Plenty of DIY electronics and reviews including vacuum tube audio, new and vintage gear, custom audio cables, headphones, experiments, and retro video gaming.
Likely the most popular tube amps in existence, Dynaco has
many fans both young and old. Given how well they have stood the test of time,
I hope they continue to be sought after for many decades to come. There are a
number of companies offering kits resembling the originals with modern bits and
pieces, but if you want to build or restore a Dynaco as close to the spirit of
the original as possible, Dynakit Parts offerings are a great choice.
Dynakit Parts is a company based in Paramus, New Jersey and
has an impressive selection of reproductions of all the most popular Dynaco vacuum tube products. The kits are about as close to the originals as you can get, from the
can capacitor to the screw-down terminal strips but have the benefit of using
freshly made transformers and other modern parts.
If you are familiar with the older Dynakits from the 60s,
you will remember they featured RCA inputs that are a little different than
what we typically see today. On the pre-amps and integrated amps, they were
spaced very close to one another and it is hard to find RCA cables that fit
them now-a-days, not to mention middling quality (though good for the time).
There have been several attempts to reproduce the RCA board
on the PAS using higher quality materials, however because of the spacing, not
all the RCAs were able to fit in these offerings. Now that we're in 2020 that is not the case. Dynakit Parts
has created a very nice-looking solution for those of you looking to feature
all the inputs of the original while also providing a high-quality
solution. Best of all, you do not have to cut or modify the existing chassis in any way, the kit fits like a glove.
Above you'll see the items that are included in the kit. You get the handsome brown polyimide board with integrated grounding scheme, nine pair of gold RCAs, two pair of half-watt resistors, a bag of hardware, some detailed instructions, and a nice decal to identify the connections. The cost is a very reasonable $75, especially considering how many gold-plated RCAs are included. Last I checked they run about $6 a pair at PartsExpress, so you can do the math.
Something else that $75 bucks buys you is you don't have to wire up all the ground connections. Just tightening on the RCAs makes electrical connection with the integrated grounding. The image below gives you a better look at the traces.
So what do you have to do to get this puppy ready for install? Simply populate the RCAs onto the board and lightly tighten with a socket tool of your choice. Next you add the resistors and solder in place, and viola! you are done. So simple you can knock it out in 5-10 minutes. You'd then install on your PAS chassis using the provided hardware, or if you prefer a permanent install you can use pop-rivets to fasten the board in place.
Here is the other side of the board when completed.
...and here are a couple of images of the unit installed in a chassis (the nice ground lug is included in the parts):
So what's the verdict? If you are looking for an RCA board that features all the connectivity of the original PAS, this is a no-brainer. It's well made, easy to use and the gold RCAs look like they'll withstand the test of time. You could make a similar board yourself by drilling out a FR4 board, but you'd end up paying the same or more and it probably wouldn't look as nice.
Now, if you're not a purist and don't need every connection available for the PAS, Dynakit Parts makes a nice kit with a couple less RCAs for a very reasonable $50 (as of this writing).
If you're interested in the PAS RCA board or other Dynaco-related items, whether building a kit or performing a restoration, Dynakit Parts should be on your short-list. I've ordered quite a few things from Kevin over the years and everything has been superlative.
Hope you enjoyed the write-up. For more PAS related entertainment, you can check out this restoration I did several years ago.
Review disclosure: Dynakit Parts provided a reviewer's sample of the PAS RCA Board to DIYAudioBlog for an honest review.
I've been quite busy over the last couple of years and haven't been able to build as much as I would have liked, but now that I'm getting back into the swing of things, there are a number of DIY providers who have left the space, so I wanted to put together a list of all the great companies who still offer DIY kits, boards, transformers, etc. I hope it's of use to you, and please try to support the DIY community in any way you can. There's not much money in selling just boards, it's more of a community service in many cases. This is a growing list and I'll be adding more as time goes on. If you have one you'd like to see here, please add in the comments section.
