[Bill Thomas]

Recently, I decided to start getting rid of some of the old Dynaco gear I had acquired over the years, but before I do, I want to be certain it will operate properly, reliably, and MOST importantly, SAFELY!  I grabbed three Stereo 70's out of my Self-Storage facility and brought them to the bench.  I also brought back boxes and boxes of NOS parts and supplies (that should come in pretty handy during this rebuild)!  As it turns out, of the three Stereo 70's I retrieved, only ONE is a candidate for a new home - but NOT in its present condition!  (One of the other units was "punched" at the fuse holder and would need to be straightened.  The other one had been heavily modified and would require further analysis.)

Our surviving candidate appears to be a *fairly* stock Stereo 70 with a few twists.  First, it was fitted with an incorrect tube cage.  While you might think that any Stereo 70 tube cage will fit any Stereo 70, you'd be wrong.  Actually, there are TWO different chassis arrangements and FOUR different tube cage/covers.  Factory-wired units used two additional "safety" screws to hold the bottom cover onto the chassis.  A corresponding notch had to be cut in the lower "ears" of the tube cage in order to clear these two screws.  Unfortunately, the tube cage this amplifier came with was NOT the "notched cover" version, while the chassis and bottom plate WERE designed for the extra two screws.  Fortunately, I brought a correct tube cage/cover from storage.  Ahh, but there's more!

Remember I mentioned factory-wired?  Well, forget THAT!  It appears that someone went to the trouble of replacing all the tube sockets, and then went ahead and replaced ALL the connectors and terminal strips, and THEN did a dangerous rewiring job.  On top of that, whoever rebuilt this thing mounted the PC-3 circuit board on TOP of the chassis.  They also mounted the Output terminal boards on the INSIDE of the chassis.  Both of these moves were REALLY bad ideas!

Upon removing the bottom plate, one look at the wiring job told me that it would be a few days before THIS amplifier would be back in reliable working condition.  It was pretty obvious that the amplifier had been totally re-wired.  Whoever wired it used something similar to "bell wire" and wasn't very careful about wiring dress.  The connections themselves were minimal, at best.  Instead of a good mechanical connection, our "amplifier builder" used the "tack solder" approach; using the solder to make the actual connection.  He also failed to tin the transformer wires, so the integrity of *their* connections was poor.  I found little tiny "solder balls" all over the amplifier.  To top it all off, the fuse holder's center terminal was oriented toward the bottom plate with minimal distance between them.  One slight flex of the bottom plate and the chassis would become electrified.  Oh yeah, the fuse holder contained a 5 Amp fast-blo fuse and someone had replaced the AC plug with a generic plastic hardware store plug.

On the positive side, the chassis itself is in fairly nice condition with only minimal evidence of any pitting.  But the BEST part of all is that the transformers are the more desirable cloth-covered wire versions.  Things were looking up!

Fortunately, all of these problems can be corrected with the right parts and a little time.  The goal will be to convert this jumble of scary wiring into a safe and reliable stock Stereo 70 (with a *couple* of modifications for reliability).  I call this one "Nothin' Fancy" because I won't be using any OFC wire, or gold plated tube sockets or connectors.  Instead, I'll use NOS Dynaco wire (honest!) and bone-stock RCA jacks and Output terminal boards.  (New, of course, but absolutely stock.)  I'll leave it to the next owner to go the "upgrade" path where connectors are concerned.  In the interest of reliability, I WILL be replacing the plastic tube sockets (except the "biaset sockets") with brown micanol Cinch sockets.  They are simply head and shoulders above the original black plastic sockets and won't distort after years of heat like the plastic ones do.  (The sockets will be nearly invisible when the tubes are installed).  I will also replace the selenium rectifier with a silicon diode.  This will require changing the value of the 10K resistor that connects from the bias potentiometers to ground, to a value of 4.7 K in order to expand the range of the bias controls.  Other than that, this thing will be a Stereo 70 the way Dynaco designed it.  (Well, I may have one or two more little "upgrades" up my sleeve, but we'll cover that later.)

First things first.  It's time to strip the chassis to a bare shell and see if it can be cleaned up.  There is a *tiny* amount of corrosion on the front panel.  It would be nice to minimize that somehow and polish the rest of the chassis before reassembly begins.  (I'm open to suggestions here.  At the moment, I am using "Nevrdull" to attempt to clean and polish the front panel, but if someone knows anything that would work better, easier, faster, cheaper, etc., I'm all ears!)

I'll report back later when I have new details.  Hopefully, I'll be ready to begin assembling a Stereo 70 tonight.

Sincerely,

Bill Thomas

[avahifi]

Just wanted to let you guys know that Dynakitparts.com has absolutely awesome original St-70 metal clones available, actually much nicer than new (along with everything you need to build a complete new unit).

