First - Bob's one tube transmitter is one of the finest I've come across. It's a controlled carrier transmitter which helps prevent over-modulation and under-modulation. It's amazingly linear and now, with the addition of the RF output stage shown below, it's a bit more powerful.
Keeping a tube type TX linear is the challenge and of course the more linear the more true to the original the broadcast will be. Bob's done something special here.
Now, if you are a 'newby' to this site and you are thinking about building your first transmitter from a print found here, I'd recommend you build this one. It's easier to build than some. It only has two vacuum tubes (I chose to use a 1mhz glass crystal from the Ukraine so it looks like 3 tubes) and yet the quality of the audio is impressive, just as Bob mentions on his site.
http://electronbunker.ca/eb/AM_StereoXMTR.html
How does the 'Goldberg' transmitter compare to this TX? Well, the Goldberg modulates better but is not quite as linear. Keeping a vacuum tube linear seems to limit it's output level, thus the lower modulation. So, I feel that the increased power output of the Goldberg makes easier to listen to. However, its always interesting to compare different models.
Modifications:
- Changed Bob's 6CQ8 tube to a 6GH8A. The 6GH8A triode section has a slightly higher amplification factor (46 vs 40) which modulates just a bit better.
- Added an RF output section using a P-C70-OSC coil for an increase in the power output.
- The power transformer used is the Hammond 262F6.
- I've installed a small circuit to produce negative voltage to improve the bias on the grid of the EL84, B-.
- Cabinet size is 5" X 7".
Please also see my "Lessons Learned" at the bottom of this page.