ACTIVE BUSINESSES FOR THE DIYER
AMB Labs (Solid State board and part provider): Ti Kan has lots of great solid state options including headphone amps (desktop and portable), power amps, DACs, power supplies and more. The famous β22 (Beta 22) balanced headphone amplifier is here, as well as the portable Mini³ headphone amp which shouldn't be too hard for beginners. Tons of boards and parts available and a good forum. Antek (Transformers): My favorite provider of toroidal transformers, Antek has fantastic pricing and a large variety of values, most of which are typically in stock. You'll find tube-friendly 6.3V and 12.6V heater secondaries here as well.
Audio Note Kits (Tube amplifiers, Preamplifiers, DACs, etc.): A little higher end than the typical DIY kit, ANK has quite a few nice offerings, most of which are based around universal chassis that work with several different configurations. Also one of the very few kit providers that offer a DAC, perhaps the only tube-based one I am aware of. Prices range from $1,300 - $6,850 per kit, so these may be out of reach for the budding audiophile, but I still consider them a good value for the parts that are being provided. One thing to note is that the tubes are hidden within the chassis on some models (many of us like the tubes out in the open because they not only look nice but it's easier to roll them).
AudioSector (Chip Amps): Nice simple circuit PCBs designed around Texas Instruments LM3875 and LM4780 chips.
Beezar (Tube Amp Kits): Tom (Tomb on HeadFi) runs Beezar and offers great, proven designs from ECP Audio (The Torpedo I and II headphone amps) as well as other DACs and headphone amps. Tom puts together a great kit and the documentation is unbelievably good. Every step is illustrated similarly to the Bottlehead kits. Highly recommended and beginner friendly.
Bottlehead (Tube and Solid State Kit Provider): Bottlehead provides mostly tube builds, from headphone amplifiers, to low-wattage speaker amplifiers, to pre-amps, etc. This is a good company for the budding DIYer as the directions are very explicit with color photographs and the support is quite good. Most of the builds are point to point with a few small circuit boards here and there for constant current upgrades usually. I'm certain they've sold a truckload of Cracks, a very affordable headphone amp offering. Below is Tyll Hertsens of InnerFidelity (great site) building a Crack.
Cinemag (Transformers): Very nicely made signal conversion and moving coil step up transformers.
DigiKey (General Supplier): Carries all the various passive and active components you'll need to fill a circuit board (resistors, capacitors, op-amps, sockets, etc.). If they are out of a product, try Mouser.
DIYAudio Store (Solid State PCBs, Kits, Chassis): Never to be overlooked, DIYAudio.com has its own store with some great little accessory PCBs like the soft start board, a universal power supply, as well as some legendary Nelson Pass designs and big hulking chassis you'll need to house them in. Pricing is very reasonable.
DynakitParts (Tube Kits, PCBs, Transformers): Kevin at Dynakit parts is an incredible guy. You can build Dynakits just like you used to back in the 1950s thanks to his incredible efforts. Dynaco, if you aren't familiar, is likely the most legendary audio kit manufacturer of all time. The ST70, their 35 watt-per-channel amplifier is a ubiquitous favorite, and their other models, including the ST35, MKIII, MKIV, and PAS pre-amp all have a very strong following. Dynakitpart's uses the original circuits, which are excellent, but perspective buyers may also use alternative boards from a variety of producers (and will need to in the case of the PAS pre-amp and SCA35 integrated). The newly designed boards have some nice attributes in some cases, but are becoming harder to obtain lately. The Quad-caps are very cool and vintage looking, but some people prefer cap boards on the inside instead. The beautiful thing is that this is DIY, and you can do whatever you like.
Edcor (Transformers): Affordable custom-made "E I" transformers with iconic blue bells. Everything is made to order at Edcor and takes 6 weeks to get produced. If you need something immediately, Hammond may be a better bet. ElectraPrint (Transformers): Well-made custom "E I" and moving coil transformers at reasonable prices and excellent customer service. Check out the ebay page for ready made units.
Glass Jar Audio (Solid State Kit Provider): Jeff is kind enough to assemble kits with designs from AMB, Cavelli, Toole and others. It's much easier to order through him than sourcing all the parts all over the place and the pricing is very reasonable.
Glassware Audio Design (Tube PCBs and Kits): Glassware has an impressive collection of designs, mostly around the Aikido circuit. You'll find pre-amps, phono pre-amps, headphone amps, volume attenuators, and power rectifier boards. Nice really thick PCBs that are well laid out and some offer the ability to fit bypass capacitors and large coupling capacitors (for DIYers who have strong opinions ;) ). Manuals and schematics are included with purchases. Not as friendly for beginners, start with Bottlehead unless you have some experience under your belt.