Kevin owns a sheet metal shop and fabricates these with a labor of love.

It kind of makes stripping and cleaning up tired old rusty units pretty futile in comparison.

Regards,

Frank Van Alstine

[Bill Thomas]

Good Morning, Frank!

     I am quite familiar with Kevin and his excellent work.  That wasn't the goal here.  (The amount of pitting is really quite minimal.)  The goal here is to restore this particular amplifier back into an attractive, desirable, reliable and SAFE amplifier.  If this unit actually had a great deal of pitting, I would not waste much time in rebuilding it.  (Remember, Frank, I'm trying to "clean out the closet" here and get RID of some of these Stereo 70's.  I'm getting tired of warehousing so much of this stuff.)  When the rebuild is complete, this will be a fine "stock" (well, *mostly* stock) Dynaco Stereo 70 that will either provide years of trouble-free service, or else it will provide a good foundation for someone ELSE'S "mods and upgrades."

 Sincerely,

Bill Thomas

[Bill Thomas]

Update:

     The amplifier has been stripped of all components and is now a bare chassis.  Cleanup of the chassis has begun and will take a few more hours before I can begin re-building the amplifier.

More to come.

Sincerely,

Bill Thomas

[Bill Thomas]

Update: Well, the chassis has been cleaned up (as much as I am going to clean it).  As it turned out, there was a hidden rust spot under one of the Output Transformers.  The final result on the chassis shows a front panel that has a few *tiny* flecks of discoloration, but is actually quite pretty.  The top of the chassis shows some tiny pitting over most of the surface, as well as the "hidden" area that was rusted.  The back panel has a good deal of discolored "pits" surrounding the "Left Output" terminals, but otherwise looks quite nice.  Most of the discoloration and pitting has been improved greatly by chemical stabilization, and then some SERIOUS polishing!  Overall, I would say this chassis started out as no better than a C-, but is now a solid Grade "B" chassis.  It will look REALLY nice once the amplifier is assembled (and the top of the chassis is covered by the tube cage/cover).

Now, the FUN begins!  Time to begin re-assembly with NEW (or NOS) components.  More to come!

Sincerely,

Bill Thomas

[acwd1950]

Hi Bill,
Im interested in buying the amp when you get it done let me know when you are going to sell it and how much.

Thanks
Steve

[Bill Thomas]

The latest update:

     After spending more time on the chassis than I planned, I'm a *tad* behind schedule on the rebuild.  At this stage, all of the mechanical assembly has been done using NOS (stock) input and output connectors, a new fuse holder, new NOS Cinch Micanol tube sockets, new switches and a new Filter Choke (the old one had "leaked" wax all over the bottom panel).  I re-used two of the black tube sockets for the front panel "biaset" sockets (for aesthetic reasons).  These sockets were in VERY good condition and my stock of sockets is not *quite* cosmetically identical to the originals.  One area where I am "upgrading" the amplifier has to do with the bias adjust potentiometers.  The originals were the "cheapest of the cheap" variety.  I have replaced them with Spectrol 10-turn goodies.  They will make it MUCH easier to tweak the bias and there is no question that they will remain reliable for decades!  I have also upgraded the filter capacitor to an 80-40-30-20 uF/525 Volt version.

     Now, on to the actual wiring.

More to come!

Sincerely,

Bill Thomas

[Bill Thomas]

Update:

     The bulk of the chassis wiring has been completed.  The Left and Right Output sections have been wired, as has the Power Supply.  The bias supply remains to be completed and the driver board has yet to be installed and wired.  I will complete the bias supply and then move on to replacing any out-of-spec components on the driver board.  The amp should sing by tomorrow night.  Burn-in will follow all day Sunday.

     The next update will cover parts replacement on the driver board and final installation.  I'll update things just after the "smoke test" Tomorrow.

Sincerely,

Bill Thomas

[Bill Thomas]

Final Update:

     The Stereo 70 is now complete and is currently being "burned in" using a NOS Mullard-manufactured, GE-branded GZ34/5AR4, two matched NOS RCA 7199's and a factory-matched quad of Electro-Harmonix EL34EH Output tubes.  The amplifier is absolutely silent under "no-signal" conditions.  Bias is very stable, varying no more than a few hundredths of a Volt.  The Spectrol 10-turn potentiometers make setting the bias a LOT easier than the standard 7/8ths turn potentiometers.

