Dynaco Mark VI Project
This project began with the purchase of a pair of modified Dynaco Mark VI amplifiers. These amps were purchased through e-Bay. The modifications were the addition of individual bias controls and cathode fuses for each of the four output tubes. The meter switch was rewired to allow metering of each tube’s cathode current, however the VU functions were lost. Additionally, zener diodes were placed in the screen grids of the output tubes to reduce the screen voltage to a safer level, increasing tube life. The amps were strong, but lacked adequate definition and detail. Screwdriver slots for the gain, bias adjustments, and metering, were quite a nuisance. One of the amps, as purchased, is shown in photos 1-4.
The addition of the fuse holders and the small circuit board with the bias adjustment controls can be seen in the bottom view (photos 3 & 4). The “rats nest” of wiring can also be seen. It was decided that the amps had merit, however they needed to be completely rebuilt and the VU functions restored.
The objectives of the project were to make the amps more user friendly, clean up the wiring, utilize Auricaps for the signal coupling capacitors to bring out the definition and detail, improve the power supply regulation, add shunt capacitors to the power supply outputs, and do some body work on the chassis. In order to make the amps more user friendly, front panel controls with knobs for the gain and metering, changing the meter switch to allow full metering of each tube’s cathode current as well as restoring the VU functions, and the addition of a toggle switch on the front panel to allow selection of two input signals. The toggle switch was placed in the position of the original bias adjustment. With the addition of individual bias controls, the original bias adjustment was no longer required.
Considering the scope of the project, it was decided to completely strip down the chassis. All parts and hardware were removed. One of the chassis had rust bubbles, which required stripping and repainting. The other chassis was simply repainted. Only the top surface of each chassis was repainted (photos 5-7).
The old parts of one amp are shown in photos 8 & 9. The small circuit board with bias pots can be seen along with the original front panel controls on the metal bracket.
While the metal work was being done, the printed circuit boards were stripped down, cleaned, and then reassembled. Some of the original resistors were used, and some were replaced. The original tube sockets were replaced with ceramic tube sockets. All compensation capacitors were replaced along with the screen grid bypass capacitor, the orange drop. Another modification to the amps as purchased, a trim pot had been added to allow adjustment of the phase inverter balance. This trim pot was originally on the back of the PC board (photos 3 & 4). It is the green object above the surface of the PC board. With the addition of individual bias controls, four bias resistors and signal coupling capacitors were required instead of two, for each amplifier. These components are now located underneath the chassis and mounted on terminal strips (photos 10-12).
Once the metal work was complete, it was time for reconstruction of the amps. Mechanical assembly was first; tube sockets, terminal strips, fuse holders, bias pots, input connectors, output barrier block, printed circuit board, and the power supply filter capacitors (photos 13-20).
Some things to notice in these photos are; chassis mounted bias pots with shafts, larger power supply electrolytic capacitors, ceramic tube sockets, and although not noticeable, stainless steel hardware.
Once this stage of assembly was completed, the power and output transformers were mounted (photos 21 & 22).
The next step is wiring. After the transformers were mounted, the leads were connected to their terminal strips (photos 23 & 24).
The wiring harness was installed first, followed by installation of the components (photos 25 & 26). Notice, the PC board is installed and connected to the wiring harness. The wires were connected to the component side of the PC board. A service loop in the wiring allows the PC board to be flipped over adjacent to the front panel. The PC board was also installed with threaded spacers, simplifying its removal. In the original Mark VI, long screws and unthreaded standoffs were used to mount the PC board. The screws also held the bracket with the original gain control, meter switch, and bias pot. As a result, when the PC board was removed, everything fell apart.
After installation of the wiring harness, the components were installed (photos 27–32).
The orange drops adjacent to the PC board are shunt capacitors for the low voltage power supply outputs. The orange drops in the middle of the chassis are shunts for the bypass electrolytic across the screen grid zener diodes. The large white capacitor is the shunt for the high voltage power supply output. The large yellow capacitors are the Auricaps used for signal coupling to the output stage.
Once the components were installed, the front panels were reattached to each chassis. With the front panels in place, the power switch was installed and wired, along with the meter and the front panel controls for gain, metering, and input selection (photos 33–36).
Photo 33 shows the bottom of the chassis with the front panel attached and the power switch installed, photo 34 - the rear of the front panel and the wiring of the front panel controls. Photo 35 is a front view of the front panel with the controls installed. Photo 36 is a close up of the front panel controls and the meter.
As of the writing of this article; the amps, driving Klipsch Belles, have been in service for about three months. They sound great, good imaging and detail, with very good tonal characteristics. Vocals are natural and smooth, strings sound sweet with no harshness. Bass is full, tight, and not muddy. Reed and woodwind instruments are a joy to the ear.
The objective of making the amps more user friendly has been accomplished. Knobs are much easier to use than recessed screwdriver slots. Full metering also allows close monitoring of amplifier operation. An added bonus is the ability to select one of two sources. These amps are now a pleasure to use and listen to.
Shown below are photos 37 before reconstruction, and photo 38 after reconstruction before the front panel was installed. Photo 39 shows an unmodified Mark VI as originally wired from the factory. Only the bottom views showing the chassis wiring are presented.
We hope you have found this presentation informative and interesting. The project has been very rewarding, an extremely transparent sounding pair of Mark VI’s if you can still call them a Mark VI, and for visual effects, they peg the eye candy meter. May we build one for you?