Hammond (Transformers and Chassis): Hammond offers a great array of transformers, inductors and chassis which are perfect for small to large DIY projects. Hammond is carried by a number of suppliers such as Angela, Digikey, Mouser, Allied, and others.
Magnequest (Transformers): Some of the best custom wound "E I" transformers for audio. Magnequest transformers are used in designs by Wavelength Audio, Curcio Audio, Alessandro, and other well-known makes. Check out the site for Bottlehead upgrade transformers as well.
Mouser (General Supplier): Need to populate a PCB? Mouser likely has everything you need aside from tubes and traditional tube sockets. If they don't... try Digikey.
Neurochrome(Tube and Solid State PCBs): The Modulus86 seems to be a popular design based around the LM3886 chipamp with some performance enhancements. Very low total harmonic distortion, if you're into that ;) Also a very cool 300B SET (single ended triode) PCB set is available.
Oddwatt (Tube Kits): Oddwatt is a two person team (Bruce and Rodney) offering a number of tube amplifiers and a preamplifier with simple but effective circuitry. The kits include Edcor transformers and custom chassis and PCBs. I don't know too much about the company, but there is a nice write up on DIYAudioProjects.
Pete Millett(Tube PCBs): Tons of great designs for tube amplifiers, pre-amplifiers, headphone amplifiers and the like. Some beginner friendly designs like the Butte and Jonokuchi headphone amps, as well as expensive, esoteric amplifiers designed around unusual tube compliments. Definitely worth checking out!
Transcendant Sound (Tube Kits): I havent purchased a kit from Transcendant Sound (yet), but have heard many good things. From what I see they look to be a tremendous value. The Grounded Grid pre-amp is quite popular, and the OTL (output transformerless) tube amp designs are said to offer treble clarity rivaling solid state.
Triode Electronics (Tube PCBs and Kits): Dynaco-based amp kits as well as some guitar amplifier kits. Very nice selection of parts and tubes as well if you are building a vintage amp. Directions not quite as perfect as Tubes4HiFi or Beezar, but still very good.
Tube Audio Lab (Tube Amp Kits and PCBs): Great and not overly complex tube-based point-to-point kits and PCBs using the classic 300Bs, 45s, 2A3, etc. Parts selection based on your preferences.
Tubes4HiFi / Bob Latino (Tube PCBs and Kits): Roy and Bob run a really great DIY amp and preamp site. There are models based on the Dynaco ST70 and MKIV, as well as some higher wattage versions if you have some pretty demanding speakers or a big room to fill with sound. Lots of great kits, excellent directions for most offerings (some of the pre-amps have more basic schematics, etc).
Great products at reasonable prices from my experience.
NO LONGER PRODUCING DIY PRODUCTS / IN HIATUS
ChipAmp (Solid State PCBs and Kits): Allen at Chipamp has a fantastic array of chipamp / gainclone builds based on the LM1875, LM3886, and LM3875. He also has some great power supply, volume and switcher PCBs. Very simple and easy builds. Beginner friendly except the power amps use high amperage and would be dangerous if safety measures aren't followed carefully. UPDATE: per communications at DIYAudio.com and HeadFi.org, it seems there may be issues with order fulfillment in 2016-2017.
Classic Valve (Tube and Solid State PCBs): Geek at Classic Valve is getting out of the PCB business to shift his attention to building turn-key amplifiers. I encourage you to buy up what he has left. There are a number of very unique Dynaco boards for the PAS, ST70, MKIII and MKIV as well as power filters and other great stuff. UPDATE: It looks like the supplier CK Magnetics is also clearing out their leftover PCBs, so grab em while you can: http://www.ebay.com/usr/ckmagnetics
DIYTube (Tube PCBs): Shannon at DIYTube is getting out of the PCB business to shift his attention to building turn-key amplifiers. You will still find excellent documentation on DIYTube.com and a semi-active forum, however the boards are no longer available.
