digital sources and a turntable/cartridge that set me back a mortgage payment it's worse than useless...I dunno. Sometimes I forget that Dynaco and Dave Hafler provided hi-fi for the masses and somewhere for geekish kids like me to spend our paper route money; it wasn't meant to be high end, just an affordable step up from Magnavox and Sears Silvertone for stoneage audio nuts. After a lifetime of upgrading my sound system every four or five years and hearing improvements with every step I'm now listening to the amps I used to drool over in the latest Allied Electronics catalogue. There's some irony there, someplace.

Thanks for your last email, I agree that the ST35 likes a boost up front but others think differently...I'll probably go with a simple active line amp and an eye towards adding a phono section later. Glass Audio has several cool gizmoes, John Broskie's CAD approach to audio design results in PCBs with nearly limitless user options. As a non-engineer I wish he'd just say which approaches SOUND best but that'd take away the fun and sense of ownership, I suppose.
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Postby wicked1 » Tue May 24, 2011 2:03 pm

I think this is an obvious question, but I haven't thought much about electronics in a couple of years...
My tubes aren't matched... I don't know how off they are, but it's a set of NOS soviet tubes. (I'm running the st-35 board w/ individual adjustments for each tube)
So I'm guessing I should implement the 4 individual grid pots to adjust them. Is that correct?

I guess a better question is, is it needed or worth adding the individual adjustments for each tube?

If so, what wattage pots should I look at? Just the standard 1/2 watt trimmer pots, similar to what are already used in the st35?
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Postby dcgillespie » Fri May 27, 2011 6:17 pm

Wicked -- It is always best to use matched tubes for a number of reasons, be it improved performance within a given channel, to helping match performance between the channels. This assumes of course that the tubes are properly matched, both staticly (at a close quiescent operating point), and dynamically (with an AC signal applied).

If you have an odd lot group of tubes to use, using the individual biasing method would certainly help, by at least allowing the quiescent currents to be matched. While this might not achieve the best performance possible, it is certainly preferred over no adjustment at all, as it will enhance OPT performance at the lowest frequencies. To properly use odd lot tubes, not only should the quiescent currents be matched, but the AC drive to the output tubes should be adjusted (by an AC balance control) to match the dynamic currents as well.

In any event, if you do use the individual biasing method, the individual controls can be very small fractional watt devices, as the current through these components is quite small.

Thanks for the interest, and good luck with your amp!

Dave
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Postby wicked1 » Wed Jun 01, 2011 12:49 pm

dcgillespie wrote:If you have an odd lot group of tubes to use, using the individual biasing method would certainly help, by at least allowing the quiescent currents to be matched.
Dave


By "odd lot" do you simply mean untested? All the tubes are from the same lot, or batch, I'm sure. So, is that probably close enough, vs being from different batches/years of tubes? Or do you mean simply an untested set of tubes? (I've got 16 of them, but no tube tester.. Maybe coming up w/ a tester is the next logical step... it's always something :) )

thanks again.
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Postby dcgillespie » Wed Jun 01, 2011 3:32 pm

Wicked -- I apologize, as I should have been clearer. By "odd lot", I simply mean tubes of various use levels, age, bias requirements, manufacture, etc. All tubes that are good mind you, but simply all different from each other.

While using tubes from the same manufacturer is surely a good place to start, tubes, like any other manufactured device, all have tolerances that can cause their specific action in a circuit to vary slightly from even the tube manufactured right before, or right after it. Therefore, rating them, and then picking two or four of the same rating is what produces matched tubes.

To do this process well, tubes should be matched under realistic real world static (quiescent) conditions -- where tubes requiring the same bias voltage to produce the same current flow are chosen -- as well as under large signal (dynamic) conditions, where tubes are chosen that are capable of producing the same level of power output. Tubes that are matched under both of these conditions are truly matched, so that it really doesn't matter what their age is, use level, origin, or whatever.

For new tubes, it is worth the money to buy well matched tubes from a reputable vendor like Jim McShane. You will be guaranteed good results from tubes that he sells. For existing tubes, a good tube tester will help, but its ability to help match tubes is very limited as virtually all tube testers test power tubes at rather low operating voltages (typically no more than 175 vdc), and at very low signal levels as well. Actually, it is not at all difficult to use the converted amplifier itself to help match the tubes for you.

