---   The Final Amplifier   ---




I've been waiting for several years, spent a lot of time, and a bit of money, getting to this point, where I have just this one more deck to rebuild. Tomorrow is the first of September, and although I'm planning on being away for a couple of weeks on my annual jaunt to photograph fall color, I nonetheless think that BigRig may yet be on the air around or about the arrival of the new year.

When rebuilding the final has been completed, I've got some "overhead" work yet to do. I've got to make two cable fixtures to use in fabricating the two inter-deck cables, and I've got to do the modifications to my rack. The rack is a bit too tall for the rig, and a bit too tall for my hamshack ceiling. A few inches off the top of the rack, and it'll be suitable for both.

The last deck to work on has two stages on it. It contains the final amplifier, a single-ended 810 triode, and it contains the buffer/driver for the 810, a single 807. The 807 will supply the 40-50 watts of drive needed by the triode final, and is in turn driven by the output of the Millen exciter. I'm becoming increasingly hesitant about calling it a "Millen," given the history of it all, but that's what I've been doing for abt 60 years now, and I'm too old a dog to learn any new tricks.

I'm a little nervous about the final tank capacitor. It's perhaps the most impossible-to-replace part of the entire project, and I'm going to have to be extremely careful in how I handle it to prevent any damage. I'm resolving now to be wide awake, not preoccupied with other matters, and stone-cold sober, whenever I'm handling that thing. And, I'll totally ignore the ringing phone. I'd hate to have the BigRig project lying dormant for the next hundred years whilst I search for a replacement tuning capacitor.

Ok, enough for now. I'm off to tear down the old and build up the new!

December 16:       Work on the final-amplifier rebuild is progressing, albeit slowly. Today, I finished up the component layout drawing. (See the picture in the final-amp section.) Over the months, a couple of prominent local radio amateurs who have seen the layout drawings, either finished or in progress, have asked why I do them. They ask "Don't you have a schematic?" Yes, I do indeed have a schematic, often several(!), but I do the layout drawings to ensure that the component placement and the resultant wiring meet several needs. I want to ensure:

1.      adequate separation between components to avoid harmful heat transfers, and,

2.      adequate separation necessary to avoid high-voltage arcovers, and,

3.      control of lead length in RF circuits to minimize stray inductance, and,

4.      the necessary separation to minimize unwanted capacitive coupling between circuits, and,

5.      ease of construction, and,

6.      ease of eventual service, and,

7.      it should end up looking nice!

To make all of that happen, you can't just throw wires at a chassis willy-nilly, or the chassis winds up looking like the proverbial rat's nest - what the old timer called "haywire." To escape any reputation for producing haywire, a lot of thought must go into the layout. For example, the final-amplifier layout drawing in this section took well over 20 hours to produce. It's certainly gratifying when a fellow ham comments on "the nice workmanship" of the completed project, but that ham may not realize how much of the workmanship was determined by the planning pencil, not by the soldering iron.

Onward!

Hot Dog! Today, the final got finished, at least until I fire it up and it makes smoke and I have to take it all apart again. Perhaps more in this deck than in any of the others, I had to fight the instinct to, in the words of the honored but defunct Telrex Corp., "make new inventions" and to "improve" the transmitter. Restore won out in most of the battles, but reform did conquer in a few others. Nonetheless, BigRig is largely and essentially, the same transmitter it was when it was built in 1945 and when retired circa 1961. Part of the reform was the addition of a Phone-CW relay and an additional adjustable grid-bias resistor, so that the 810 bias would be automatically optimized for A1 and A3 modes. There had been a compromise single resistor before. Another reform was the addition of 3 test points, intended to facilitate bias adjustment and other testing, when on the air.

For all my planning, I didn't foresee until too late, that I was painting myself into a corner, near the 807 buffer coil socket. It got crowdeder and crowdeder in there. (After the umteenth soldering-iron burn and related injuries to the hands, one is permitted to coin new words.) Another problem was that the addition of the relay, and the addidional resistor, resulted in a bunch of screw heads on top of the chassis, that interfered with the mounting of the inter-stage shield. Those screw heads required that the shield to be mounted an eighth of an inch or so above the chassis.

I was distressed to note the difficulty of link adjustment caused by the too-flexible flexible-shaft mechanism that I had installed in my youth. It's embarassing to see how lousy my teen-ager's engineering had been. I've been contemplating replacing it with a gear-drive system, but decided that the deviation from original would be too great, so it'll remain original as an example of BigRig's dark side.






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