Transcendar(Transformer Producer): Once an extremely high value "E I" transformer producer, this maker appears to no longer be producing new products.
The Fine Print: DIYAudioBlog and Zynsonix Audio, LLC does not represent or have any business affiliation with any of the above businesses or individuals. Building/modifying circuits can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.
The Dynaco Mk III monoblock was introduced by Dynaco was back in 1957, just after the Mk II and the PAM-1 pre-amp and two years before the famous ST-70. The amp was designed by David Halfer and Ed Laurent, and the kit sold for a mere $79.95 in 1950s dollars. Each monoblock makes use of a pair of KT-88 power tubes, a 6AN8 driver tube and a GZ-34 rectifier tube. Later in the Mk IIIs life, the power tubes were switched to 6550s. The output power is rated at a very health 60 watts per channel, which should easily drive all but the most demanding speakers.
After building slightly modernized variants of the Dynaco ST35 and ST70, I felt that the Mk III was the next logical step. Of course there are some deviations from the original design that I will be pursuing. The first is based out of a concern that was addressed on the SDS Audio Labs page, "The capacitors in the [mkIII] power supply are not rated at a high enough voltage for the power transformer. This results in the quad cap being run over it's rated voltage every time the amp is turned on. The first section of the original quad cap is rated at 525 volts and the B+ goes as high as 585 volts during start up. This is due to the fact that the rectifier heats up and begins conducting before the power tubes heat up and start drawing power from the high voltage supply." Thankfully, SDS Labs offers a solution to this: a capacitor board that can handle anywhere from 630-800 volts depending on the configuration. The SDS Labs board is currently available from the folks over at Triode Electronics, and even populated, it doesn't cost much more than a typical can capacitor.
SDS Cap-board left, Triode driver board right
The second deviation would be a more modern driver board for the Mk III. There are currently several on the market, one called the Poseiden from DIYTube, one from Tubes4HiFi that incorporates additional electrolytics on the board its-self, one from Curcio Audio and one from Triode Electronics. All four boards make use of two driver tubes as opposed to the stock version's single tube. Since the SDS cap-board was being utilized, I didn't feel that the Tubes4HiFi board's additional capacitors would be necessary, and the Curcio board makes use of 6922s, which have been used for audio for many decades making it difficult (read: expensive) to get vintage tubes. The Triode Electronics version uses a pair of 12AU7 (or others depending on the configuration) and the Poseiden uses a 12AU7 and 12AX7 (or others as well). Both seemed to be perfectly up to the task, so the Triode version was chosen as I was already putting an order in with them for the cap-boards.
Schematic for the MKIII w/ Triode Electronics driver board
Finally, in order to make the build a bit more unique, a gorgeous pair of 10x13 blank chassis were ordered from eBay seller po1019. These arrived with walnut wood sides ready for finishing. Holes will need to be drilled to accommodate the tubes, transformers and other various and sundry switches and connectors, but this will offer a bit more room to arrange things within the chassis.
Some of po1019s fine chassis work
The first step was to begin arranging the chassis and drawing out where the boards, tubes and transformers would be sitting. This would prepare the chassis for accurate holes to be drilled on the drill press. A permanent marker was used in this case as the chassis would be powder-coated, however light pencil markings would be used if the chassis would be left as bare aluminum.
Next would be the population of the capacitor boards. The way the capacitor board is set up, it runs the caps in series, which halves the capacitance but doubles the maximum voltage rating. Two 100μF 400V caps in series is 50μF 800V, as long as the proper dividing resistors are used between them to split up the load effectively. Therefore the whole board offers four banks of 50 micro-farads. I used some 30mm tall by 25mm wide Panasonic (Matsushita) snap-in caps to fill up the banks and some Koa Speers as the dividing resistors. There is also room for bypass capacitors on the board, Solen 0.1μF 630V caps were used for that purpose.
Populated SDS capacitor board
The Triode Electronics driver boards were then populated with a variety of Kiwame two watt carbon film resistors, silver mica capacitors and a pair of gold-plated ceramic tube sockets. Kiwame carbon film resistors are chosen for their warm and natural sound compared to modern metal film. Also, the values don't "drift" over the years like sweet-sounding carbon composition resistors, which is another benefit.
