Hidden Treasure: Bringing Out The Very Best of Dynaco's SCA-35

[dcgillespie]

This thread presents a significant work for this amplifier, examining all of its virtues and warts, providing explanation of all the various issues, and presents real modifications for resolve. It will be presented in a number of sections as the thread develops, which will allow for questions or discussion between the sections from those who might be interested as the material unfolds. The work will be posted at a later date on the Tronola site as one complete piece, so that easier reference may be made to its various sections.
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The SCA-35 has always intrigued me ever since I built one as a boy. As an easy to build kit, its high quality transformers, UL operation, and straight forward design made it the ultimate culmination of Dynaco’s long standing design philosophy of getting the most out of the least. With it’s a timelessly good looks, it was yet another of Dynaco’s success stories from the day it was first introduced in 1963.

But good as its foundation and good looks are, it didn’t come without issues. If I was aware of them when I first built one, they were insignificant against the wonderful sound produced from a working kit I built myself. 60 years later however, the issues it has are more apparent to me than ever.

I started looking into its issues about 15 years ago, with the first result being the development of the unique EFB™ biasing concept that produces so many benefits over the bias arrangement it replaces. That effort was laid out in my white paper entitled “ A New Look At An Old Friend”. And while the problems it solves and performance improvements it provides are significant, it still only represents one link in a chain of issues that exist within the amplifier. Since a chain is only as strong as its weakest link, then even with the addition of EFB, the amplifier is still often only considered to have achieved an “also ran” status.

Like the issues EFB tackled, the remaining issues are also due to its extreme simplicity, coupled with costs cut to the bone. With fresh thinking however, many of its issues can be rather easily resolved, making for a significant improvement in listening room conduct and overall impression. A little modern technology can then finish off its remaining issues (if need be) in ways its designers never could. This latest effort then dovetails with my original EFB work by addressing those issues that remain, so as to achieve the full potential of which the amplifier is capable.

Because of the SCA’s unique design however, this has not been an easy project. Not so much because the work has been hard, but because it’s been very tedious and time consuming: as an on-going project, it’s taken years to complete and often times, was just plain frustrating as well. Efforts for improvement in a given area would often result in either incomplete resolve or new problems that didn’t exist before, which would then send an effort back to the drawing board (again). Or, new ideas and better approaches would come to mind along the way to effect a restart as well. New tube testers had to be devised: one specifically to test for the heater/cathode noise that triode/pentode tubes can display (that conventional tube testers cannot detect) when their heaters are operated on AC voltage. Another was devised to specifically measure the AC gain in the pentode section of the driver tube in the SCA design. Along with the time required to just ponder and develop the ever changing approaches used within the overall project then, there was also the significant time involved in wrangling out all the numerous details that presented themselves along the way. Even this writeup has been a number of months in the making. Testing and retesting was seemingly endless, but necessary if the modifications developed were not only to be successful, but successful enough to be well worth the time and effort to implement.

In the end, while all of the modifications developed are quite effective, some may seem to be so simple as to either be obvious, too simple to be effective, or beg the question as to why they took so long to be developed. Some were obvious. Some are incredibly simple. But others were anything but. If they were all so obvious and easy, they surely would have been presented by now. Some have in one form or another. Others are entirely new with this work. To make for a truly improved amplifier however, all of the areas must be addressed in a single effort to strengthen all of the weak links in the amplifier’s design chain. Otherwise, any remaining weaknesses will continue to define the SCA, of which a number still do to this day. That has been the amplifier’s history that this article seeks to change once and for all, and is the real reason that collectively, the modifications took so long to be developed. From this work effort, they were either all going to be presented as a complete package for resolve, or not at all. The SCA doesn’t need anymore piecemeal efforts to address its weaknesses in a one at a time, here and there fashion.

What follows then is a bible of sorts, discussing all the various issues identified in which the amplifier’s history, and the realities of today’s vacuum tube world have found the original design to be wanting. Individually, some of the issues are rather small or seemingly insignificant, but together they all add up to
collectively take their toll on the impression the amplifier produces and performance it delivers.

The issues have been divided into two basic groups: Those in Group 1 address five basic issues with the original design but importantly, resolves them in a very Dynaco like fashion: simple modifications are presented that effectively reside within the framework of the amplifier’s original design. This group also addresses important details for two popular modifications performed on these amplifiers as well. The Group 1 modifications will therefore be of interest to all.

Group 2 addresses one area from Group 1 more extensively, and more complex issues relating specifically to phono preamp performance. It also offers an additional more general modification of the amplifier for consideration as well. The Group 2 modifications are the most complex to execute, but fully eliminate all of the economically driven design limitations of the amplifier.