One of the really nice side benefits of installing the EFB modification, is that the all important concern of never operating the SCA/ST35 without all the output tubes installed is completely removed. With the EFB modification, it is perfectly safe to operate these amplifiers with any number of output tubes installed, as bias is no longer determined by current flow through the tubes, but by the EFB regulator circuit. Therefore, it is possible to leave one channel's set of tubes installed, while using the other channel to operate and measure one tube at a time, and match your tubes at least for similar quiescent performance that way, which is the most important feature to match in a given pair of tubes. If/when you install the modification, I'll be happy to help you through that testing process.

Good luck with your amp!

Dave
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Postby corndog71 » Sat Jun 04, 2011 7:17 pm

dcgillespie wrote:For those of you who may not feel comfortable installing the point-to-point version of the EFB(tm) modification in your amplifier, I have made arrangements with Pacific Audio Regenesis to include the modification on a new power supply board they offer to replace/upgrade the SCA-35's original power supply twist-lock can caps. This significantly simplifies the installation of the modification, making it a much easier task to perform now. You can contact their website for more information
Dave


This is very cool and very reasonably priced. It's clearly made for the SCA35 but could it also be used with an ST35 circuit? I really like the DIY Tube circuit board which uses a 12AX7 and 12AU7 vs. the 12DW7/7247 but I'm not certain I could integrate it with the audioregenesis EFB board.

I'm sure I'm making this more complicated than it needs to be,
but nobody makes a single PCB of what I want. I was almost considering breadboarding the whole thing but just drawing it out gave me a headache.
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Postby dcgillespie » Tue Jun 07, 2011 10:12 am

Corndog -- Actually, interfacing the Audio Regenesis EFB power supply board with an existing diytube Stereo 35 board would not be all that difficult, but there would also be some redundancy as a result.

First, you would need to make sure you had available room for the AR board, and that there was sufficient air flow at its place of installation to allow the heat sink on the AR board to perform properly. It that regard however, the AR board is one tough cookie. Besides the other premium grade components, it uses an identical but higher rated industrial version of the LM337 commercial device I originally specified in the article, and a sizable heatsink (for the application) as well. This allows the regulator on the AR board to remain well within its maximum temperature rating as installed inside a buttoned up SCA35 amplifier, even after one hour of full sustained power output in both channels. We surely know how hot the inside of an SCA35 gets, so the requirements here are hardly stringent, but certainly should be observed.

Then, on the DIY board, some modification would need to be done to allow for the installation of the grid stopper resistors and the screen stability resistors at each output tube. Also, each output tube cathode would need to be completely disconnected from its respective cathode bias network on this board, and the cathodes of the output tubes in each channel then connected together with a jumper.

Finally, there would be four jumpers then used to connect the two boards together:

1. One from the left channel's output tube cathodes to the AR board left channel cathode connection.

2. One from the right channel's output tube cathodes to the AR board right channel cathode connection.

3. One to connect the power supply grounds of the two boards together.

4. One to connect together where the OPT B+ leads connect on these two boards.

And that would be it. In using this approach, capacitance on the AR board for the output tubes would now augment the capacitance already provided for that purpose on the DIY board, which would be an extra benefit. The test connection posts and bias control on the AR board would then be used to set the output tube bias as instructed in the article. Of course, AR builds the board for installation in an actual SCA35, so there will be no support from them regarding this type of installation, as would be expected. But you can see that the work involved would not be all that difficult if you are comfortable with such work, and there is space available to accommodate the AR board.

As you make your decisions moving forward, this at least gives you an idea of what would be required to add the EFB modification to a DIY Stereo 35, by way of using an AR power supply board with the EFB.

I hope this helps!

Dave
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Postby Dynaco_WJW » Sat Jun 11, 2011 1:50 pm

I placed a new post where a SCA-35 is 'restored' with a complete set of new boards from http://audioregenesis.com with also the new Power Supply Capacitor Board with EFB™
Photos of the process

This is the link to that post http://www.diytube.com/phpBB2/viewtopic.php?t=4818
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Postby dcgillespie » Sat Jun 11, 2011 5:18 pm

Dynaco_WJW -- A very nice job indeed! The picture documentation you provided throughout the installation shows as well on your work, as it does on the Audio Regenesis product. They did a wonderful job of taking my EFB(tm) modification, and integrating an exact copy of it into their new power supply board for the SCA35. It makes the installation of the modification extremely easy now.

