[Whitese]

This was a fun and informative read....!!

I am so glad for you guys that can fix our vintage gear in this day and age..and I hope there is a next generation brewing...for that reason I had all my Pilot gear redone.

basically, thank goodness for you guys who know shit!!!

[autoteck1]

The best cleaners I have used on vintage audio equipment is "MeanGreen" it disolves all kinds of grime including nicotine from white plastic !! and dosent seem to harm the original finish, great stuff and it's cheep!
I buy basket cases from E-*** they are usualy filthy but clean up rather nice with it. 

[Bill Thomas]

     "Mean Green" is a great product and does work EXTREMELY well.  Unfortunately, in the case of our Scott preamp, the aluminum chassis suffers from a good deal of "pitting" or "etching."  In fact, the ONLY way we are going to wind up with a nice, shiny chassis is to drill out all the rivets and do an extensive sanding, polishing and buffing job.  This changes our "methodology" a bit because we'll need to draw ALL of our chassis pictorial diagrams and remove all of the tube sockets prior to sanding and polishing.

     As you might guess, this is the reason for the delay in our next "update."  I DO hope to have some news to report within the next couple of days.  There WILL be pictures as well.

Sincerely,

Bill Thomas

[Bill Thomas]

     Just a quick note:  I have been "internet poor" for over two weeks since my cable company changed hands and they had problems during the transition.  (They disabled ALL of the DVR boxes on the system for over a week and also lost the ability to provide internet access, system-wide.  Apparently, they have most of their "issues" worked out and my internet access has returned.  Look for an update sometime in the next 24 hours.  Sorry for the delay in posting updates.  Progress IS being made (as you shall see).

Sincerely,

Bill Thomas

[Bill Thomas]

October 5th, 2009 - Update:  "Beginning To See the Light!"

     You would think that each restoration project follows the same path.  You would *think* that, but you'd be dead wrong!  Oh sure, the basic steps are similar, but each project tends to take on a life of its own, dictating "modifications" in the manner we approach things.  In the case of our H. H. Scott 121-C, our *original* idea was to attempt to rebuild it without removing any of the riveted components.  Unfortunately, the condition of the aluminum chassis precluded us from following our original approach.  As a result, the Main Chassis had to be completely stripped of all parts in order to clean up the corrosion and pitting so we could "make it prettier."  We'll hold off on showing you pictures of the results until we get closer to the end of our project.  Meanwhile, let's take a look at the initial stages of repopulating the Main Chassis with the parts we removed and are re-using.

     Here's where the re-assembly begins:  The Phono Preamplifier section.  Here's a picture showing the initial mechanical assembly:

     

     All of the tube socket pins and the terminal strip terminals have been cleaned up.  The sockets have also been pressure-cleaned and de-oxidized; cleaned again, and then treated with a contact conditioner.  It takes several hours to complete this process, but the results are WELL worth the time invested.  Since there are so many multiple connections to most of the tube socket pins, complete solder removal is ESSENTIAL to insure a minimum number of problems during reconstruction.

     While I would *normally* install all of the sockets and terminal strips before any wiring is done, the terminal strip on the left side of the chassis is *hidden* a bit by the "lip" of the chassis, so it made more sense to install the components on the terminal strip before most of the other sockets or terminal strips were installed.  Here's a picture:

             

     With that out of the way, we can add more sockets and terminal strips.  Along the way, we'll add a few of the MANY wires that run to and from the terminal strips.  In MOST cases, the wires are added first and THEN the individual resistors and capacitors are added "to the mix."  It just makes for a neater, "cleaner" result when the project is completed.  Here's a picture of the next "phase" of the mechanical assembly:

     

     Let's add more "stuff" in this picture:

     

     This picture shows all of the terminal strips installed:

     

     And here we have a picture with the rest of the tube sockets installed:

     

     H. H. Scott supplies the filaments of the two low-level tubes and the Output tube with DC for the filaments in order to keep hum to a minimum.  As a result, these three tubes are wired in series.  The orange wires connect the three tube filaments together.  The rest of the tubes are supplied with AC from the Power Transformer, but there is a "hum adjust" potentiometer thrown into the mix.  We'll show you how that all connects in our next "update."

