The completed amplifier: Now on to the building stages!! (Note: The first time we listened to this amplifier was with a Zoot Simms playing tenor sax CD and then a CD with a large choir singing Mendelssohn. The sound was crisp, yet mellifluous with deep rich undistorted sonorous bass. The image that came into my mind was the pouring cream or honey. How do you really describe a wonderful sound? I guess you can't. Wine and cheese tasters have many wonderful and colourful words. But nothing can really put those experiences into words. With audio amplifiers, the final arbiter for the experience is the sound itself and the sound from this amplifier is as good as I have heard. Truly a surprise and a wonder after bolting and soldering a bunch of pieces together!
When in comes to instruction manuals and pictorial illustrations to guide you, this is about the best I have ever seen. It was extremely easy to check each step against the very clear and detailed colored pictorial that clearly showed the wiring and postioning of the major parts.
Here is a sample page from the assembly manual. It is very well written and gives you hints and suggestions for upcoming steps at exactly the right places.
Day 1: Parts for the amplifier unplacked and laid out ready to go. The pre-punched new chassis is identical to the old chassis in size, shape and all chassis markings but instead of nickel plated steel like the old chassis, the new chassis is made of non-magnetic mirror polished type 304 STAINLESS STEEL.
Day1: Here I have layed out the small parts used for the driver circuit board. These small parts come in resealable plastic sleeves and are labelled for easy identification and retrieval. This is especially useful if you are not familiar with things like resistor codes, where resistance is indicated by having 3 or 4 colored bands (depending on the system used) on each resistor. Other bands are for tolerance, etc.
Day 1: This is the driver board for the new amplifier with no parts yet added. The driver board on the original ST-70 was a brown board with open solder traces made of a very inexpensive phenolic material. As the years went by, this board would not stand up to the many hours of hard use and would actually start to burn from the heat. On the area around and under the 7199 tubes the board would turn a blackish color on many ST-70’s. Expansion and contraction from the heat would cause, at times, cracks in the open solder traces. When a crack occurred, the usual symptom was noise and/or intermittent output usually in just one channel. The VTA (Vacuum Tube Audio) driver board used in this new amplifier was designed by Roy Mottram of Puyallup, WA. This board uses an all triode design and is a double sided military spec printed circuit board made of epoxy fiberglass. This is a very high quality material and will not go bad over time.
Day 1: The driver board with the three 12AT7 dual triode tube sockets now soldered in place. This board has an onboard bias system and separate bias controls for each output tube. The original amp had one bias control for each PAIR of tubes. This newer system allows the settings of the bias for individual output tubes rather than requiring matched pairs of these tubes at considerable extra expense. You can see the four blue bias controls on each corner of the board. Once the amplifier is completed these four bias controls are availalble from the top of the chassis so that they can be adjusted with a meter to accurately set tube bias values.
Day 2: The circuit board with all tube sockets, resistors, potentiometers and capacitors soldered in place. What looks like 4 blank spaces for capacitors (C3, C4, C5, C6) are for the 4 capacitors that have already been mounted on the underside of the circuit board. For 2008, all amplifier kits from Bob Latino come with a triode/pentode kit at no extra charge that are installed as you build the amplifier. This "retrofit" may be done to any previous ST-70 with a VTA driver board. With this added mode, the power is reduced to about 22 watts per channel. The amp will have a little more mid range projection with a very slight rolloff of the extreme upper and lower frequencies. Nice for acoustic jazz, vocals and systems with an aggressive top end.
Day 2: The underside of the chassis with only the five octal tube sockets attached. The tube sockets on the older amps were a phenolic plastic material and could burn and cause intermittent contact on the tube pins as the pins slowly oxidized with time. The tube sockets on this new amp are modern high quality Micanol sockets with cadmium plated pins for better electrical contact. Cadmium is extremely durable and will not oxidize with time.
Day 2: The two heavy duty output transformers are on the left and the power transformer is on the right. In the original amplifier the power transformer was big enough to run the amp but had no reserve and as the years went by many ST-70's started to be plagued with hum, buzzing and mechanical vibration issues that emanated from the power transformer. The new power transformer in Bob Latino's amplifier, although looking near stock, has a higher stack lamination and a much greater power handling capacity. On the old transformer both tube heater legs could output 6.3 volts AC @ 3 amps per channel (6 amps total) while the new transformer is rated at 6.3 volts AC @ 5 amps per channel (10 amps total). The old power transformer was rated at 115 volts which was the AC voltage in homes in the 1950’s while the new one is rated for modern AC voltage levels of about 120 volts.
Day 2: The output terminals on the original Dynaco amplifier were simple screw connectors. The newly designed amplifier uses gold plated 5 way binding posts to prevent oxididation over time that would eventually cause poor or intermittent contact and loss of power to the speakers. You can see these terminals in place in the top left and top right of the next picture.
Day 2: The under side of the chassis with the power transformer bolted in. The speaker terminals, the on/off switch and the fuse holder have also been mounted on the back of the chassis.