To facilitate the project, two test/development SCA-35 amplifiers were used. The first amplifier (designated as the Mark I amplifier), has all of the Group 1 modifications installed, and still contains all of its original boards and can caps, and continues to use 7199 driver tubes. This amplifier also includes a point to point installation of EFB, being the original amplifier that modification was developed on — but it goes no further. The second amplifier (designated as the Mark II amplifier), has all of the Group 1 and Group 2 modifications installed, and has been restored with all new boards, including amplifier boards that directly accept alternate driver tube types, and a power supply board that replaces the original can caps and incorporates EFB. Some modifications presented in Group 2 replace those presented in Group 1, so the Mark II amplifier has had the installation of its Group 1 modifications adjusted accordingly.

Before continuing however, a word is needed about what this project is not: It is not about addressing the subjective sound qualities of the amplifier and what components might produce a desired outcome towards that end. This is not to say that such an approach is not without merit. For example, work on this project has uncovered reasons as to why some tubes may in fact sound different from others in the phono preamp and driver sections of the SCA. The reasons uncovered allow for real understanding to be had, rather than simply assigning the differences to popular notions regarding the goodness of one tube brand or type over another. The article is also not about dressing up the amplifier with improved jacks, connectors, and power cords, or creating a new look, either. Those too can be worthwhile efforts as well, but are not a part of this project.

The SCA-35 was the result of an engineering exercise designed to produce a targeted outcome for the amplifier’s original concept. Similarly, the modifications presented here are not a list of popular subjective recommendations, but modifications developed from an engineering exercise, that are designed to produce the desired overall improvement in performance sought.


THE TARGETED ISSUES

Collectively then, the total opportunities identified in each group include:

GROUP 1

1. Power Amplifier Stability and Performance— Particularly when using alternate driver tubes with adapters, or boards designed to directly accept them.
2. Improved Preamplifier Stability — Squeals and/or putt-putt sounds emitted when the Phono or Tape Head setting is selected with open inputs at elevated volume control settings (particularly notable when the tone controls are advanced from the flat position). Also, correcting an early version selector switch issue.
3. Improved EFB — Reducing regulator operating temperature and maximizing regulator protection.
4. A Better Loudness Switch — Providing dual frequency boost.
5. A Better Filter Switch — The problems the original design causes, and a popular repurposing for its function.
6. Amplifier Hum — Produced from the use of AC to power the small signal tube heaters and pilot lamp.
7. Phono Preamps — Conversion of Tape Head Input into a second Phono Input, which is also a prelude for further improvements offered in Group 2.

Modifications in this group strike directly at the character and conduct of the original SCA-35. They also enhance tonal and source flexibility, and long term dependability as well. They resolve all of the amplifier’s notable issues in the most economical way possible.

GROUP 2

8. High Level Input Sensitivity — Adding an active line stage amplifier.
9. The Case For DC Heaters — Benefits beyond hum reduction.
10. Phono Preamps — Spotlighting significant design issues and possibilities for resolve.
11. Tape Output Drive Impedance — Too high for driving modern external SS devices, affecting Phono Preamp performance.

The first two modifications in this group impact the entire amplifier, while the last two tackle issues associated with the phono preamp section. When implemented along with the relevant Group 1 modifications, the Group 2 modifications then make for the most complete redress of the original design.

None of the modifications in either group require any chassis work, so the modified amplifier still retains its original appearance. Even more important, the design philosophy of the original amplifier is still honored and maintained, so that the fully modified amplifier still retains its identity as an SCA-35 — even down to its simplicity of execution. Finally, all the modifications are fully reversible, so any or all can be removed to restore the amplifier to its original build as might be desired.

We'll dive head long into the Group 1 issues -- next time.

Dave

Below: A tale of two Dynacos -- From a front view, both look identical on the outside, and slightly different on the rear side which you'll see later -- but neither have any of the warts of the original design. The Mark I is on top, and the Mark II on the bottom.

 

[Dswankey]

LOVE these threads!!!, thanks for sharing Dave!!

 

[peterh]

This will be interesting !!

 

[dcgillespie]

EXAMINING GROUP 1 ISSUES IN DETAIL
AND PRESENTING SOLUTIONS


1A. Power Amplifier Stability And Performance

A popular modification for the SCA is to use alternate driver tubes in place of the expensive and increasingly rare 7199. Socket adapters are available that will allow alternate tube types using the 9AE /9DC pinout configuration to be installed in place of the original 7199 tube used in this application. Replacement boards are also available that retain the original circuit, but allow for the direct installation of the alternate pinout tubes without the use of adapters. Since both of these approaches can work, use of the alternate tube types often receives little more thought than simply addressing the different pinout requirements involved. Both of these approaches do generally “work”, but they offer only varying degrees of success, as a proper conversion requires circuit modifications to ensure the original stability characteristics of the amplifier are maintained when using the alternate tubes.

The first issue involves the fact that the triode section of the 7199 was intentionally designed with a lower Gm (transconductance), lower amplification factor, and higher plate resistance specifications compared to the alternate tubes. This allows it to operate as a phase splitter that can directly drive a push-pull output stage, without generating any instability due to the significantly different drive impedance levels this type of inverter inherently presents. Using tubes with a higher performance triode section accentuates the difference in drive impedance, and therefore invariably invites instability in the push-pull connection on transient and/or high level signals when alternate tubes are used. Modifications provided with the best replacement boards designed to directly accept the alternate tubes prevent this type of instability from occurring, and will therefore always produce the best results.