Populated Triode Electronics boards
The capacitors for the board are AmpOhm paper in wax, which will be mounted separately and leads run to the board given the size. Since the design is vintage, I figured some vintage style paper-in-wax caps would be a nice feature. Other nice parts for the build include Sprague Atom electrolytics and Cardas bare-copper binding posts.
AmpOhm & Sprague Caps, Cardas Posts
One of the most time consuming parts of any build is preparing the chassis. This involves laying out the parts where you think they would look and perform the best, measuring to verify your lines are straight, marking the chassis and drilling / punching a ton of holes. Since the two 9 pin tubes sit to the right of the Triode board and I wanted the tubes to be centered on the chassis, the board is offset to account for this. The three eight pin tubes sit behind the board, and the big hulking power and output transformers will sit in the back.
Drilled and punched aluminum panel
The next bit of work is staining the mahogany wood panels that came with the chassis. These amps will feature a red and cream color scheme rather than the typical copper and antique copper colors just for the sake of keeping things fresh and interesting. So the wood panels are being stained a nice deep red with a water-based dye.
Chassis wood panels before staining
The wood panels were initially sanded with 220 grit sandpaper and would be stained red like the Frugel Horn Speakers with water-based stain. Bright Red Trans Tint was added to a clear tint base to get a nice bright red. Water-based stain has it's own set of products for pre-stain prep and finish, which you can see in the below photo.
The water-based stain lineup
The initial prep includes a pre-stain layer, which is followed by a light sanding. This prevents the grain for raising inopportunely during the staining process. This is followed by the wood stain. The TransTint is to be mixed at a half bottle per 32 ounces; in this case the whole bottle was used for a deeper, more saturated tone. Three coats of this stain were applied to get the wood evenly dark. Next, three layers of water-based satin Polycrylic are brushed on to seal up the finish. Finally, the four pieces were rubbed with a wax-based product to give them a bit of a luster.
Wood panels stained red
Both chassis and the transformer bells came back from the powdercoater, the chassis being a nice cream color and the bells a vibrant cherry red.
Custom chassis with a cream powdercoat
Transformer bells in vibrant cherry red
The bells can be installed back on the big beefy power and output transformers now that they're painted. Rather than use the existing hardware, brass screws, washers and acorn nuts were used. Since brass hardware is harder to come by than its zinc-plated counterpart, one has to make-do with what's available, which usually includes cutting longer size screws to fit. Using a small tabletop vice and a rotary tool with cutting disc, the screws are quickly trimmed to the perfect length. The process is loud and metal dust goes everywhere, so both hearing and eye protection were used.
Brass screw being trimmed down
Once a full set of hardware has been adjusted to fit, the bells can go back on the transformers.
The bells mounted to the power and output transformers
With the chassis fully prepped, the switches, jacks and other connectors could be fastened in place. Starting with the rear panel, a 3 prong power IEC inlet, heavy duty SPST power switch, fuse holder, Cardas copper binding posts and a Cardas RCA jack were installed.
The mkIII rear panel
Next came a set of PTFE / Teflon tube sockets and aluminum base plates. The aluminum tube base plates came from a company called VT4C in Hong Kong, I don't know of anyone else in the world who makes them. They are really only for decoration purposes, and were powdercoated red to match the transformer bells. Holes were punched with a Greenlee die-cut tool for the tube sockets and two small screw holes were drilled adjacent each punched hole to mount the plates. The heads of the screws for each surround were painted red using a Sharpie oil-based paint marker.
PTFE tube sockets and aluminum tube surrounds
The bottom of the base plates are compatible with aluminum rings that VT4C sells. These work with Teflon tube sockets and allow one to tighten the socket in place without using a traditional socket mounting bracket.
VT4C aluminum rings holding the PTFE sockets in place
To facilitate the soldering to the pins of the tube sockets, PCBs are fastened to the pins with solder which provides an empty solder hole for each pin. Also installed were the C354 choke which is directly under a transformer, a bias pot made by PEC, a heavy duty DPDT switch to allow for triode and pentode mode switching and a set of board-mounted solder tabs where the oversized paper-in-wax AmpOhm caps will reside. The gold-plated board-mounted solder tabs are from VT4C as well, they are the first gold-plated solder tabs I've seen that were reasonably priced.