     Initial Power-up was completely uneventful.  I first applied power using a Variac, with no tubes installed in order to make sure the chassis was not energized and that the Power Transformer was delivering the proper Voltages to the filaments and the Plates of the rectifiers.  Next, I installed the GZ34/5AR4 rectifier and gradually applied power using the Variac.  At around 70% of normal line Voltage, the rectifier began delivering some juice to the filter capacitor.  I gradually increased the Variac while measuring the B+ Voltage on the first section of the filter capacitor.  Without the normal load imposed by the tubes, the B+ Voltage went as high as 510 Volts.  The following capacitor sections also showed MUCH higher Voltage than one would find under "normal" operation.  The can capacitor took it all in stride with no sign of any heating going on.  Next, I installed the tubes I mentioned above.  Initial Power-up with the tubes installed was also uneventful.  Everything operated correctly.  I immediately set the bias for the Output tubes at the normal 1.56 Volts and applied a signal.  Again, everything operated properly with no added "drama."  Apparently, my careful parts selection has paid off pretty well.  (We'll discuss that in the next paragraph).  The two audio channels are absolutely balanced.  Switching the amplifier into Mono delivered a perfectly centered signal right in the middle of the near-field monitors.

     I mentioned my careful parts selection.  Here's the deal:  I wanted to keep the "Vintage Flavor" of the amplifier so I used NOS Allen-Bradley carbon-composition resistors in this build.  I carefully measured a LOT of resistors in order to match them as closely as possible.  As it turns out, Dynaco must have done the same thing because the 47K resistors in the phase-inverters on BOTH channels were absolutely identical.  In fact, Dynaco matched them better than *I* could from my current stock, so I left 'em in!  (I have more resistors in storage so matching won't be a problem in the future.)  The 270K bias-feed resistors were not quite so closely matched so I replaced them with more NOS Allen-Bradleys, matched to better than 0.1%.  The 1.5 Meg resistors were within tolerance, but not close to each other.  Granted, that shouldn't make much difference, but they were tossed anyway for more NOS Allen-Bradleys, matched to better than 0.1%.  In the same manner, I checked every resistor.  As it turned out, the 1 Watters were all closely matched and well-within tolerance, so I kept them; all except for the 1K feedback resistors which differed significantly from each other.  Once again, the trusty Allen-Bradley stock came through with two 1K resistors that had less than a 2 Ohm difference between them!  As it turned out, I wound up replacing all of the 1/2 Watt resistors on the driver board with NOS, carefully-selected Allen-Bradleys.  I also matched up four 1K Allen-Bradleys to replace the old "grid stopper" resistors under the chassis.

     As for the capacitors, I went with the "Nothin' Fancy" theme.  The 0.1 uF "Black Cats" were replaced with NTE .56 uF film capacitors.  The .05 uF capacitors were replaced with Illinois Capacitor MWR (el-cheap-o's).  The 82 pF and 390 pF capacitors checked out fine and close to each other so they stayed.  By the way, when the parts were removed from the driver board, it was thoroughly, but CAREFULLY cleaned using iso-propyl alcohol and a LOT of Q-Tips!  The original tube sockets on the driver board were in FINE condition, so I left them on the board and cleaned around them as much as possible.  As it turns out, this was probably the right move.

     You've heard horror stories about how fragile the driver board is.  Those stories are MORE than true.  Even using the most *careful* de-soldering equipment and procedures, there was *slight* evidence of two isolated trace pads beginning to lift!  Careful bending of the component leads provided mechanical stabilization and the solder solidified the connection electrically.  Please understand that the traces did NOT lift, but they showed the initial signs of "flex" at the junction of the pad and the phenolic substrate.  It is HIGHLY unlikely that the board could survive another rebuild.  But judging from the results of THIS rebuild, it is unlikely that the board will *need* to be rebuilt any time soon.

     On the main chassis, the selenium rectifier was tossed in favor of a silicon diode.  The 50 uF capacitors were replaced with 100 uF units.  The associated resistors were also replaced with new Allen-Bradley carbon-composition resistors.  The lower 10K resistor (that connects from the bias potentiometers to ground) was replaced with a 4.7K resistor, in order to offset the increased negative Voltage produced by the silicon diode.  The 6.8K and 22K resistors in the main B+ supply were replaced with 2 Watt units; again, Allen-Bradley carbon composition types.  While the amplifier was playing, I wiggled the tubes in the tube sockets.  Not even a HINT of static or noise!  Apparently, I made the RIGHT decision regarding the tube sockets on the Driver Board!

     I have been "burning in" the amplifier for over two hours now.  There is not a HINT of extraneous noise or hum.  Everything appears to be very stable.  Bias has drifted less than +/- 0.04 Volts.  I have listened to a LOT of very familiar music during that time.  The Stereo 70 delivers it all beautifully.  At the moment, the weakest part of this amplifier is the Electro-Harmonix EL34's, and yet, even THEY seem to be delivering reasonably good results.  I will continue "burning in" the amplifier until morning.  (At the moment, Sinatra is doing "Nice and Easy."  I really wish you could hear this for yourself!)  If things continue in the same uneventful manner, I'm going to consider this one a wrap!

