- Written by John E. Johnson, Jr.
- Published on 24 November 2011
The Design of the MD-75 Integrated Amplifier
The MD-75 has a four gain-stage circuit. The 12AT7's are dual triodes, operating in Pure Class A (most preamplifiers operate this way), with two gain stages. They produce a single-ended voltage gain signal, which then passes through the driver tubes that are phase inverters with some gain, then on to the output stage where the output tubes produce current gain. Since there is an even number of gain stages, the output is in phase (not inverted). The MD-75 would be classified as operating in Push-Pull, Class AB.
The standard version of the MD-75 comes with two 6SN7 drivers and with eight 6CA7 output tubes. However, there are several options: you can purchase the MD-75 with Tung Sol 6SN7's or Black Treasure CV-181's instead of the standard EH 6SN7's for the drivers , and you can also get Tung Sol 12AT7's (input stage), Tung Sol 6550's, Tung Sol KT88's, Genelex KT88's, or Black Treasure KT88's (output stage). Each has its own sonic characteristics and is one of the things that "tube rollers" do. The sockets are the same, and the only thing that needs to be changed is the bias voltage on the grid of the output tubes.
When I unpacked the contents of the shipping box, this is what was inside:
The tubes were all in individual boxes, with the KT88's marked as to which socket they should be inserted into (the factory sets the bias for each KT88 and its corresponding socket). There is a plastic cover with ventilation holes to use if you have small children, but otherwise, I think you will prefer it, as I do, with the cover off.
The power supply (solid state) is housed separately, and is connected to the rear panel of the MD-75 with two thick cables:
Below, is a photo of the rear panel, with the 4 ohm and 8 ohm speaker binding posts, and the power supply jacks. There is also a pair of pre-in RCA jacks should you just want to bypass the preamplifier and just use the power amplifier.
There are four pairs of RCA input jacks on the side, for Aux, CD, Tape, and Tuner. So, even the labels for the inputs are "Retro" (do you still have a cassette tape deck?) More likely, you would use them for your CD player, media player, and perhaps the output from a phono preamplifier.
The inside of the chassis has one very small circuit board, but the majority of the circuit is hand wired point-to-point. Notice how meticulous the wires are laid out. They all run parallel or perpendicular . . . and exactly so.
Once the tubes are inserted, and the power supply is connected to the main unit, you plug the power supply into the wall. This is important, as you should not plug the power supply into the wall and then plug the power cables into the main unit.
OK, so it's all together, plugged in, you take a deep breath, and flip the power on/off toggle to the on position. If you have done everything correctly, the amplifier will look like this (you can see the four sets of RCA input jacks on the right rear):
The tube filaments will all glow orange-red. A blue LED will glow on the front to tell you all is well. On the left is the volume control, which is motorized, and which rotates fully counter clockwise when you turn the amplifier on. To the right of the volume control is a toggle to select Triode Mode or Ultra-Linear Mode, then the LED, a toggle to select Power Amp or Integrated, and finally, the Input Selector knob.
The MD-75 comes with the bias voltage for the output tubes adjusted. Nevertheless, I checked and adjusted the bias using a meter and a probe that is inserted into the middle of the RCA jack that is situated near each output tube (the positive probe goes into the RCA jack, and the negative probe is attached to the chassis as the ground connection). Using a small screwdriver, I turned the slotted dial next to the RCA jack until I was within the recommended range. You will need a long, thin screwdriver with a small tip for this. I bought one at Home Depot. You probably already have a volt-meter, but if not, an inexpensive one will do just fine.
A close-up photo of the RCA jacks and bias adjustment screws is shown below. In the second photo, I am using the small screwdriver to adjust the bias voltage.