Open call for suggestions:
I own a bare 6WD chassis "Wild Thumper" by Dagu, designed by Oddbot. And I have (2) "racing packs". Those are basically 6 NiCd cells in series, providing 7.2 Volts nominally. The motors are rated for 6 V. Stall current is listed as 5.5 A.
Even the most basic speed control will need some sort of amplifier for the PWM signal. Let's start very simple. Just speed control. Just one motor. One direction. What kind of transistor shall I buy? And what kind of circuit shall I use?