       This has been a truly enjoyable experience.  Having it operate flawlessly right "out of the gate" was just that much sweeter!  It looks as though "Nothin' Fancy" has turned into "Something Special!"  At least, it SOUNDS like it anyway!  I hope you've enjoyed this little chronicle.  Sorry I couldn't include before and after pictures.

     That brings me to my final point: In addition to Stereo 70's, I also have a few PAS preamps (of various vintages), as well as some FM-3 Tuners (also of various vintages).  What should I restore next?  (Or should I just pack it all in and search for the "Old Dynaco Geezer's Home" somewhere?)  I'm interested in *your* thoughts.

     Well, it appears that my overall goals for this project have been met.  "Nothin' Fancy" was intended to be a good "meat 'n' potatoes" amplifier.  Nothing exotic, just good, high-quality parts, careful wire routing to minimize hum and just a "pinch" of "upgrade."  The result is an amplifier where the sound emerges from a completely black background.  There is simply NO trace of hum or noise with my ear right against the speaker.  Ed Laurent and Bob Tucker sure knew what they were doing when they designed this thing!

     So, I guess we can call this one done.  "Nothin' Fancy" may not be a "looker" (due to the pitting of the chassis), but "It *sure* can COOK!  (And YES, I really *did* use NOS Dynaco wire! - well, mostly.)

Sincerely,

Bill Thomas

[Bill Thomas]

     I just wanted to let everyone know that "Nothin' Fancy" is heading to a new home!  Thanks to EVERYONE who had some WONDERFUL comments on this one!  I will be re-building another Stereo 70 soon.  It will be quite similar to "Nothin' Fancy" with the addition of ceramic tube sockets, rather than Micanol.  I will also increase the coupling capacitors to full 1.0 uF capacitors.

     I will be re-building one more Stereo 70 later this month.  I will also chronicle the details of that build as well.  (Then, I'm giving my "soldering hands" a rest for a week!)

Sincerely,

Bill Thomas

[Bill Thomas]

August 23, 2008 - Update:

     While I *know* I said I wouldn't be posting a further update, a potential problem has surfaced that requires further explanation.  It all has to do with bias and different bias requirements of different tube manufacturers' products.

     In testing "Nothin' Fancy", I used NOS Mullard EL34's for the Output tubes.  With the parts values listed in the "Nothin' Fancy" rebuild article, the Mullards biased up beautifully.  I could get the Cathode Current as low as 35 mA per tube, and had no problem setting the bias so the Voltage measured at the "biaset" socket was dead-on at 1.56 Volts (50 mA per tube).  So far, so good.

     But, not all EL34's will deliver the same Cathode current with the same bias Voltage.  As it turns out, the Mullards require less negative bias Voltage than most other types in order to achieve an idling current of 50 mA per tube.  Electro-Harmonix EL34EH's seem to require about 1.5 MORE Volts (negative) than the Mullards to achieve the same idling current.  Siemens tubes require a bit over 2 Volts more negative Voltage than the Mullards.  This made it impossible for "Nothin' Fancy" to be adjusted much LOWER than 50 mA per tube when using Siemens tubes, and it wasn't possible to get as LOW as 50 mA per tube with a few individual tubes.  Apparently, the "bias expansion" modification was just a *bit* TOO effective.  This means that in MOST cases using the stated values in the bias circuit will result in an amplifier that will have potential difficulty in setting the bias for a few "challenging" tubes.  That is simply NOT satisfactory!

     Here's the solution:  In the stock Stereo 70, the fixed resistor values in the bias circuit are both 10K at 1 Watt.  In "Nothin' Fancy" I changed the resistor that goes from the bottom of the bias potentiometers to Ground from 10K to 4.7K.  That worked GREAT for the Mullards, but not so great with the other tubes.  Therefore, it is necessary to increase the value of that lower resistor from 4.7K to 8.2K.  This still allows SOME expansion of the bias range, but not TOO much.  It's a simple fix, but it's a fix YOU need to know about before restoring your own Stereo 70.

     You may find that your results are *slightly* different, but 8.2K is a *reasonable* starting point for that lower resistor.  You may have to go back to the original 10K, OR you might have to go a bit lower, like a 7.5K resistor.  But it seems that 8.2K is a good place to start, rather than 4.7K as listed in the "rebuild chronicles."

     I hope this will help you complete YOUR Stereo 70 restoration with a minimum of frustration.

Sincerely,

Bill Thomas