Chassis Details
Notes and photos from Mitch about how he builds the amps.

After the board and faceplate are made, the next step is working on the chassis.

These chassis are supplied by metal fabrication companies who do the bending and often weld the corners together. They usually are made from aluminum although steel can be used, too. Aluminum is easy to work with and it's also non-magnetic. This is important when large transformers are used to minimize interaction between the power transformer and the output transformer.

Drilling the chassis is a boring job but it has to be done. I begin by marking the center for all the holes using the layout drawing (see the Boards page for more about the layout drawing). I drill the holes with a drill press, using a step drill bit to cut the larger ones. Sometimes I use a chassis punch for the larger holes, too. Here's the Aberdeen chassis after the holes were drilled.

Aberdeen Chassis

Aberdeen Chassis

Once the holes are drilled, the jacks, switches, potentiometers, transformers and tube sockets can be attached. The front panel and rear panel (if the amp has one) are held in place by the jacks, pots and switches.

Aberdeen Chassis

Aberdeen Chassis

Here's what the inisde of the Aberdeen chassis looked like at this stage.

Aberdeen Chassis

The AC wiring between the power connector, the fuse, the power switch, the pilot light and the power transformer is done next. The transformers use stranded wire which is quite flexible. It tends to not stay where it's put it so I dress it down with cable lacing. It's an old school way to dress wires but it looks way better than nylon zip ties.

The heater wiring is done at this time, too. It's done with solid core wire. I usually twist two lengths of wire together using a power drill and make enough for all the heater wiring in the amp in one go.

Here's the Aberdeen chassis with the AC and heater wiring done. Also the secondary for the output transformer has been wired to the impedance selector switch and the output jacks.

Aberdeen Chassis

Aberdeen Chassis

Here's the heater wiring for the Custom 50, which has more tubes to connect than the Aberdeen and it was done Hiwatt style.

Custom 50 Chassis

The input jacks are wired next. Sometimes it's easier to wire the jacks outside the chassis where there's more room. I made a jig to hold four jacks in position while I attach the wires and resistors they need.

Input Jacks

Input Jacks

Here are the input jacks installed in the Custom 50.

Input Jacks

Any remaining wiring for the pots and jacks on the front panel is done, as shown here on the Custom 50.

Front Wiring

Now the board, which was assembled earlier, is installed and its flyoff wires are attached to the pots and tube sockets. In the following photo, the preamp board is installed in the Custom 50 and has been wired to the tube sockets. The flyoff wires for the pots have yet to be attached.

The Custom 50 has a separate smaller board for the power amp and the power supply. In the photo it has already been wired to the output tube sockets and the DC high voltage wiring has been completed.

Board Wiring

Here's the Custom 50 with all its board wiring done.

Front Wiring

Front Wiring

Front Wiring

Now all that's left is tidying things up by dressing down any wires that aren't in place yet. The following photos show the completed chassis for the Custom 50.

The Custom 50 is a very accurate reproduction of a Hiwatt DR504. The originals were built in England by a fellow named Harry Joyce and his company. He also did work for the British Navy. He used many of the same military-spec wiring techniques on the amps he built for Hiwatt as he did on his naval equipment. That's why Hiwatts were built so neatly and ruggedly. I emulated Mr. Joyce's wiring as closely as I could on this amp and use many of his techniques on all the other amps I build.

Custom 50 Chassis

Custom 50 Chassis

Custom 50 Chassis

Custom 50 Chassis

Custom 50 Chassis

Here's the completed chassis for the Aberdeen.

Aberdeen Chassis

Aberdeen Chassis

Aberdeen Chassis

Aberdeen Chassis

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