     "Back in the day", so-called "bumblebee" capacitors were considered to be high-quality parts.  Of course, we have since learned otherwise.  They may have worked well initially, but they didn't remain that way for long.  Here's a picture showing *most* of the removed "bumblebees":

                         

     You'll notice that a couple of them are physically broken in two.  Most of the rest have cracked cases.  Needless to say, the chance of ANY of them working correctly is close to ZERO!  Our modern replacements will go a LONG way to insure that our preamplifier will have another long lifetime ahead of it.

     That's it for this update.  But, we'll be posting MANY more updates in the days to come.  We'll finish up the filament wiring and begin the long process of repopulating the unit with all new passive components in our next update.  We'll also install our completed front panel to the chassis and begin the task of sorting out "which wire connects where" so don't give up hope just yet.  There's a LOT more to cover before we're "making music" again.  Stay tuned!

Sincerely,

Bill Thomas

[Bill Thomas]

October 8th, 2009 - Update:  "The Plot Thickens!"

     With "point-to-point" wired units, the FIRST wires to be installed are the filament wires.  These wires should be as close to the chassis as possible; ESPECIALLY the wires carrying AC.  We have added one extra little step:  We have twisted the AC wiring that feeds the various tube sockets.  This little step accomplishes two things.  First, we reduce the AC radiated by the filament wires and Second, we also reduce the susceptibility of noise pickup by the filament wires.  Granted, this is a SMALL contribution, but when you are dealing with high-gain devices, ANY reduction in hum and noise pickup is a potentially GOOD thing.  Let's follow the trail of the AC filament wiring.

     We start off with the AC filament wiring at the 6X4 rectifier:

           

     Pins 3 and 4 are the filaments.  These pins will be fed directly from the Power Transformer (which will be installed a bit later) and the brown and gray wires from these pins then feed the hum adjust potentiometer here:

                           

     The wires from the 6X4 are on the right.  The wires on the left then feed pins 3 and 4 of the 6AV6 bias rectifier and amplifier for the "Dynaural" noise reduction circuitry.  Here's a picture:

                                         

     The wires on the bottom of the picture come from the hum adjust potentiometer.  The wires on the top of the picture feed the two 12AU7's for the high-frequency and low-frequency Dynaural circuitry AFTER the brown wire goes to a 2.7 Ohm filament dropping resistor here:

                             

     This 2.7 Ohm resistor reduces the filament Voltage to the two 12AU7's by a VERY small amount.  I'm not quite sure why this was done, but it was certainly not uncommon to see something like this in all SORTS of applications during the 1950's.  Needless to say, we're not going to second-guess the design engineer!

     Anyway, after the resistor, the filament Voltage is fed to the two 12AU7's, first V3 as shown in the previous picture, and then to V5 as shown here:

               

     And that's the end of the line for the AC filament wiring!

     With THAT little chore out of the way, it's time to mount the Front Panel to the Main Chassis and begin to wire up the whole "shootin' match."  The accepted practice for "point-to-point" wiring is to connect the actual wires to the terminal strips first, THEN we add the passive components.  (Of course, there are always a *few* exceptions, but in GENERAL, this is how it's done.)  That means that we had to connect the wires from the left side of the Front Panel first.  Here's a picture showing that wiring:

         

     In this "bundle" of wires, there are three green wires, three black wires and a blue wire.  Needless to say, it is CRUCIAL that these wires all connect to the proper locations.  Sure, you can trace where they go on the schematic (if you have one), but it's a LOT easier to use the photos and the pictorial diagrams we drew up when we took it all apart.  Still, before you cut ANY of the wires, double and triple-check the wiring locations!  (Measure twice - cut ONCE, right?)  You should know that even with ALL of the photos and ALL of the pictorials I drew up, there was *still* a bit of ambiguity involving two of the black wires.  They are both ground wires, but one connects to the center of tube socket V1 and the other connects to lug #4 of the top terminal strip, along with the third black wire.  Moral:  you can NEVER take too many pictures or draw up too many pictorials!!!

