I have done a strait replacement of the IRF510s with RD16HHF1s.

Here are the before and after values I got.

All tests done with uBitx VR1 drive level in the same position of approx 60% of range.

1. IRF510s and main board at 12.1V. PA idle current checked at 0.20A total (factory setting) so assume 100mA in each final.

(For info, Rx currents: 164mA no volume, about 209mA "normal" volume).

- At 7.1Mhz: 10W, total current: 1.79A, of which PA current: 1.31A, therefore main board current 0.48A

- At 14.2Mhz: 5.5W, total current: 1.39A, of which PA current: 1.0A, therefore main board current 0.39A

- At 21.2Mhz: 2.2W, total current: 0.95A, of which PA current: 0.53A, therefore main board current 0.42A

- At 28.1Mhz: 1.3W, total current: 0.95A, of which PA current: 0.53A, therefore main board current 0.42A

2. IRF510s with 16.5V, 13.8V for main board. PA total idle current checked at 0.21A.

(For info, Rx currents: 188mA no volume, about 230mA "normal" volume).

- At 7.1Mhz: 19W, total current: 2.65A, of which PA current: 2.09A, therefore main board current 0.56A

- At 14.2Mhz: 11W, total current: 2.20A, of which PA current: 1.80A, therefore main board current 0.40A

- At 21.2Mhz: 5.5W, total current: 1.40A, of which PA current: 1.00A, therefore main board current 0.40A

- At 28.1Mhz: 2.2W, total current: 1.02A, of which PA current: 0.60A, therefore main board current 0.42A

I haven't found a definitive reference for the safe and optimum values of the RD16HHF1s idle bias current but it seems to range from 200 to 500mA. So I would not recommend long term usage of the 500mA bias I used for these measurements.

I will reset mine to probably the 400-450mA value I read from some articles.

3. RD16HHF1s and main board at 12.1VDC, 250mA idle bias each (Total 0.5A PA idle current).

- At 7.1Mhz: 10W, PA current: 1.20A

- At 14.2Mhz: 9W, PA current: 1.21A

- At 21.2Mhz: 4.5W, PA current: 0.65A

- At 28.1Mhz: 5.5W, PA current: 0.95A

4. RD16HHF1s and main board at 12.1VDC, 500mA idle bias each (Total 1A PA idle current).

- At 7.1Mhz: 10W, PA current: 1.18A

- At 14.2Mhz: 9W, PA current: 1.26A

- At 21.2Mhz: 5W, PA current: 0.71A

- At 28.1Mhz: 6W, PA current: 1.11A

5. RD16HHF1s and main board at 13.8VDC, 500mA idle bias each (Total 1A PA idle current).

- At 7.1Mhz: 13.5W, PA current: 1.95A

- At 14.2Mhz: 13.5W, PA current: 1.93A

- At 21.2Mhz: 6W, PA current: 1.38A

- At 28.1Mhz: 9.5W, PA current: 1.79A

Interesting observations:

A. The RD16HHF1 produces a much flatter power curver over frequency (in my device), although it shows a dip somewhere near the 15m band.

B. The IRF510 can produce some nice power in the lower frequencies when increasing the PA supply voltage, but it comes at the price of a steep power drop at higher frequencies.

C. The bias does not seem to influence the efficiency of the finals at full power with RD16HHF1, since biasing at 250 and 500mA produces essentially the same output for the same DC power input. Assuming distortion reduces with higher bias, can we assume
a higher bias (within limits) is preferable? Any risk of thermal runaway?

D. The board main current (which includes the current in the driving stages of the power amplifier) does not seem to change with frequency from 20m onwards. Is this because the gain is pretty constant? If so, most of the drop in power with increasing frequency
seems to be in the IRF510s, supporting the results obtained with the RD16HHF1s.

E. With the current uBitx PA circuit the RD16HHF1 seems limited in output, although not having the proper test equipment I can't say where the limitation occurs.

F. When I increased the drive through VR1 I noticed that at around 40% for the lower frequencies and at around 60% for the top frequencies I get a compression effect and the output does not increase much more from there on. I left it at 60% and got a positive
feedback on the voice quality on my first QSO on 40m. Therefore I assume that the compression/clipping is not significant at that level (but I can't measure the sprectral purity).

So since my target was around 10W on 10m and 10 to 15W on 40m minimum I can say I have reached my goal just by changing the finals to RD16HHF1s and supplying the board with 13.8VDC (below the 15.2/15V stated in the respective datasheets of the RD16HHF1
and TDA2822).

To replace the finals I simply cut the legs of the IRF510s about 3mm above the board and correspondingly cut and crossed over the drain and source pins of the RD16s to match, then soldered in place.

It would be interesting to compare these results with others who performed the finals swap on the stock uBitx.

Next is the installation of the TPA2016 audio amplifier with I2C controllable AGC.

This is a lot of fun.

All the best,

73, John (VK2ETA)

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