Secondly, in the pentode section, the alternate tubes are generally accepted to be similar enough in characteristics to that of the 7199’s pentode section to allow for direct circuit substitution without any changes other than those addressing pinout. That’s typically true for conventional circuit design, but the SCA’s driver circuit is anything but conventional. Unlike most other tri/pent phase inverter/driver designs, the SCA’s driver stage not only employs global NFB like other designs, but also includes a DC feedback loop, and a positive AC feedback loop (PFB) as well. That changes things considerably. Small differences that exist from tube type to tube type produce insignificant differences in the final outcome when conventional design approaches are used. When PFB is employed however, any differences are greatly magnified. The alternate tubes therefore need to perform similarly under the dynamic conditions as presented by the SCA’s driver stage design for their substitution to be successful.

Assuming the alternate triode characteristics are accounted for, tests show that fortunately then, most tubes within the alternate pinout groups will in fact work very well in the SCA’s driver circuit with the addition of a small step network connected to the plate of the pentode section. This network acts to contain a slight parasitic oscillation that can form because — as with the triode section — the pentode section of the alternate tubes are also higher performance devices designed primarily for RF/IF work, and are therefore more prone to developing such oscillations in the SCA’s driver circuit. Again, the best boards that are designed to directly accept the alternate tubes address this issue accordingly.

With appropriate compensation provided for both sections of the tube then, the 6GH8A, 6U8A, 6EA8, and 6LN8 all perform well, producing similar AC gain and distortion levels to that of the 7199 in the SCA’s driver design. One tube however clearly does not — that being the 6BL8. This tube invariably displays instability in spite of measures taken and often, outright oscillation when installed in the SCA’s driver position because under PFB conditions, the gain of this tube is excessive, being enough to cause the circuit to spill over into sustained oscillation. To make matters worse, I have a whole sleeve of Amperex 6GH8A tubes that oscillate wildly in the SCA-35. This is not because of any manufacturing issue with the tubes, but because they are not 6GH8A tubes at all. They are tubes that had their tube type and manufacturer’s logo painted on as was such common practice during the end times of the tube era, when one tube was often chosen to be the replacement for many tube types. In TV sets and conventional circuit applications where most of these tube types were destined for, this one size fits all approach typically works just fine. But under the PFB conditions used in the SCA’s driver stage, the 6BL8 — and tubes manufactured to its specifications but presented as other types — will either not remain stable, or at best only marginally so. Therefore for many reasons, make sure the purchase of any driver tubes comes with a solid return policy in case they just don’t work correctly. The Amperex 6GH8A tubes I have were purchased decades ago, with tests now showing that they act and perform exactly like a 6BL8. Who knew back then? As the old saying goes however, if it walks, talks, and quacks like a duck, then guess what.…..

This emphasizes an important point then: The above alternate tube recommendations are based on original production tubes of bogey specifications for the tube types listed. If tubes are used that are identified to be within the accepted group but are really part of a “me too” re-labeling effort, then how well such tubes perform will directly relate to what tube was used as the origin for the relabeling process. Tubes with painted on information should always be considered to be part of such an effort, and used with caution until otherwise proven appropriate.


But there are also three final points to address:

A. Within any acceptable tube type — 7199 or the alternates —because of the normal manufacturing tolerances that exist during the production of any tube type and the acceptable range in characteristics this produces, and because the SCA circuit uses ~ 10 db of positive feedback, it means that the effects of these tolerances are also greatly magnified. As a result, the AC Gain produced by the pentode stage in the SCA’s driver circuit can normally vary by as much as nearly 90% (or nearly 2X) from one tube to the next within a given tube type. The circuit handles these variations in open loop gain (OLG) just fine, remaining perfectly stable with tubes providing the greatest amplification, while still producing rated distortion with tubes producing the least amount of amplification. The global NFB loop irons out the different sensitivity levels this would normally cause, so in the end, nobody’s the wiser. Except that such differences can impact the sound for those with sensitive ears, because the wide range of AC Gain levels also affects the damping factor of the amplifier as well. Such differences are small within the range of NFB produced, but for the best performance then, the driver tubes in the SCA — be they original or alternate types — should be matched for Gm to achieve the most balanced performance between the two channels. Gm does not directly relate to AC Gain produced, but is close enough to eliminate major performance differences.