     NOW we can begin to repopulate the Main Chassis with new components.  We start things off by installing the passive parts around V1; the 12AU7 used for the equalized preamplifier section.  Here's a close up of the completed wiring at the first tube:

               

     You'll notice that we are using a "mix" of Allen-Bradley 5% carbon-composition resistors and metal-film or metal-oxide resistors.  The "rule of thumb" is to use metal resistors for Cathode and Plate resistors, while retaining carbon-composition resistors for grid resistors and other locations where audio passes through them or is "shaped" by them.  This way, we retain the "warmth" attributed to the Allen-Bradleys, while reducing the potential for noise in the Cathode and Plate circuitry associated with the tubes, since appreciable current is drawn through these circuits.  Of course ALL of the resistors were checked to make SURE they were well within 1% of nominal values.  (Most were EXACTLY as marked!)

     You'll notice that we have a LOT more room than the original parts allowed.  Modern capacitors are a LOT better in quality and a LOT smaller than those old tubular "bumblebee" capacitors.  Having a little extra room allows us to "dress" the leads for a neater "look" while still keeping the lead length short.

     Here's a picture of our Main Chassis at this stage of completion:

     

  What's done looks pretty nice.  Now we just have to "fill in" the rest.  We'll cover the following stage in our next update, coming up in a day or so.  Don't tarry at the waterin' hole TOO long.  You'll want to have a clear head to make sense of the next "bunch of parts" that will be installed in our next update - coming up VERY soon!  Stay with us!

Sincerely,

Bill Thomas

[avahifi]

Bill, I suggest use Dale Vishay RN60C metal film resistors for your plate resistor applications.  RN60D types or similar tend to be very noisy with high voltage across them.

Regards,

Frank Van Alstine

[Nels Ferre]

I love watching Bill's work. 

[oldmp3]

Bill,

Anyone who ever picked up a soldering iron has great admiration for your work!  The intricate attention to detail, both function and "looks" in your work is amazing.  That and your ability to preserve the essential best of a classic really top it off.   

I'm only sorry the art and science of preserving and updating classic electronics is not more well known and practiced. 

Mark

[smbrown]

As usual, amazing work, Bill. Any chance for a peak at the top of the chassis now that you've cleaned it up?

[Bill Thomas]

Thanks for the comments, guys.  It helps when you're knee-deep in "now WHERE did that wire go?" stuff!

Frank, I agree with you regarding Vishay/Dale resistors, but in this particular circuit location, the Voltage seen by any of the resistors is not much more than 100 Volts.  Most of them see MUCH less than 50 Volts.  For example, the 150K resistor in the Plate circuit has approximately 103 Volts across it.  The 82K resistor sees about 57 Volts.  Of course, the current is less than one mA.  These resistors *should* work just fine here and they should certainly be quieter than carbon-comps.  I generally don't see a noise problem until the Voltage goes past 250 Volts and the currents are MUCH higher.  Of course, the proof will come when we "fire up the beast."  Still, there WILL be some Vishay/Dales coming up further in the rebuild.

As for pictures of the top of the chassis, they are "in the works" and WILL be posted a bit later.  I *will* tell you that the pitting was VERY deep in the aluminum, so not ALL of it was completely eliminated, but it looks much better than the corroded wreck we started with.  The final result, while not a perfect "mirror finish you could shave with", is still quite acceptable and will be in keeping with the "Vintage" look of the unit when all is said and done.  I just didn't want to "spoil the surprise" just yet.

