The GTELs had 20-notch controllers. The rationale for this notch-count is succinctly covered in this excerpt from a Diesel Railway Traction (DRT) article on the GTEL-8500 in the 1960 November issue:
“The 20-notch throttle introduced on the 4,500-h.p. locomotives in place of the conventional 8-notch diesel-electric throttle has been retained, as it gives finer control of the high horsepower per axle that must be handled.”
The GTEL-8500 had a transistorized control system in which the Lemp curves were constructed electronically. This was described in DRT 1959 May.
The GE 16-notch control system introduced with the U25B was somewhat enigmatic, and it does not seem to have been given detailed treatment in the rail fan press (which generally although not universally appears to avoid anything too technical.) The best discussion I have seen was on the GE Locomotives Yahoo groups back in 2005, where there was a contribution from a retired GE staffer including comment as to its rationale. Available GE manuals provide some information as to how it was done, but do not cover all of the details. Nevertheless, with some deduction and extrapolation one may construct a reasonable narrative. Overall it was a system intended to have specific rather than general utility, and apparently also to have maximum commonality with the standard 8-notch control system.
On the face of it, one might have expected the 16-notch control to have been needed more in the export market, where power-to-adhesive weight ratios were often quite high, and track conditions were sometimes less-than-ideal. But then a wider ranging version that provided finer graduation of running power as well as of starting and low-speed tractive effort would probably have been desirable. For example, both Hitachi and Mitsubishi used such 16-notch controls on some of their 1960s and early 1970s export models.
The case for finer control was illustrated in this excerpt from a 1975 GEC (UK) catalogue, which included the statement “Continuously variable power control eliminates the loss due to notching troughs.
Evidently, in the 1950s and 1960s at least, GE saw the standard 8-notch control with the optional continuously variable humping control as best addressing the export market requirement for fine power control. As diesel-electric control system technology developed, I suspect that it became easier to provide gentler ramping between the notches, thus reducing the size of transient tractive effort peaks. At the same time, improved wheelslip detection and control systems lessened the chance that any such peaks would initiate sustained wheelslip. So perhaps from the 1970s or so, finer power control became less necessary.
I don’t know if any GE export models of the 1970s were fitted with the humping control. But it was still in vogue as a fine power control during the 1970s. Queensland Railways (QR) had it fitted to its 1970s acquisitions of Clyde-GM locomotives, models GL26C and GL22C et seq. Early on it was described as a hump control, but later as a manual power control. By the 1978 edition of the Operating Manual, it could be switched between MU and lead-unit-only mode.
Returning to the 1960s, GE did get at least somewhat involved in a wider ranging 16-notch control, being that used on the SP’s Krauss-Maffei hood unit diesel-hydraulics (and also retrofitted to the earlier cab units). These used the same KC-99 master controller as the U25B, although whether GE was involved in anything downstream of the master controller is unknown. Be that as it may, the system provided for 15 steps of power control (with 15 corresponding engine speeds) and 16 steps of hydrodynamic braking. So in respect of power control, both running power and starting tractive effort were finely graduated.
Going the other way on notch count, the 7-notch number found on some GE industrial switchers was also found its way on to some exports. The QR 1170 class, a 70-tonner variant, had a 7-notch pneumatic throttle control, possibly similar to that used on the US Gypsum 54-ton pair. The standard 70-tonner in non-MU form had a 7-notch mechanical throttle, whereas the MU version had a 7-notch electric throttle. What was fitted to the C+C export version of the 70-tonner, MU-fitted, that went to several South American operators is unknown. Returning to QR, curiously the earlier GE-built 1150 class had a non-MU 8-notch pneumatic throttle.