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Tuesday, March 22, 2011

Making of a 211 amplifier, Part 2: Assembly


The first article about this amp outlined the decision process which let to the circuit design and parts choice. Now after the circuit design is done the assembly can start.

The amp shall get a chassis in the usual style. All parts mounted on a metal plate which will be inserted into a wooden frame. In order to design the metal plate, the arrangement of the parts needs to be found. Easiest way to do this is simply placing the parts and rearrange them until the result looks good.

The biggest parts will be mounted on the upper side of the plate, capacitors, B+ mains and output transformer and of course the tubes. The transformers will be placed under metal covers. Some trials let to the arrangement as pictured on the left. A row of capacitors, 10 in total. These pretty much determine the length of the chassis (more than 50cm). Signal tubes besides each other in the front, the rectifier tubes behind the two transformer covers. All input and output connectors and switches in a row on the back side.

Based on this arrangement a metal plate gets designed and manufactured:

The plate already comes with all holes for the screws which are needed for mounting the parts and to fix the plate later in a wooden frame. In the photo, speaker terminals, RCA and mains inputs, fuse holder and switches (on/off and ground lift) are already attached to the plate.

First the capacitors get mounted:

In the photo also the filament transformers are already mounted as well. Two transformers are needed in series simply because no suitable transformer was available with the required voltage. The transformers provide 10VAC/5A each. Due to the choke input supply and voltage drops across chokes and diodes, about 20VAC are needed. Some pre wiring is already done to the caps. Also some ground wiring is visible.

Next, the chokes are mounted in a second tier above the capacitor row:

In this photo the tube sockets are already mounted too and some more of the wiring is completed.

A view from the other side:

As can be seen, the B+ power transformer and also the output transformer are already mounted as well.

The next two photos show the top side of the plate:

The output transformer needed to be mounted sideways, otherwise it would not fit under the transformer cover. A frame construction was necessary to firmly hold the OPT.

The sockets of the 6HS5 driver and 6CG3 rectifier tubes are not directly attachedto the metal plate. They are a bit recessed below the plate. This is to have larger creepage and clearing distances from these sockets to any metal parts. The high voltage present on some of the pins makes this necessary.

The last photo shows the amp completely assembled and wired, ready to be tested:

Part 3 of this series of articles will show the completed mono amps in their wooden frames and describe the testing and first listening.

Best regards


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