It's funny how these restorations go.  My original time estimate from start to finish was for the "project" to take about two weeks.  Hah!  Not even CLOSE to what was eventually involved.  But this little preamp simply DESERVES the extra time involved in "making it right."  Besides, I couldn't LIVE with myself if I really "cut corners" to meet a self-imposed time estimate.  The TRUTH is, it takes as long as it takes to do the job RIGHT!  As I have ALWAYS stated, my goal is to wind up with a unit that will serve the end-user faithfully for ANOTHER lifetime of use.  Cutting corners is simply NOT an option!

My "better half" always complains about how long it takes for me to do the dishes, but I GUARANTEE that you will NEVER see a SPECK of ANYTHING on ANY dish *I* wash!

In the midst of all this, I'm STILL having some "issues" with that prototype FM-3 tuner with new circuit boards.  I'll be taking a small break from the preamp in the hope that a fresh perspective will FINALLY get to the bottom of the alignment problem on the IF board.  I'll be posting another update on the preamp during the weekend and I *hope* to have more information on the FM-3 tuner "issues" before the weekend is done as well.  MUCH more to come.

Sincerely,

Bill Thomas

[avahifi]

Bill, the noise issue reared its ugly head with our original Super Pas rebuild for the Dyna Pas-3 preamp.  Even with a 100V drop or less, we had to go to RN60C plate resistors throughout to be sure of long term quiet operation.

One of these days I hope you can get to that rebuild kit too.

Regards,

Frank Van Alstine

[Bill Thomas]

Friday, October 16th, 2009 - Update:  "Loose Ends and a Few Observations."

     While things are progressing a good deal slower than I originally planned, progress IS being made.  But before we look at more parts and wiring, I wanted to conclude our look at the can capacitors.  First, a little "Visual Detective Work" will show exactly WHY we had to rebuild the can capacitors.  Here's a picture of the Bottom Cover for the preamplifier:

           

     This is a "before" picture (before cleanup, that is.)  Notice the two "spots" or "stains" on the bottom cover?  These "spots" are directly UNDER two of the three can capacitors.  These "spots" are due to electrolyte leakage from the can capacitors above these locations.  If you see this sort of "staining" on ANY piece of electronic gear, REPLACE THOSE CAPACITORS.  Don't even THINK about trying to reform them.  An electrolytic capacitor that shows ANY sign of physical leakage simply MUST be replaced.  There is NO other choice.  This leakage is proof positive that a capacitor is (as Jerry Clower used to say), "Graveyard Dead!"  If it is working at all, it will fail completely VERY soon.  Why take a chance on trying to use a component that is BOUND to fail?  It makes NO sense at all!  Take a closer look at these "stains" and YOU decide.  Here's one of them:

   

     Notice that you can even tell where the electrolyte leaked out from the two terminals directly above the "staining."  Now here's a close-up of the other "spot":

                 

     Again, you can barely see an indication that the electrolyte leaked out from two of the terminals on the capacitor.  Not good at ALL!

     Now, here is a picture of our three "re-stuffed" can capacitors after complete assembly:

     

     Yes, there is a visible "parting line" where the cans were cut open, but it's really not *that* visible.  We used good old J-B Weld to permanently "glue" the can back onto the base.  In order to make SURE the epoxy will hold, a bead of J-B Weld was applied to the base and another bead of J-B Weld was applied to the inside of the can top.  The can was then replaced onto the base and the assembly was held together with large rubber bands to maintain alignment.  The bead of J-B-Weld inside the can will gradually descend onto the bead we applied to the base, fusing the two parts together.  By the way, we used the original J-B Weld formula which takes about 12 hours to cure.  This gave us plenty of time to assemble all three capacitors before the epoxy "took a set."

     I should mention that while J-B Weld labels one of the two tubes of their epoxy "Steel", in reality there is NO steel or metal of any kind in regular J-B Weld and it IS an insulator.  (It says so right on the package!)