B. The given component values for the driver stage are critical to its proper operation. For conventional designs, components with a tolerance of as much as 20% will likely upset audible performance slightly between stereo channels, but will still typically allow the circuit to otherwise perform acceptably well. Such is not the case however with this design, and most particularly for those amplifiers still using original boards and components. As mentioned, one of my test/development amplifiers falls into this category (Mark I amplifier), but its components have been carefully checked and/or replaced as necessary to ensure close adherence to the correct value — not to be just within the tolerance allowed for the component, but to be well within it. Before this was done however, test were made with all of the original carbon composition resistors still in place. These types of resistors invariably grow in value with age and heating. Tests showed that for the original resistors associated with the feedback networks for the pentode stage, an upward rise of just 9% in some values can produce over a 3X increase in AC Gain for the stage with some tubes. Therefore, ensuring close tolerance components are used is not only critical to the operation of the design, but also more important than ever in ensuring matched performance between the channels. It is no wonder that Dynaco supplied these what would otherwise be easy to assemble boards as preassembled and tested pieces with their kits.

C. It should also be noted that the use of alternate driver tubes — either by use of adaptors or boards designed to directly accept them — will generally cause an increase in hum when the hum balance controls are adjusted as instructed by Dynaco. This issues is fully addressed in the section dealing with reducing amplifier hum.

Next up, instability in the phono preamp secton.


Dave

Below: This works for a number of amplifiers, but it's really not a good idea with the SCA-35 (alternate driver tubes with adapters, but using the original boards):


Below: Oh, it usually works pretty well at low power levels (about 1 watt shown here):


Below: But if you push it a bit (scope is not calibrated in this or last shot), this is all to often what usually happens. It is usually better in amplifiers that have been modified with EFB (as my Mark I amplifier has as is shown here), but without the proper modifications, this is usually what you get anywhere north of about 5 watts power output:


Below: How nice it would be if this were in fact a 6GH8A tube, but alas, that is not the case. This tube by all measurements is a 6BL8A, and turns either of my test/development amplifiers into a power oscillator:

 

[daveshel]

Excellent. I started an SCA-35 earlier this year, but the project stalled when I got busy when my work season cranked up, but that's over now. I installed all the new boards and the EFB board. I had started with the tone control bypass but I screwed it up. Now I'm glad I waited.

 

[spantou01]

Yay! Looking forward to this series. I also have a SCA-35 that requires some work, so this is perfect timing for it.

 

[peterh]

The russian 7199 caused oscillation during power up, now i have that explained.

 

[dcgillespie]

2A. Improving Preamplifier Stability

Because of the higher gain involved, the instability noted this section is most notable in the Tape Head setting, but can also occur in the Phono setting, particularly when the Bass and/or Treble controls are advanced. This is really bad behavior and something a well designed amplifier should not display under any condition of use.

The problem can be traced back to an economic error of omission: The phono preamp tubes need to be shielded: At high volume settings, the high gain and close proximity of the power amplifier and preamplifier circuits allow unwanted coupling to take place between them. When that happens, anything from squealing to motor boating can take place depending on how the controls are set, which can vary between the channels, and from one amplifier to the next. Installing grounded shields on the preamp tubes prevents this from happening. But shielding the tubes may require some ingenuity on your part.

Back in the day, tube shields were manufactured that worked specifically with PC board tube sockets. The socket employed a grounded stem that extended up from the socket, running up the side of the installed tube. The shield itself was a ribbed compression type, that was pushed over the tube and the stem, the stem then compressed between the tube and the shield, with the stem then grounding the shield. The sockets Dynaco employed had provisions for installing the grounding stem but they were not included, nor was the board designed to accommodate them either: when installed, the stem was grounded by an extra (center) pin on the socket, so the board had to be designed to accommodate and ground the extra pin — but PC-11 boards do not include these accommodations. Therefore, other approaches must be used.

Compression tube shields are still the best answer for shielding the preamp tubes since they fit securely once installed. They can often be found at auction, or in discarded tube radios from the late 50s that employed PC Board construction. Conventional non-compression shields can also be used (but fit loosely over the tube), or tin available from hobby shops can be scissor cut, and rolled and soldered into a proper size for a correct fit. But in any case, without a ground stem in place, a short wire needs to be soldered between the shields and their respective ground terminals on the Input Selector switch to ground them (R Ch = wafer A rear terminal 3, and L Ch = wafer B rear terminal 3). Wafer A is towards the front of the amplifier. Be sure the ground wire is long enough to allow the shield to be completely removed from the tube so that it can be changed when desired. Once installed, the shields will prevent the unwanted feedback, and produce stable operation at any volume or tone control setting on any low level input.


Early Version Input Selector Switch

The earliest units released included an input selector switch that had short fingers installed on the rear side of wafers A and B and terminal 2 (A2, B2). The purpose of these fingers was to short the output of the phono preamps to ground when they were not selected, to help prevent any bleeding between selected and non-selected sources. Due to the unique design of the preamps however, this usually results in the preamp circuits going into oscillation, which can then be heard very softly in the background when any of the high level inputs are selected. Dynaco very quickly corrected this issue by simply removing the two fingers in question (only the rear side fingers!), which eliminated the problem. Since the low level input jacks signals are also shorted to ground when not selected, the fingers in question were redundant in their purpose, so their removal solved the issue without introducing any new ones.