I hope you'll report back on the sonic impact it provides once you've had a chance to really give a listen to it. In the mean time, enjoy the greater power, lower distortion, and longer tube life the modification provides.

Thanks for the interest, and for sharing your work!

Dave
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Postby Dynaco_WJW » Sun Jun 12, 2011 5:38 am

Mr. Dave Gillespie -- Thank you for your compliment. In fact the nice job was already done by Audioregenesis. He made these beautiful circuit boards! The only thing I had to do was to solder the leads on the boards. I have to make you a compliment also for the EFB 'invention' !

I bought a SCA-35 in 2005 I guess, I immediately liked the design of the cabinet, now knowing what kind of electronics was under the hood! I was looking on the Internet and found lots of articles about 'making the SCA-35 better'. Like bypassing things and use other capacitors. This is what I wanted to do....until...I found the website http://audioregenesis.com I was a bit surprised why I didn't found this website earlier! The way how Dynaco's where restored are fantstic. So I didn't hesitate to buy a set of circuit boards for the amplifier. One of my SCA's had already a power board from audiolabs I believe. I remember when I put that audiolabs board in a SCA, I couldn't reach the holes for soldering anymore so I had to remove the board again and first attach the wires. This is the difference with boards from Audioregenesis. That particular SCA-35 I traded with a brand new PAS-3X in the original box so I was happy !!
To mount the new set of boards is very easy! In a few hours work I have a amazing amplifier. My next project is restoring a ST-35.

About the EFB. I compared just one song yet. Madonna - The Power of Good-Bye. First I listened the song a few times with SCA with EFB, then I listened the song on another SCA. The difference between these SCA's is the power board (with and without EFB) and the output boards (PC-10A against PC-10). The volume is at about 80% (3 o'clock). What I heared is that without EFB there is much more noise. I guess that is distortion. As if there is much more sound what doesn't belong there. After listening a few times without EFB I went back to the SCA with EFB. What I heared is, that there is more 'rest' in the song! The voice is more clear. The low frequencies sound fantastic on my concrete http://rauna.com speakers. It sounds as if it's no problem for the amplifier at all to bring up the volume. Maybe the most important thing is to hear that the sound is just so pleasant for your ears! So smooth, like honey.
Right now I'm listening in my 'listening room' (4 x 5 meters). It's a great pleasure!

Here is a photo of the difference between the two amplifiers. Greetings from The Netherlands, Europe.

Image
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Postby dcgillespie » Sun Jun 12, 2011 3:07 pm

WJW --Thanks for the kind words, and for adding your comments on the sonic impact the EFB(tm) modification made to your system. It mirrors my own observations, and those of others that have installed the modification as well. With it, the increase in available power and reduction in distortion can easily be measured, and from all published observations to date, can just as easily be heard in the listening room.

You have an impressive collection of Dynaco gear! Good luck with all of it, and keep on enjoying the music!

Dave
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Postby Dynaco_WJW » Sun Jun 12, 2011 4:33 pm

Dave -- Thank you! My next project is restoring a ST-35. I already disassembled it and I will do a new post for this project. I struggle a little bit with implementing EFB in the chassis. I have plans to bring the heat of the voltage regulator to the chassis via a piece of thick copper. I don't want to drill in the chassis. Pity there is no (not yet?) Audioregenesis board available for the ST-35 with EFB. I bought the PC-13 Output Board Set for this amplifier.

About the new power board with the (military?) voltage regulator. It has a heat sink what keeps it quite cool. With the cap removed and the amplifier on full power I easily could touch the heat sink, not hot at all. Thanks to the special regulator and the heatsink there is plenty of headroom left.

How can I measure the signal? Can I do it with a scope and sine generator? Where to measure? just across one speaker?
This afternoon I listened a lot, and my ears didn't get tired at all! lovely!
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