     By using a small hobby lathe to cut the cans open cleanly, the size of the "parting line" is kept to a minimum.  When using the J-B Weld to "glue" the cans back together, you *might* have some "oozing" from the parting line.  A Q-Tip dampened with lacquer thinner will clean things up *IF* you use it before the epoxy begins to harden and "cure."

     Well, that's brings us up to date on the can capacitors!  We'll take a look at the latest wiring on the main chassis in our next update, which will be posted VERY soon!  We'll show you the completed installation of the Front Panel with ALL of its associated wires and parts connected to the Main Chassis.  (That's over TWENTY wires and five leads from parts that connect to the terminal strips on the Main Chassis!)

     There's still a lot of "fun" left to enjoy, so don't touch that dial *just* yet!

Sincerely,

Bill Thomas

[acwd1950]

Bill,
That one spot on the bottom cover where the caps may have been leaking looks more like a arc burn. I say this because thats what Aluminum looks like when its not clean and you try to Tig weld it. Black, sooty and porosity. Just my opinion. Keep up the outstanding work!

Steve 

[hdspeakerman]

Bill,
Beautiful work as always.  Almost as pretty as the one of kind preamp you did for me.  Good luck.
Howard 

[Bill Thomas]

     Thanks for the kind comments, guys.  Steve, you're right that the damage DOES look like an arc, but I can assure you it is chemical in nature.  There is simply too much distance between the capacitor terminal and the bottom cover for there to be an arc.  If the cover had been pushed against that terminal, it would have been SERIOUSLY bent!  There is ZERO trace of any bending or warping of the bottom cover.

     Howard, I am SO glad you are still enjoying your unit, both visually and audibly.  I only wish I had more time to devote to these "projects" so they would be completed a bit faster.  But in a way, they are a bit like a fine wine.  They are ready when they are ready and rushing them would spoil the end result.

     A lot of folks wonder why these projects take so long.  The answer is simple:  "Because they do!"  In the case of this Scott 121-C, the lack of documentation means that for every connection that is made, I have to refer to a pictorial that I have drawn, a picture or ten that I have snapped of the original unit, and the flawed photofact schematic that I was able to locate.  It takes a LOT longer to install EVERY component or wire up the connections throughout the unit.  I should also add that point-to-point wiring is MUCH more time-consuming than "stuffing" parts onto a printed circuit board.  But, I truly *hope* the end result will be worth it!

     And with THAT out of the way, let's proceed with our next "update!"

Sincerely,

Bill Thomas

[Bill Thomas]

Thursday, October 22nd, 2009 - Update:  "More, More, More!"  ("How Do You Like It?")

     While I truly doubt there are many Disco records on 78, it's time to add more, more, more parts to our preamplifier and complete the wiring of the second tube in the chain.  Our previous wiring update showed the completed first tube stage - a 12AU7 that is used as the equalized preamplifier stage, now let's complete the second gain stage that utilizes a 12AX7.

     The 12A?7 family of tubes are actually two tubes in one envelope.  The 12AU7 has a gain of 30.  The 12AT7 (not used in this amplifier) has a gain of 70 and the 12AX7 has a gain of 100.  It's a bit unusual for a 12AU7 to be used in the equalized preamplifier stages.  In most other preamplifiers, a 12AX7 would be used for more gain.  Dynaco did things that way, as did MANY other manufacturers.  But some people find the "sound" of a 12AX7 to be a bit "strident" when used at maximum gain.  Not much chance of that in THIS preamplifier.  The second stage tube (the 12AX7) is run WELL below maximum gain!  The first half of the 12AX7 is used to amplify the output of the equalized phono preamp stage, since the 12AU7 used there doesn't output the necessary audio level to match the line-level sources.  The first half of the 12AX7 also acts as a "buffer" to isolate the load on the phono preamp stage.  If there weren't a "buffer", changing the setting of the "preamp gain" control *might* upset the equalization circuitry.  We CERTAINLY don't want THAT result!  Neither did the folks at H. H. Scott!

