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Sunday, January 23, 2011

Making of an 45 / 2A3 amplifier


Some photos of an amplifier that uses all directly heated triodes:

This amp can be used with 45 or 2A3 output triodes. The settings for the tube are selected by a switch. Also different input tubes can be used, either 801A and similar (10Y, VT25, VT62 ) or 26 (also UX226). In the picture above 45s and VT25s are plugged in.

This picture shows the amp with globe 45s (UX245) and globe UX226. The amp is not finally fixed to the frame, so some screws are missing. Also the bottom plate with feet is not attached yet.

This series of photographs illustrates the assembly process.

For easy handling the metal plate which carries all parts is mounting into an assembly fixture. This allows to turn the whole assembly upside down or sideways without the danger of scratching the transformers or capacitors.

First the tube sockets, connectors (RCA inputs, loudspeaker outputs and power supply connector) and switches are fixed to the metal plate.

Then the capacitors and transformers are mounted. The second picture shows how the capacitors are fixed to the plate. The transformers used are Tango XE20S for the output and the Tango NC20F interstage.

This picture shows the wiring of the first level. All signal wiring is done with teflon insulated solid core silver. For mechanical protection of the fragile silver wire it is pulled through an extra insulation sleve.

The parts have been arranged for short signal paths. Therefor the input tubes are at the back, nearby the input jacks.
After completion of the first level wiring. All parts which are inside the wood chassis get mounted on a frame made of aluminum profiles. These are fixed to the main plate by metal stand offs.

The driver tubes filaments are supplied with DC. The output tubes are AC heated. Therefor a filament transformer for the output tubes is placed inside the chassis (in the back row, center).
Other components inside are: 4 B+ chokes, one for each stage and each channel for decoupling. Also 2 filament chokes for the drivers and another 2 capacitors for which there was not enough room on the top. And finally cathode resistors and also some resistors which get switched in for correct setting of filament and B+ voltages and operating points for the various tubes which can be selected.
This picture shows how the components are arranged in a 3D-fashion and interconnected.

All the wiring is completed and the amp is ready to be tested.

The power supply still needs to be built. It will be placed in an extra chassis and built up in a similar style.

Here are a few more shots of the amplifier:

Best regards


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