Hi Everyone,
I have begun evaluating the 20W RF power amp by K9HZ (Dr. Bill Schmidt) with T41 to address the increased T41 V12 frequency and CW capabilities. Some of what I have found is as follows:
Frequency Response
A Bode plot of the amp stand-alone is shown in the first figure below. The response at both 5W and 15W (RMS) is essentially flat from 1MHz to over 50MHz. This covers the 6M band that the original T41 amp could not reach.
Power Output
Note that Bill (K9Z) rates the amp at 20W PEP, but it is current limited to about 16W (RMS). I tested the amp with two equal amplitude 7MHz tones but1KHz apart in frequency. at 16W RMS. The PEP of that is well over 20W and a scope plot of the LP filtered PA output (through a 50-ohm attenuator), showing the clean out, is contained in the next figure. A rating of 20W PEP represents a real-world SSB condition, since we rarely transmit sine waves on SSB, but rather complex speech waveforms.
I have used the amp quite a bit over the past week, and it performed flawlessly. No overheating at full power for extended periods on the 3.5"x1.5"x1.25" heatsink, with no fan. No thermal run-away. Also, because of the included current limiting, it is not possible to drive the output stage transistors to destruction. Since each final RD16H MOSFET is rated at 16W max dissipation, the amp is conservatively designed.
For anyone concerned about?less tha20W output, note that the difference between 16W and 20W is about 1dB, or 1/6 of an S unit on the receiver - not much at all.
SSB Signal quality?
That is currently under test and will be reported in the near future (waiting on some hardware components).
CW signal quality
As contrasted with SSB signals, CW is always a single tone, which is limited to 16W RMS (for sine waves 16W RMS is equivalent to16W PEP).
The first CW plot is a narrow band spectrum of a 7.2Mz CW tone from the T41 V12 into the K9HZ amp. The plot is centered on 7.2MHZ with a 20KHz span. Within that bandwidth the signal is almost 80dB above the noise, with no other spurious components. The next plot is a wide band spectrum from?3Mz to 30MH?of the 16W 7.2MHz amp output, through the T41 LP filter. The second harmonic is at -51.5dB relative to the 7.2MHz fundamental, well below the -43dB limit.
Efficiency
The amp draws about 620mA with no signal and 2.46A at 16W, indicating a class AB mode with about 45% efficiency at full power.?
For reference, a typical 300WH portable 12V battery would provide well over 15 hours of SSB operation with the T41 and K9HZ amp.
Bottom line
Jack and I fully recommend the K9HZ 20W amp for use with T41, especially for those wishing to have an extended frequency response to 6M, with V12 T41 boards.
For readers of the T41 book, please note that we plan to include this material and more regarding V12 performance in a revision of the book to be added in the near future. These additions are reflective of changes suggested by builders of the T41. Afterall, the T41 was designed to be an experimenter's platform and that is exactly what has happened, both in hardware and software innovations. Thanks to all who contributed.
For those who already have the book, we will post the additional material to this site in the files section at the same time the book revisions are completed and uploaded.
K9HZ 20W power amp Frequency response
K9Hz amp output at 16 W with Two-Tone input
Narrow-band spectrum T41 CW mode at 16W
Wide- band spectrum CW output at 16W