     The second half of the 12AX7 provides *some* gain, but it also acts as a buffer, or driver for the tapped inductor that feeds the Dynaural noise reduction circuitry.  Here's a picture showing the completed first and second tube stages:

               

     Notice that the top terminal strip is now fully wired and all terminals have been soldered.  The vacant terminal is not used in this application.  The terminal strip on the left side of the picture is *almost* completely wired.  Three of the four terminals have been soldered.  The fourth terminal (located at the bottom of the strip) has one more small electrolytic capacitor to be added, so *that* terminal has yet to be soldered.  We'll add that part once the can capacitors have been re-mounted to the chassis.

     In the picture above, you can see the inductor I was talking about earlier.  It is mounted to the Front Panel and is used somewhat like a crossover inductor to assist in splitting the audio into a low-frequency and a high frequency "band" which is then processed by the Dynaural noise reduction stages.

     Now, let's jump ahead to the Output Stage tube.  It is also a 12AX7.  One half of the tube is used to amplify the Output of the Tone Control circuitry, while the other is used as an Impedance reducing circuit called a "Cathode Follower."  Here's a picture showing the initial wiring of that 12AX7:

               

     The reason we have "skipped ahead" has to do with the completion of the wiring at the right-side front terminal strip.  If you look closely, you'll see that all four terminals have been soldered.  This means that ALL of the terminal strips adjacent to the Front Panel have been wired and soldered!  Also notice that the terminal strip to the right of the Output Tube has three of its four terminals soldered as well.  The fourth terminal will have a few resistors added once the can capacitors are installed.

     Do you notice a pattern here?  In order to make CERTAIN that all connections are soldered, our "course of action" was dictated by the terminal strips, NOT the tube sockets.  By systematically going "down the line" of terminals, our preamplifier will be properly completed.  We WON'T have any connections that AREN'T soldered.  We'll continue in the same manner throughout the rest of the preamplifier.

     So, once we complete the wiring and componentry around the Output Tube, what's left?  The most interesting circuitry in the entire preamplifier:  The three tubes involved in the Dynaural noise reduction section.  Here's a picture of the tube socket used for the High Frequency "Gate"  (It's the tube socket on the Left side of the picture):

               

     We also have the Low Frequency "Gate" and the Bias amplifier left to wire.  Here's a picture of those two tube sockets:

     

     Finally, here's an "overall" shot that shows what has been completed, and what remains to be done:

               

     Yes, things are *definitely* shaping up, but there's still a good deal of "fun" left.  We'll cover much of that in our next "update", coming soon!  We'll also install the three can capacitors and begin the "clutter reduction" of many of those wires that are currently "waiting in the wings" for their new home!

     Don't miss it!  It's going to go a LONG way toward the completion of our preamplifier "project."  Stay with us!

Sincerely,

Bill Thomas

[Bill Thomas]

Sunday, October 25th, 2009 - Update:  "It's Getting Better!"

     No, we're not ready to play that unopened vinyl original pressing of "Sergeant Pepper's Lonely Heart's Club Band" *quite* yet, but we ARE getting closer "all the time!"

     Let's continue with our restoration of the Output Stage.  Here's a picture of the Output 12AX7's tube socket:

                   

     While it isn't *quite* complete, this shows the bulk of the construction for the Cathode Follower section of the Output Tube.  We have made a *slight* change in the parts used.  In the "stock" unit, the Input Network for the Cathode Follower used dipped silver-mica capacitors.  We have replaced them with the same values in polystyrene, rather than mica.  Both types of capacitors are OUTSTANDING capacitor, but some people feel that polystyrene capacitors are a bit "kinder" to audio.  Since ALL of the audio flows through these capacitors, (and since we're not in a high heat environment), polystyrene capacitors will perform admirably!

