I made this amplifier in the period 2003-2004. When I moved overseas I put it in storage since it was a bit too heavy to transport.

This amplifier has been playing music for a couple of years now, but the building time took a couple of years. I can now understand why handbuild things are so expensive… The price is not in the parts, but in the labour.

 

The amplifier is spread over four boxes: 2 amplifiers, 2 powersupplys. I wanted to have a dual-mono setup, but even then each side would be too massive/heavy to work on. Hence the split between power supply and amplifier. And then you end up with an umbilical between amp and supply. This isn’t simple: the GM70 needs 20V at 3.5amps, there are two high voltage supplys at 450V and 1200V.
The tubes that are used in this amp are classics in the DIY scene.
In the amp itself: 76 to 300b to GM70.
The 76 has current source as load, the 300b a choke, and the 300b is directly coupled with the GM70. This allows the amp to go nicely into A2. the power supply uses 866’s for the 1200V, an 83 for the 76-300b stage, and a 6x5gt for the bias supply for the 300b.

I bought the transformers for these amps at AE. They can do any transformer you ask for, at very reasonable prices. There are also some nice mA meters available, I modified mine with blue leds for the backlight.

 

 

 

In my first iteration, the 300b was capacitor coupled to the gm70. This gave an easy 20W of power. Hooked up to a computer I could easily measure the point where the distortion went up. The cap coupled stage simply couldn’t let the gm70 go into a2. 20W seemed a bit low for such a massive amplifier, so I decided to hook the amp up a little bit different.

The 300b power supply and the gm70 power supply are separate, which allowed me to stack them. the gm70 is biased with a large resistor on the cathode. the gm70 is then connected directly to the 300b anode. Thanks to the decision of choke loading the 300b, this worked nicely.

At about 20W you could see the amp going into a2 mode, with a rising third harmonic. Usable power turned out to be about 40W!

Of course, in normal listening it’s only that first watt that’s really important.

Also, the startup of these amps is controlled with a micro controller in each power supply. it’s completely separate from the audio part, with it’s own power supply. there is a single button in the front and a blue indicator light. When the amps are not in use, the blue led pulses softly. press the button and the transformer for the heaters and bias supply is switched on. After a couple of minutes of heating, (you need to be careful with mercury vapour rectifiers), the high voltage transformer is turned on, and music plays!

if you turn it off, and then turn it back on within a couple of seconds, it doesn’t need to go through the several minutes of heat up, so the start up sequence is shortened. While heating up, the blue indicator led blinks slowly, when the high voltage is turned on, the blue led turns off, as not to distract from the impressive glow of al those tubes. They are almost enough to be able to read a book at night, and definitely heat up the room.

 Power supply construction

The boxes are constructed from mdf, holes drilled for the tubes. tube sockets are attached to a piece of blank pcb, as a form of sub chassis.

Chokes and capacitors are attached to a sturdy aluminum subframe, which is then bolted to the mdf box.

The micro controller circuit and the dc conversion for the heater elements is done on a pcb on the top part of the box.