My Mark I amplifier is an early unit that included the problematic selector switch. The extra fingers in this unit have been carefully removed, and a pic is provided showing the end result.

Next up, an additional improvement to EFB.

Dave


Below: Old compression tube shields from late 50s and early 60s vacuum tube equipment that used PC board construction are perfect. Note the wire soldered to each shield, and connected to terminal 3 (ground) on each wafer. The shields absolutely kill all the instability otherwise noted on open low level inputs at high volume settings:


Below: Early selector switches had fingers on both the front and back side of each wafer at terminal 2. This caused the phono preamp tubes to go into oscillation when they were not selected. The pic shows the finger on the rear (ONLY) side of the wafer carefully removed to prevent the instability from occurring.Terminal 2 is the terminal with the green wire connected to it. Most units don't have this finger, but if yours does, it should be removed:

 

[Mr. Bissness]

Subscribed! Thanks for sharing.

 

[radioactive]

Yay! Looking forward to this series. I also have a SCA-35 that requires some work, so this is perfect timing for it.

I have one as well and will be following.

 

[analogadict]

Yesterday I had taken down my SCA35 from the storage rack and looked at it, wondering what to do with it. Thank you Dave.

I have some novice questions. Is it not possible to split the inverter and driver functions to separate tubes? This may require a single stereo board. IIANM such a board is offered by someone.
Also, about using adapters for alternate tubes with the original boards, the problem is the adapters themselves or something else?

Regards,

Anwesh

 

[peterh]

Yesterday I had taken down my SCA35 from the storage rack and looked at it, wondering what to do with it. Thank you Dave.

I have some novice questions. Is it not possible to split the inverter and driver functions to separate tubes? This may require a single stereo board. IIANM such a board is offered by someone.
Also, about using adapters for alternate tubes with the original boards, the problem is the adapters themselves or something else?

Regards,

Anwesh

Some adapters are high enough that the tube tips touches the case, thus risk to damage them.
One solution is to move the boards to the underside of the chassies, adding small distances between the board and
the steel.

 

[dcgillespie]

Anwesh -- If you start thinking new board configurations, then really anything is possible from separating the existing driver design into separate tubes, to using more tubes (not having to use a pentode section), etc. It just depends on how far you want to go with it. Of course you quickly run into other limits such as physical space and available heater current, so it just depends on how far (or how willing) you are to deviate from the original build to accomplish a goal -- and then at what point does such an effort cease being an SCA-35? Every person has to define their own parameters for that when modifications start becoming so major in scope. For this project, the boundaries were set at keeping everything at least within the realm of the original design and build to provide the most help for the most folks.

As for the alternate tubes, the problem is the tubes -- not the adapters. The higher Gm of the triode section causes instability when output tube grid stoppers are not used in the design, and the pentode section has slightly lower internal internal capacitance that can cause them to produces a higher open loop frequency response, requiring the need for additional step network to account for that. These measures are hardly a major redesign of the driver circuit, but modifications required none the less for the alternate tubes to perform properly in the original design.

Dave

 

[MRL_Audio]

Dave - thanks for sharing this information and your expertise. I love reading these threads and learn so much along the way. It’s certainly a big part of why AK adds so much value to the hobby.

 

[dcgillespie]

3A. Improved EFB™

An additional update for this circuit has been included in Group 1 because so many of these amplifiers have been modified to include it. An update was previously released a little over a year ago involving the installation of 4 diodes to help protect the EFB regulator. The diodes eliminate a regulator startup quirk that is unique to the EFB application which apparently, a (very) few examples of the regulators could not hold up to over time. Installation of the diodes completely eliminates the startup transient otherwise produced, and therefore ensures long term dependability of regulator. The instructions for adding these diodes have been included in this thread. It is highly recommended that they be added to the EFB installation if not already installed.

Recently however, an additional improvement has also been developed that further improves the dependability of the regulator. The improvement promotes much cooler operation of the device, and adds significant protection to it should an output tube fail catastrophically. The modification is simple enough, and places a 200Ω 4W resistor across the IN and OUT terminals of the regulator. Without the resistor, the regulator is handling the full current flow of the two output stages, which is optimally 108 mA under quiescent conditions (with EFB). In that scenario, the regulator is dissipating nearly 1.8 watts, which even with a large heatsink, still produces elevated (but quite safe) device operating temperatures inside a covered SCA that has reached full operating temp in a nominal ambient environment. With the resistor in place however, it now handles the lion’s share of the quiescent current flow (~ 82-83 mA), which then reduces regulator dissipation to ~ 0.42 watts, or just 23% of the previous dissipation level produced without the resistor in place. With the resistor installed, performance of the EFB circuit itself is completely unaffected: The regulator still acts to hold output tube bias voltage quite steady (other than for adjustments it makes due to EFB action), regardless of power output conditions occurring at any given time. This modification also allows for the more readily available LM 337 regulator device to be used, rather than the LM 237 often specified. A pic is provided to show how the new resistor can be easily installed.