     Now, *finally*, as promised, let's add our "re-stuffed" can capacitors to "the mix" and begin connecting the wiring to the respective units.  Here's a picture:

       

     With the can capacitors added, we can begin to reduce the "clutter" by connecting some of those loose wires to their respective terminals.  Here's how things start on the Right Side of the preamplifier:

             

     Here's the other can capacitor, located on the Left Side of the chassis:

             

     Several of the loose wires will connect to this capacitor, but first, let's finish off the Output Stage.  Completed, it looks like this:

               

     Pretty "busy" around the Output Tube, isn't it?  This is, for the most part,  unavoidable when point-to-point wiring is used.

     It's interesting to note that with point-to-point wiring, components are added in a different order than when restoring Printed Circuit Boards.  With Printed Circuit Boards, you (generally) start with the lowest profile components like resistors and diodes.  Then you add the next larger component until the largest parts are mounted.  But with point-to-point wiring, the larger components (mainly the "tubular capacitors" are generally mounted first, followed by smaller and smaller parts.  This technique assures us that there won't be any parts hitting the bottom panel of the preamplifier when we are finished.

     Now that our Output Section is complete, let's "fill in the middle" by wiring up the first of three tubes associated with the Dynaural Noise Reduction section.  Here's a picture of the completed High Frequency "gate":

               

     Before we leave this area, let's take a closer look at that little 1 Meg resistor.  This gives you a good example of how the component leads are formed and how we measure the length of the heat-shrink tubing that covers the component leads:

     

     This 1 Meg resistor will wind up connecting to the Low Frequency "gate"  and the Bias Amplifier tube circuitry (as soon as we can get to *those* sections!)

     Let's bring this update to a close by taking an overall look at the Main Chassis wiring to this point.  Here it is:

     

     For all practical purposes, the top half of the preamplifier is now completely wired!  In our next "update", we'll begin wiring the bottom half of the preamp, starting with the Can Capacitor located on the Left Side of the Main Chassis, followed by the wiring of the Bias Amplifier/Bias Rectifier tube (the 6AV6) located just to the Right of the Can Capacitor.

     Well, we're getting closer and closer to a completed preamplifier, but there's still *quite* a bit remaining.  Don't "bug-out" on us now!  This is just *beginning* to get really GOOD!  More to come VERY soon!  Stay tuned!

Sincerely,

Bill Thomas 

[AudioSoul]

   This is a total work of art. I love it, thanks for taking the time to show how it should be done.
   I cant wait to see it finished and hear your impressions on how it sounds......

[Bill Thomas]

Thursday, October 29th, 2009 - Update:  "For the Benefit of Mr. Scott"

     (Sorry, I couldn't resist with all the Beatles "Hoopla" going on lately.)  But with THAT out of the way, let's get down to business!

     Our H. H. Scott 121-C is getting closer and closer to the "smoke test."  As I mentioned in the previous "update", we're wiring the bias amplifier/rectifier for the Dynaural circuitry in this installment.  You would *think* that wiring up five more tube socket lugs would be "no big deal."  But, with the number of small components that had to be added here, along with the crowded conditions at several of the tube socket pins, it isn't exactly a "walk in the park."  But "slow and steady wins the race."  Just take things in order and work your way up from the connections that are closest to the chassis, then the "mid-level" connections, and finally, the "top-level" parts.  What initially appears to be an IMPOSSIBLE task, winds up being a series of *teensy* little steps that are quire possible.  But don't let me keep you in suspense here.  Let's take a look at the finished wiring around the 6AV6 tube socket, the associated Quad Capacitor to the left of that socket, and the connections to the various terminal strips "in the neighborhood":

             

     As you can see, things are pretty crowded around those seven pins!  If it weren't for the MANY photos and SEVERAL hand-drawn pictorial diagrams, I would have NEVER gotten through it.  In case you're wondering, this was about three night's worth of work!  (Yeah, I'm REALLY slow, aren't I?)  But each connection requires CONSTANT checking against the pictorials, the photos AND the photofact schematic diagram to make CERTAIN everything goes where it should.  It's a LOT easier to make SURE things are correct when you are BUILDING it, than to try to troubleshoot it and "make it right" AFTER the thing is complete.  ("Measure twice, cut once", right?)