Socket PCBs, choke, DPDT switch and solder tabs installed
The board was installed next using a set of aluminum standoffs, screws and locking washers. As noticeable in the picture below, the board is offset so the 9 pin tubes end up in the middle of the chassis. The AmpOhm caps were mounted in such as way as the smaller 0.22uF caps were on one side and the larger 1uF caps were on the other. It worked out to be a perfect fit, which was carefully verified before the caps were ordered ahead of time. The caps were arranged so the lead would be soldered to the solder tabs, then a piece of Kimber TCSS wire was run to the proper place on the board. The bottom of the board was labelled in red with the cap locations for ease of installation. Each capacitor was aligned so that the outer foil and inner foil were in the proper place.
AmpOhm capacitors installed
The big hulking transformers were then fastened to the chassis. A bit of the powdercoat was sanded off around the screws on the transformer bells and the chassis so that the bells would be properly grounded. Rubber washers were also added between the transformers and the chassis to ward off a little bit of the vibration. Nearly every inch of the chassis is being used, so the SDS cap board and C-354 choke are mounted underneath the transformers. A very handy turret tag board was fastened to the middle of the amplifier, this would be used mainly for the bias circuit.
Transformers mounted to the chassis
The amp can finally be wired up now that everything is in place. The transformers from Triode Electronics feature nice silver plated copper in Teflon wire, which really beats the typical PVC covered wire that comes with most transformers. Not only is there less loss from the dielectric, but you don't have to worry about it melting when soldering things that require a lot of heat like the binding posts. Nearly all other wiring was done with Kimber TCSS wire (19 gauge copper in Teflon). This was doubled up for the power inlet (aggregate 16 gauge). Shielded Cardas 2x24 was used for the signal input to help prevent EMI interference. As pictured below, nice quality Sprague and Solen caps were used for the bias circuit.
MKIII almost fully wired
Compared to the original layout in the 9x9 chassis (pictured below), the custom 10x12 chassis allows for more air around the parts and a more logical topology in my humble opinion. The transformers were able to be moved further away from one another, the bias test point was able to be conveniently moved to the top of the unit, and the tubes were able to be arranged so that the amp has a definitive front and back, which was harder to identify in the original version. Also the gargantuan AmpOhm caps would never have fit in the original chassis.
Example photo of an original MKIII
The pilot light was then installed, running off of the 6V tube heaters. The SDS cap board was also grounded in a couple of different places.
Wiring on the MKIII complete
Now the tubes could be installed and the chassis decorated with brass plates. A relatively low-cost tube compliment will be used with these monoblocks, the Tung Sol 6550 power tubes, Sovtek 5AR4 rectifier and JAN 5814 (12AU7 equivalent) driver tubes.
MKIII unit front
MKIII unit rear
All that's left to do is finish up the bottom plate prep. A number of holes were drilled for the sake of ventilation, which is especially important for tube amps as they can get pretty warm. A number of holes were drilled to go directly underneath the power and rectifier tubes, as well as under the chassis where the transformers sit.
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The Fine Print:
Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.
The ST35, also known as the Stereo 35, was released by Dynaco in 1963. It's the baby brother of the Dynaco ST70 and tends to be overlooked by many vintage amplifier aficionados. I think the main reason this is the case is that there isn't a large price difference between the two units, and with a tube rectified power supply, power choke, bias system and twice the wattage, it's hard not to want to step up to the ST70. For those of us with relatively sensitive speakers, the ST35 is a nice little unit with a small footprint and more than ample power (17.5 watts x 2). I've also read from more than one source that the ST35 is the best sounding of the Dynaco units stock, for whatever that's worth ;)
Because there are a lot less ST35s floating around, there aren't really any boards with modern circuits that fit into the original chassis. Shannon Parks of DIYTube has created a brand new circuit board that is similar to the ST35 circuit with some modern sensibilities if one would like to go that route. I prefer the looks of the original ST35 chassis myself, with the exposed PCBs on the ends. For this, I turned to Dynakit Parts. Dynakit Parts is a company based in Clifton, New Jersey and has an impressive selection of old school reproductions of Dynaco products. The kits are about as close to the originals as you can get, from the can capacitor to the screw-down terminal strips.