Next up: The Loudness Switch

Dave

Below: The new regulator bypass resistor can easily be installed with a couple of T-strips mounted via the mounting screws for the EFB Power Supply Board. The left T-strip's isolated terminals has a lead connecting it to one of the ground points on the EFB power supply board. The right T-strip has a lead connecting its leftmost isolated terminal to the Hum eyelet on the EFB power supply board. These two points are electrically the same as the IN and OUT terminals of the regulator, so the new regulator bypass resistor is connected between them. In this case, the resistor is a 220Ω 2 watt device, versus the 200Ω 4 watt device mentioned in the text. That's because this picture is taken from the Mark II amplifier that also has Group 2 modifications installed, and when that is done, the recommended value of the bypass resistor increases to the 220Ω value shown installed. Other items seen in the pic will be discussed in later sections. Not sure why the flash produced a red hue from the chassis in this pic; it's certainly not there on the amplifier itself:


Below: Instruction for making and installing the Diode Protection Pack modification released last year. It is very highly recommended that this modification be installed on any SCA amplifier that has EFB installed, but has not yet had the protection pack installed. The diodes prevent transients from occurring at startup and shutdown that a very few regulators apparently have trouble dealing with over time. For those rare instances where a regulator has failed for no apparent reason, this modification has stopped that issue dead in its tracks.


Below: A schematic of EFB with both modifications shown:

 

[dcgillespie]

4A. A Better Loudness Switch

This issue is largely the result of the way audio consumers ultimately found this feature to be most appealing: While sound is certainly subjective, most folks who use the loudness feature like it best when both the low and high frequencies are boosted when the loudness switch is engaged. Dynaco however (as did other manufacturers of the day), chose to boost only the low frequencies with its loudness feature, which makes for an uneven presentation at more normal listening levels, and particularly so for those who have lost a fair amount of their HF hearing.

The circuit can easily be modified to boost both sets of frequencies by adding a 220 pF cap (preferably Mica or NPO ceramic) between the unused terminal of the loudness switch, and the “top” terminal (where the signal is applied) of the relevant volume control section (terminals 3&6 by Dynaco’s designation). That is, when adding these caps, make sure they do not cross connect between the channels, but connect within the loudness circuit of each channel.

The modification makes for a more even sound presentation when the loudness switch is engaged, and provides for greater effective range of the treble control when the volume control operates within the range where the loudness function is active. When turned off, loudness boost for both low and high frequencies is eliminated.

A schematic showing this change is provided in the next section coming up.

Next up: The Filter Switch

Dave

Below: In this pic, you can see the two dark brown silver mica caps connected between the top terminals of the volume control, and the unused terminals of the loudness switch (closest to the Stereo/Mono switch). The cap with green sleeving on its leads is for the left channel, while the cap with red sleeving is for the right channel. When the schematic presented in the next section is examined, sharp eyes will note that the build of this kit does not match the Dynaco schematic. The kit was built according to Dynaco's instructions, and the original components shown on the schematic presented do in fact reflected the way they are shown on the Dynaco schematic. Connected either way it works just fine; it's a rare example of un-clarity in Dynaco's otherwise well written Assembly Manual:

 

[dcgillespie]

5A. A Better Filter Switch

Back in the day when this amplifier was produced, any amplifier presenting itself as a high fidelity device had to have filter switches. The intent was to control record changer rumble on the low end, and record scratch on the high end. Dynaco, true to its goal of simplicity, combined both low and high frequency filter functions into one switch — therefore, you either got both, or none. Vinyl play is making a comeback these days, but such issues are now more properly handled at the source (better TTs, tone arms, and record handling), rather than by squeezing the frequency response of the playback signal at both ends of the spectrum. As a result, such filters are an anachronism in today’s audio world. Not only that, but because of where the switch is physically located in the amplifier, it causes problems with modern high output low impedance sources (like a CD player for example) in that these types of sources won’t fully “turn down” when the volume control is set at minimum. This happens because the signal level presented to the filter switch is quite high originating before the volume control, while those appearing in the surrounding tone control circuits operate at a level that occurs after the volume control, which is then further reduced by the tone controls themselves. When the volume control is at minimum then, bleeding can occur between the filter and tone control circuits, so that the amplifier never fully quiets with these types of sources. When the switch’s (lack of) usefulness is coupled with the problems it causes, the easiest and best answer is to simply remove the filter circuits, so that the signal from the Stereo/Mono switch then directly drives the volume control. This surely solves the bleeding problem by removing the high level signals from the tone control area —but then begs the question — is there anything else the switch could be used for that would be more useful?