Day2: Top of the chassis showing the power transformer, 2 output transformers, the main filter capacitor and the 5 octal tube sockets. You can also see my working finger prints on the bottom left corner of this beautiful non-magnetic stainless steel chassis. The A-470 output transformers, on the upper left and upper right, are brand new custom production transformers. They use the same interleaved/layer winding technique as the original cloth lead A-470's and incorporate the same high quality M-6 grain oriented laminations. They are dimensionally accurate to the original design and have a durable heat resistant black epoxy finish.
Day 2: You can see the dramatic increase in the number of wires that need to be dealt with after the installation of the 2 output transformers and the choke coil.
Day 3: Arnold back at it and working away soldering wires on the underside of the chassis. The assembly manual is very well written and extremely easy to follow (a refreshing change from most sets of instructions)!! The included, and very clear, colored pictorial of the wiring on the underside of the chassis, makes it really easy to check each step you make against that pictorial. (Photo by Lise)
Day 3: The wiring of the chassis begins. Towards the bottom right you can see the choke coil (C-354) that will help filter the ripples in the DC voltage. On the bottom edge: the left speaker terminals, fuse location, power switch and right speaker terminals.
Day 4: My workspace on day 4. There is the desirable cup of coffee, light for working on dark days, flashlight when working in dark corners, meter for checking the integrity of contacts, tape measure for measuring wire lengths and almost at the top corner just to the right of the wires, a 20x magnifying glass for checking the the quality of solder joints.
Day 4: Most of the wires for the power transformer, the output transformers, the choke and the electrolytic capacitor have be soldered in place.
Day 4: Most of the chassis wiring and part mounting completed. The circuit board has now been installed but only partially connected to parts on the chassis.
Day 5: The mulit-sectioned electrolytic capacitor for filtering the DC current (along with the choke coil) is the shiny can exactly in the centre of the photograph. The original Dynaco 70 had 30, 20, 20, 20 microfarad (μf) capacitors (total 90 μf). This amount of capacitance was enough to run the amplifier at lower power levels, but would cause the audio to break up prematurely at higher volume levels. The replacement capacitor shown here with 80, 40, 30, 20 (170 μf total) has a much greater charge reserve, allowing the amplifier to run at higher output levels without distortion.
Day 5: Around 4 pm and ready for adjustments and testing. The finished amplifier is moved into our family room and connected to the speakers. The power is turned on and the 4 bias voltages adjusted using the multimeter connected in turn to 4 octal socket connections on the front edge of the amplifier. You can see 2 of the blue potentiometers used for the bias adjustments on front edge of the circuit board. Two days after I completed the amplifier, I realized that the analog multimeter scale of 3 volts I had used was too large to measure 0.4 volts accurately, so I purchased a good digital multimeter and reset the bias voltages.
Day 5: The completed underside of the chassis with the circuit board lit up by daylight flooding through the window.
Day 5: Completed and resting in its final place on top of our DVD/CD player and the NAD receiver used as a preamplifier.
Day 5: The view with the lights turned down low, relaxing and listening to Zoot Sims (tenor sax) playing "For Lady Day", a tribute to Billie Holiday. Recorded April 10 and 11, 1978.
Day 5: Up and running and making very beautiful music in our family room. A British NAD 7250PE Stereo Receiver is used for the preamplifier with the input to the Dynaco ST-70 fed from the line output of the NAD. The power amplifier of the NAD is not used. The two speakers on each side of the room (you can see the left speaker behind the rocking chair) are Celestion DL10 8 ohm speakers that handle 10 to 190 watts of power with up to 90db sound level. The sound is so beautiful it makes you weep. The other outstanding part is that our Kawai digital piano (off to the right of the picture) sounds better than it ever did with the our old amplifier.
Stage 2: Adding the triode/pentode switches. These are the parts for the new kit (as of 2008, now included with the Dynaco 70 kit). Addtion of these parts took around 3 hours.
The vacuum tubes have been unplugged and placed in a safe place so that they won't roll on the floor and break. You can see the two holes for the switches and the small hole in between for mounting the 2 two lug terminal strips.
The top of the chassis with the 2 triode/pentode switches in place.
The underside of the chassis with the 2 DPDT (double pole double throw) switches and the 2 terminal strips in place.
The completed wiring for the left channel switch.
The wiring is now complete. The bottom cover was replaced and all tubes reinstalled. With the amplifier back in place, everything worked perfectly. Our wonderful sound was back after the upgrade.
The new ST-120 Amplifier is now available. It has an output of 60 watts per channel with both channels driven from 17 Hz to 30,000 Hz at less than 1 % harmonic distortion. Contact Bob Latino at 508-347-9120 or Email at > Bob01605@aol.com for more information about the amp or to purchase an amp.
ST-120 amplifier internal wiring.
Comparison photo of a stock ST-70 and the ST-120 showing the differences in the transformer sizes.
This composite photo shows a square wave comparison between the old Dynaco 60 WPC Mark III with the original 6AN8 driver board and my 60 WPC ST-120 amplifier with the updated (and modified VTA driver board) at 10 KHz. There are a few changes that have been made to the components on the ST-120 VTA driver board as opposed to the driver board on the VTA ST-70 amp. These changes in a few components better deal with the higher voltages and larger current needs of the ST-120 amp. The comparison photo shows a quicker rise time, wider bandwidth and lower phase shift of the new amp.
