John VK2ETA has done a strait replacement of the IRF510s with RD16HHF1s in his uBITx. You can see from the photo above that he has them installed cross-legged. John says, “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”.

What follows are the before and after values of output power and PA current that John measured. All tests were done with the 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

John hasn’t managed to find a definitive reference for the safe and optimum values of the RD16HHF1s idle bias current but it seems to range from 200 to 500mA. He would not recommend long term usage of the 500mA bias used for these measurements and will reset his idle current to the 400-450mA range.

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

John made the following observations:

A. The RD16HHF1 produces a much flatter power curve over frequency (in his 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, but without the appropriate test instruments he can’t say where the limitation occurs.

F. When he increased the drive through VR1, he noticed that at around 40% for the lower frequencies and at around 60% for the top frequencies he gets a compression effect. The output does not increase much more from increasing the drive level. John left the drive gain at around 60% and got positive feedback on the voice quality on his first QSO on 40m. He assumes that any compression/clipping is not significant at that level (but he hasn’t measured the sprectral purity).

So since his target was around 10W on 10m and 10 to 15W on 40m minimum, he is pleased to have reached his goal just by changing out the finals to RD16HHF1s and supplying the board with 13.8VDC. This is below the 15.2/15V stated in the respective datasheets of the RD16HHF1.

#### DK5LV experience with RD16HHF1s

Henning Weddig DK5LV thanked John for his intensive research on the PA stage and commented that in his experience, “the RD16HFF1 really needs a very high quiescent current of about 500 mA each, which is not good for a QRP design”.

He goes on to say, “The output transformer plays an important role in the design. Normally a 1 to 4 impedance transformation (12.5 ohms to 50 ohms) is sufficient. Each transistor “sees” half of that impedance i.e. 6.25 ohm. The windings of the transformer must be capacitively compensated and the leakage inductance mimimised on the windings.”

“Another big issue is the choke for the supply voltage: the commonly used centre-tapped transformer without the choke is not recommended. Ashhar Farhan uses two isolated chokes, and in my experience a bifilar wound choke is the better choice.”