The usual and most popular answer is to convert the switch to a tone control bypass switch. While virtually never done correctly, this is a popular answer for those who complain about the performance of the original tone control circuits, and regularly report how much better the amplifier sounds without them. While there is some truth to their claims, in reality, the most notable effect with the tone controls bypassed is the improved sensitivity of the amplifier. Things just happen more quickly when the volume control is rotated, so the amplifier presents itself as more dynamic, responsive, and powerful sounding.

But things usually happen too quickly when the tone controls are either completely removed, or bypassed without some attenuation. It also destroys the original gain relationship established between the phono preamps, and the (now) new high level input sensitivity, so that the volume control — more responsive as it is — now operates down in the weeds. After all, the basic power amplifier circuits have an input sensitivity that requires just 0.16 vac RMS to drive them to full power output (0.168 vac at the top of the balance controls), which is often too sensitive for some high level sources to provide a reasonable volume control response. After the losses of the tone controls (~17 db), sensitivity at the high level input jacks rises to the 1.0 volt figure that Dynaco published — which can then be too low for other high level sources. So with the controls included sensitivity is too low, and when simply removing them, it’s too high.

The answer of course is to install a compensated attenuation network, so that when the bypass function is selected (or permanently installed), the signal is attenuated either by the same amount as the tone controls themselves (when set for flat) for a true tone control bypass function or, to some intermediate value to take advantage of some of the much needed extra gain that’s available when the tone controls are out of the circuit. Either way, any attenuation network installed must be compensated to prevent the significant loss of HF response that otherwise occurs were compensation not included. If this last option is chosen, the tone controls won’t be available of course but extra gain would be — and without the volume control operating down in the weeds.

A high level input sensitivity of about 0.25 vac is typically very satisfactory in satisfying the needs of a great many high level sources — both new and old, so this value is a good target to shoot for. For those interested in converting the Filter switch into a Tone Control Bypass switch then, here are two simple networks that are useful in doing so:

To maintain the same attenuation as the tone controls when they are bypassed, network values = 3.3M strapped with a 18 pF 5% silver mica cap.

To produce an increased input sensitivity of 0.24 vac in the bypass mode (12 db gain over stock), network values = 330K strapped with a 180 pF 5% silver mica cap.

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04/2024 UPDATE: It has been determined that the suggested network values of 330KΩ/180pF are only correct when using an original Dynaco PC-10 power amplifier board. When using a PC-10A replacement power amplifier board, the correct network values are 330KΩ/360pF. Please see post# 157 for further information.

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Frankly however, the first network is not very useful as in addition to providing no extra (but needed) gain, due to the high value of attenuation resistor required, the network introduces some notable additional noise under quiescent conditions at minimum volume (hiss). A lower impedance attenuator would eliminate the noise but cannot be used in this instance, as it would also cause excessive loading on the volume control, exaggerating the already slow response the control displays in the stock design.

The second network however is very useful, introducing an appropriate amount of needed gain, with only a very slight amount of additional noise. It also allows the tone controls to now operate as a pseudo second loudness network. That is, since loudness functions are most needed at lower volume levels, and inclusion of the controls reduces sensitivity, then switching the tone controls into the circuit allows for the most flexibility of low level listening — meaning there’s really a very good reason to not just categorically throw out the tone controls as is sometimes suggested. With the controls bypassed, then sensitivity is improved to a level that will allow for notable foreground listening without the volume control operating down in the weeds, yet still well within range of the loudness switch function. Therefore, with the increased sensitivity, a (now) improved loudness switch function (when used), and use of properly compensated bypass attenuation networks, the sonic presentation in bypass mode typically leaves little to no desire for any further tone control adjustment.

Because the second network increases high level input sensitivity however, it too compromises the gain relationship between the phono preamp and the high level sections of the amplifier. As a result, phono sensitivity increases to requiring just 1 mV to produce full power output when the tone controls are bypassed with this network. Being able to restore the original gain relationship when needed then is just one more reason to retain the tone controls in the amplifier.

In either case, the networks are installed by connecting the Filter switch so that the input to the Balance control selects either the output of the tone control circuits as in the original design (Filter position), or the output of the new bypass networks (Off position). The input side of the networks connect to the wiper terminals of the volume control, along with the original leads feeding the input to the tone control PECs.

As for the tone controls themselves, the manual’s instructions for installing the tone control knobs are pretty accurate, although the best results will always be had by installing the knobs so that they flat at a 12:00 noon position when the bass and treble controls produce the best 200 Hz and 2 kHz square wave displays respectively. The overall tone control circuits are actually pretty accurate as well, although limited in range — but the improved loudness switch really helps to improve their audible performance in that regard. They can have their issues — although it’s more common that they don’t. Again, square wave testing will show off any discrepancies very quickly. Operating properly and with their knobs correctly installed, they will produce very acceptable square wave displays at the target frequencies when set for flat performance.