     You may wonder why I used polystyrene capacitors.  As it turned out, I either forgot to order .0033 film capacitors, or I "left them someplace."  (Hey, we're ALL human!)  Seriously, I looked and looked, but they weren't to be found ANYWHERE.  But I DID have these lovely polystyrenes available, so in they went!  (A .0033 microfarad capacitor is the same value as a 3300 picofarad capacitor; it's just expressed differently.)  Polystyrenes are actually more stable than typical film capacitors, so let's just call this an "upgrade" (and conveniently ignore my sloppy "parts organization", ok?)  Seriously, polystyrene capacitors are considered IDEAL for use in audio circuitry.  They have extremely low leakage, are usually "spot-on" in value and their only "downside" is that they don't like to play in hot environments.  No problem with that here.

     You'll also notice that we've completed the wiring of our nearby quad capacitor.  Two sections of that capacitor are used as Cathode Bypass capacitors and the other two sections provide part of the B+ filtering for the preamp.

     So, what's left?  Well, we *still* have one more tube used in the Dynaural noise reduction circuitry.  Here's a picture of the "work to be done" around that tube socket:

     

     Since five connections are already completed at that socket, there are really only four tube socket lugs that need to be wired.  Piece of cake, right?  Well, we'll see about that in our next update.

     There's one more area that needs to be completed:  The rest of the Power Supply and the associated quad capacitors.  Take a look:

             

     Since selenium rectifiers are so "last century", we're using a more modern silicon bridge rectifier to replace that old selenium rectifier in the filament supply, see?:

                         

     Selenium rectifiers may have been OK "way back when", but they have several drawbacks.  They start deteriorating as soon as they are manufactured, (If I could even FIND a NOS replacement today, it would most likely be bad after sitting on a shelf for 50 years!)  they have a fairly low current capacity, (the original was rated in milliamps), they really *STINK* when they fail; generating fumes that can actually be poisonous, and simply weren't that good at rectifying AC to begin with.  Modern silicon rectifiers are DEFINITELY the way to go here!

     This silicon bridge rectifier is rated at several AMPERES!  We'll have NO problem powering the three tubes that are fed from our DC filament supply.  But there IS one "gotcha!"  Since silicon rectifiers have a lower forward Voltage drop than selenium rectifiers, we will *probably* be delivering higher Voltage to the tube filaments than they are rated for.  Since the tubes are in series, we need to deliver no more than 18.9 Volts to the filaments.  With the silicon rectifier in place of the selenium rectifier, we will *probably* be delivering closer to 22 Volts to the filaments!  No good can come from THAT, so we'll "juggle" the resistor values in the filament supply in order to bring the Voltage back down to something closer to 18 Volts.  That's right, I said 18.0 Volts!  By running the filaments slightly BELOW their typical ratings, we will extend the life of those tubes by a significant amount.  That tiny reduction in filament Voltage will cause NO problems "in-circuit", but *could* help our valuable tubes live a few YEARS longer!  (Honest!)

     As I usually do, here's a picture of our overall progress to this point:

     

     I'm calling it about 85% complete.  But we STILL have some "fun stuff" ahead.  We'll be "improving" the cosmetics of the Power Transformer and wiring it in, getting the rear panel ready to re-mount to the main chassis and THEN - the ever-popular "smoke test!"  (I can feel the anticipation building!)

     My sincere thanks go out to everyone who has posted their comments or e-mailed me regarding this "project."  A *special* boatload of thanks go out to the owner of this preamplifier who has shown INCREDIBLE patience during this VERY slow rebuild.  We'll soon see if that patience will be rewarded!

     There's still a bit of "fun" coming, so don't stray too far from Audiocircle!  (Hey, you've stuck with me THIS far, do you REALLY want to miss the "exciting conclusion?")  Stay tuned!

Sincerely,

Bill Thomas