Dynakit Parts ST35 Kit
The original Dynaco ST35 Schematic
Rather than purchasing the whole kit, I purchased a number of the parts a la carte, as I was interested in adding a little bit of customization to the kit with a power choke, power switch, triode / ultra-linear switches, a C8 power inlet for a custom C7 removable power cord and a number of boutique parts. I also added the ST35-BCU bias control upgrade kit which fits nicely above the chassis. As stated on the Dynakit site: The ST35-BCU employs an adjustable fixed bias circuit which allows for user adjustment of the bias setting of each (EL84) output tube via (4) separate on board precision bias pots. This allows you to not have to purchase a matched set of power tubes. And even if you did purchase a matched quad, power tubes tend to age differently, so they can be adjusted after this occurs.
Kevin of Dynakit Parts was working on a brand new brown PCB board for the ST35 when I contacted him. I really appreciate it when a company isn't resting on their laurels but investing in new products and innovations. The new PC-13 boards from Dynakit is made from aerospace grade 240 degree Centigrade brown Polymide mil spec material and features silk screened component designations. The brown will go very nicely with the vintage patina powdercoat scheme that I had planned. I wish the bias board was made of the same nice brown material, but we can't have it all, can we? ;)
Brown Polymide boards with a few parts in place
I elected to populate the small driver / power boards with Kiwame carbon film resistors, Obbligato and Mundorf Supreme film capacitors and Teflon and ceramic tube sockets (the Teflon sockets even have beryllium copper contacts). The caps fit pretty much perfectly, although the Mundorfs will have to be mounted once the boards are mounted due to sizing constraints. The Dynakit build calls for nicer silver mica capacitors in place of the ceramic disc capacitors that were in the older units. These can be found just about anywhere (Mouser, Digikey, Partsconnexion, Handmade, etc.).
I chose to go with the cadmium metal chassis as opposed to the newer stainless steel version as the chassis is going to be powder-coated; no need to pay extra for that chromed-out finish when no one's going to see it ;) Below you will see the chassis, I've enlarged the holes to fit the Cardas binding posts and RCAs, and the center hole has been enlarged to hold a piece of FR4 material that will then hold the C8 inlet in place. I then gave the chassis a nice sanding and labelled it for the powder-coater.
A few chassis mods and a bit of sanding prep
One thing I would be painting myself was the power choke. A choke is a nice cheap upgrade for the ST35 and helps reduce ripple in the power supply. A number of people have used the C354 to replace the 50 ohm resistor, I chose a Hammond 156R with 56ohms DCR, which is pretty much the equivalent. Chokes tend to be on the ugly side, so if they aren't hiding in the chassis (there's absolutely no room in this case), they need to be dressed up a bit. I was prepping two at once, one for the ST35 and one for the Millett Jonokuchi.
The first step in this process is to sand down the metal frame on the choke. Then painters tape is cut and applied to mask off the inside. I've seen some builders just paint the whole thing, but I don't really care for that look.
Chokes masked off with painters tape
Next I apply two layers of primer, one per hour, then multiple coats of Hammertone paint. It can be difficult to get the Hammertone paint to lay correctly without making the coat too thick. Any over-spray is corrected using black permanent ink.
Chokes painted with copper Hammertone paint
Once dry, I measured and cut some strips of black leather and used some epoxy to keep them in place, wrapping around the spool of the choke. I use a couple layers of leather when possible to give the spool a fuller look, it's more aesthetically pleasing.
Completed chokes with leather wrapping the spool.
UPDATE (6/19/11): The parts arrived for the ST35 power / driver boards, so I went ahead and populated everything except for C4 and C5, which will be 0.1uF Mundorf Supremes that will hang off the board just a bit and will need to be installed once the boards are fastened to the chassis. C1 was not used per Bob Latino's suggestion as its purpose is DC input blocking and nearly all modern-day preamps already have a DC blocking output cap, so I used a pair of thick leads to short C1. As you can see below, the Teflon tube sockets have blue PCB boards installed to ease the soldering process. The sockets had a rather long ground pin that I trimmed down and sanded smooth as there would have been a clearance issue in the height challenged chassis.
