Tuesday, 10 February 2015

'29 MOPA Progress

Another few small steps were taken this past weekend in the construction of my 1929-style MOPA, previously described in earlier blogs. Now that all of the parts have been either procured or manufactured, I've been playing with developing a suitable layout.
I've always liked the longer, skinnier appearance of breadboard rigs compared to the short, deeper ones and have combined this look with my love of the early 30's style. This was a transitional period in construction methods, as fabrication slowly changed from breadboard to metal chassis construction. The period in between involved mounting a metal sheet on top of a wooden breadboard, taking on an attractive 'art-deco' appearance, also popular in this period. A good example of this type of construction is seen in George Grammer's landmark 'tritet' oscillator rig which I duplicated several years ago, in a much modified version, for 10m CW.

The new MOPA will take on a similar appearance, with an aluminum plate over a mahogany breadboard. The tentative parts layout is shown below, minus the metal plate. I used the photograph to plan the wiring and to make layout notes.

Amplifier on left - oscillator on right
Although this early 30's style is visually attractive and offers the advantage of shorter and more efficient ground connections, it is a bit of a nightmare to construct. Many of the parts must be both mounted and insulated on the groundplane, while other connections must go through and under the breadboard chassis ... but I think the overall result will be worth it.

In order to mount all of the tuning capacitors above ground, a set of hardwood spacers were turned on the lathe. The spacers have been coated with two coats of spray-on lacquer to finish them in appearance as well as to keep moisture from penetrating


One of the hardest vintage components to source are era-appropriate fixed resistors. Even when found, so many of the original ones have drifted in value and cannot really be trusted to stabilize. Accordingly, I have  manufactured all of the ones needed, using more modern brown-body resistors.

First, the color bands were sanded off and then reproduction "OHMITE" labels were made, very similar to the original style.

The labels were then glued to the resistor bodies and given an orange color-wash using highly diluted acrylic paint. It would be nice to find good originals but these should work as a suitable substitute and will certainly maintain their value for many years.

The next job will be to fasten all of the components to the test-bed, wire it up and test everything out before stripping and rebuilding the finished version ... lots of work still ahead.

My old single-tube Hartley is starting to look not so bad after all!

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