Modifying the Tait T199 Power Module for Class AB1
Firstly, I have a confession to make. My earlier statement that the power output from my BLY89A-based 4m PA was 35W was grossly optimistic. In hind-sight I can see where the error came from . . . When measuring the current drawn by the PA, I had omitted to take various other devices into account . . . And, well . . . The actual output power was only 15W . . . NUTS!
Some time ago, I began contemplating a high power 4m PA employing a pair of BLW86’s. Unfortunately, this was a project that was for some reason, blighted with problems. Using John, G3WZT’s excellent guidelines, I came up with a design. John even checked my calculations and was kind enough to point out an error. I was very confident that it would work. But alas, no! I suspect there might actually be a problem with the BLW86 at 70MHz. It is interesting to note from the manufacturer’s data sheet, that the input and output characteristics are given in two parts . . . above 70MHz and below 70MHz . . . Makes you wonder! I blew two devices, or the same device twice, if you know what I mean? I convinced myself that a push-pull amplifier was a non-starter so I knocked up a single stage amplifier having done fresh calculations for the input and output matching networks. I was about to congratulate myself, until I discovered that it was producing almost as much ‘hoot’ as desired power . . . NUTS again!
. . . Finally, I turned my attention the the T199 PA blocks available from www.technoloya.co.uk. The advantage with these units is that since their heritage is ‘land-mobile’, they operate from 13.8V, unlike the BLW86 which requires 28V. Another distinct advantage is that they are essentially ‘no-tune’ power blocks. Tait have spared no expense in employing high quality Semco mica capacitors throughout.
Unfortunately the amplifier is biased for Class-C operation and close examination of the circuit diagram showed that as well as L503 and L510 DC-grounding the bases of each of the two transistors, the driver (SD1012) has a 47R resistor (R502) ‘slugging’ the Base-Emitter junction and the PA device (SD1019/11) has a 22R resistor (R504) in a similar position. It is clearly evident that these two resistors play a critical role in impedance matching and therefore cannot be removed without affecting the overall performance of the amplifier. The solution is to lift the ‘earthy’ ends of each resistor and fit a 1uF chip capacitor between the resistor and ‘ground’. This way the resistor is only effective at RF. This done, the next step is to bias the two transistors for Class-AB1 operation. There are several ways of biasing bipolar transistors but the most effective and reliable circuit is shown below.
The bias output voltage is the base-emitter junction drop of Q1 plus the drop across R4.  R1 must be selected to provide sufficient base drive current for Q2, set by it’s hFE. The only requirement is that Q1’s VBE(f) at that current must be lower than that of the RF transistor at it’s bias current level. The maximum current capability depends on Q2 and R2. The power dissipation of Q2 can be up to a few watts and in most cases should be heat sunk, but must be electrically isolated from ground. The value of R2 can be calculated as (VCE-VCE (sat))/Ib. 2 of these circuits are required (one for each transistor) for the T199 board.