When selecting the tone control bypass position, the network providing increased sensitivity (X4) does cause a slight increase in signal bleed at a minimum volume setting. This was traced to leakage through the volume control that was not due to any cleanliness issues but rather, just the nature of its construction. When selected, the greater loss through the tone control circuits makes the leakage virtually inaudible, but when the gain producing bypass networks are selected, their lower loss then allows it to become softly audible. It is hardly problematic for me — even with the use of high level low impedance sources (nothing like the bleed produced with the original build), and would take a new control of modern construction to eliminate. For the small amount of bleed produced, I don’t feel that is warranted so therefore, I have not pursued it any further with the project. I’m simply noting it, explaining how it’s occurring, and why.

For full assessment, I have both types of bypass networks installed in my two test/development amplifiers — simple 12 db gain networks described here in the Mark I amplifier, and low impedance type 0 db gain networks described later in the Mark II. The low impedance networks installed in the Mark II amplifier are different in their makeup from either of the two simple networks presented here, but are completely noiseless, allow for a true 1:1 comparison of the tone controls when they are in or out of the circuit, and are made possible by the installation of the line stage amplifier presented in section 8A in Group 2. For those interested in installing the line stage amplifier, I would encourage you to check out the information on that modification first before installing either of the two bypass networks presented here, as they are not recommended for use with the line stage amplifiers when they are installed. This will prevent having to undo the installation of either of these networks if it is decided to install the line stage amplifier at a later date. Or, since the increased gain provided is the same with either the 12 db bypass networks or the line stage amplifiers, installation of the 12 db gain networks at this point might be a good intermediate step to take for those wanting to see if the increased gain provided by the line stage amplifiers is worthwhile for their needs. This will help them to decide then if installing the line amplifiers is the way to go with their project, without actually having to build them.

Finally, for those committed to absolute purity by the complete removal of all controls except for (possibly) a selector switch, power switch, and (say) a stepped volume control, 15K stopper resistors should be added between the wiper of the volume control and the input of the power amplifier board (eyelet 7) in each channel to ensure complete stability under all dynamic conditions.

Next up, slaying the hum dragon.......

Dave

Below: A schematic showing the revised signal path from the Stereo/Mono switch, the caps added to the Loudness switch, the tone control bypass networks, and the Filter switch repurposed as a tone control bypass switch:


Below: The leads from the Stereo Mono switch now go directly to the top of the volume control, rather than over to the Filter switch as they did in the original build:


Below: The heat shrunk packs are the 12 db gain tone control bypass networks I chose to install in my Mark I amplifier. The green/black sleeved pack is for the left channel, while the red sleeved pack is for the right channel. The left channel pack runs down low in the chassis while the right channel pack runs high in the chassis to prevent any cross talk between them:


Below: The Filter switch is configured so that center terminals feed the balance control, the terminals closest to the power switch are from the bypass networks, while the terminals closest to the loudness switch connect to the leads from the tone controls:


Below: A top side view. The small resistor on the rear bass control ensures that both controls track the same at the flat position. This is the only amplifier I've ever needed to do this on:

 

[gadget73]

200Ω 4W resistor across the IN and OUT terminals of the regulator

Any idea how small of a resistor can be used before the regulation starts getting funny? Just curious if this could be used in setups that use tube heaters as the cathode resistor.

 

[dcgillespie]

Gadget -- I considered that application back when I first started working with the idea of implementing the use of a bypass resistor. Tests showed that a minimum of about 15 mA should be maintained through the regulator nominally for it to have enough "wiggle room" for its proper operation under various quiescent conditions. This is enough current to allow for variations in AC line voltage and output tube characteristics. The bias control allows for the proper setting of the quiescent current, but because different tubes require different bias voltage levels to achieve the correct current, the current through the resistot then changes, which then changes the available quiescent current passing through the regulator as well. Maintaining a minimum of 15 mA then provides the necessary wiggle room so that these variables don't impact the regulator's operation.

Does it work? Well yes, but I didn't really go forward with the concept as the whole idea of using output stage quiescent current to power preamp tube heaters is so dicey to begin with. To minimize hum while also allowing somewhat compromised tubes to operate properly, and also without affecting warmup time too significantly, 12AX7 tubes require about 140 mA of heater current with the 12 volt connection, producing about 11.5 volts of drop across the heater. This current level is really beyond what the 6BQ5 family of tubes should draw in a push-pull stereo setting, and just at what the 7591 family should draw, leaving really nothing extra to establish the regulator's base current draw. Also, the configuration is only good for two preamp tube heaters, since a third tube starts getting very near the limits of the voltage drop that the regulator can safely handle across it. So yes, it can be made to work if you lower the heater current below optimum levels, but for me it began to become such a compromise that I didn't pursue it any further, or publish any comments on the idea.

Dave

 

[gadget73]

Thanks, I thought something like that may have been the situation.

As for the max voltage across it, could part of that be made up with a zener diode? LM337 is good for an absolute max of -38v. Could you put say a 15v zener from input to ground to get another 15 volts worth of max voltage at the output terminal while still being within the max drop across the regulator itself?

no idea why I might need to do such a thing, just sort of thinking out loud.