ST35 boards mostly populated and ready for installation
UPDATE (7/8/11): The chassis is pack from the powder-coater so I was able to fit the boards in place. I opted to install them rotated 180 degrees as I wanted the RCAs on the rear of the chassis and there wasn't sufficient clearance with the power tube sockets blocking the way. I took some generic gold-plated RCAs from my parts bin and trimmed off the back with a cutting disc, rendering them nonfunctional but small enough to squeeze between the tube socket and the chassis front, simply for the sake of aesthetics. I was also able to position the painted Hammond filter choke, attach the Cardas copper binding posts and install the Cardas rhodium plated RCA jacks. Also installed was the large silver multicap and the bias control board from Dynakitparts.
Getting started assembling the parts on the chassis
In the interest of making things convenient, I installed a small 6A SPST toggle switch on the front panel of the chassis so the amp can be powered on and off conveniently.
A 3 Amp power switch installed on the front of the unit
The ST35 Bias Control Unit from DynakitParts makes use of an adjustable fixed-bias circuit, allowing for adjustment of the bias setting of each EL84 power tube via four separate precision bias potentiometers.
Wires run to the Bias Control Unit from DynakitParts
The initial wiring was completed using Kimber TCSS 19 gauge stranded copper in teflon dielectric. I opted to drill out the area originally designated for the fuse holder and the cable in the interest of fitting a C8 power inlet. Because the re-purposed real estate on the chassis back was a little too large for the C8 inlet, I mounted it to a piece of FR-4 material and then mounted the combo to the chassis. This adds a degree of convenience, allowing a removable C7 power cord to be used to power the amp. The fuse was then moved inside the unit and mounted to one of the chassis braces, really a perfect place for it in my humble opinion. Grounding lugs were added to both chassis braces to connect to the RCA grounds (the Cardas RCAs are insulated from the chassis unlike the stock Dynaco RCAs) and the negative connector of the speaker binding posts.
Initial wiring of the ST35 complete
UPDATE (7/28/11): The transformer bell-ends arrived back from the powdercoater and were reattached to the iron with brass hardware. Rather than using the cut transformer wire, I prefer the Kimber TCSS Teflon wire for all extraneous point-to-point wiring. The dielectric doesn't melt, it's a little thinner so it's easier to route, and I'd imagine the copper's of a higher purity. I opted to add a set of three 1uF 630V Solen film capacitors as bypass caps for the can capacitor. The Solen film caps are only a couple bucks a pop and have the potential to add refinement to the power supply. It's a narrow fit in the ST35 case, no room for zip tie mounts for the caps.
Transformers wired in place, Solen bypass caps added
The wiring was finalized by adding a pair of DPDT switches to manage the triode and ultralinear switching. This was wired based on the following advise from Bob Latino and Tazsmonn on the DynacoTubeAudio Forum: Remove green or green white wire from pin #9 of each tube and solder to separate poles of DPDT switch on the same end, Solder wire from pin #9 to center pole of DPDT switch, Solder 100 ohm [resistor] from pin #7 to DPDT switch opposite end from green or green white wire, All wiring to and from switch side must be to same tube.
Triode mode switches wired up
Now that the wiring is complete, a final checkout of the circuit is performed, tubes and installed, the unit is powered on and the tubes biased using the ST35-BCU. The tubes installed were Mullard EL84s and Phillips EGC 12DW7. The sound is quite excellent, very resolving and holographic imaging. It actually sounds quite "hifi" for a circuit from the early 1960s. I could use a tiny bit more bass, but I was listening on a small set of Tektons with 4" Fostex full range drivers. Here's some additional progress pics:
Dynaco ST35 front view
Final Dynaco ST35 back view
Dynaco ST35 side view
UPDATE 8/8/2011: I was able to have a pair of custom engravings made for the unit. A 1" gold circle with the word "Dynaco" engraved in black and a small rectangle with a Dynaco Stereo 35 logo that I made in Adobe Illustrator. These are the final touch.
Final Dynaco ST-35 front view
Final Dynaco ST-35 top view
Engraving detail on quad-cap
Final Dynaco ST-35 rear view
To recap, the customizations to the kit were as follows:
Teflon tube sockets with Beryllium copper contacts
Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.
