I can see why. For the low number of output stages on the amp, they had to push those tubes pretty hard. Probably created a grid heater issue. With the endurance of the Tracker and the mission profiles, they were going to be maintenance hogs.
LISTING OF APS-38 COMPONENTS RT-282/APS APS-38 RF Unit, 50 KW-Pk
RT-575*/ APS-38B RF Unit
AM-938/ APS-38B Amp-Synch. w/ fallopian tubes
AS-567/APS-33B APS-38 Antenna Dish
C-1204/ APS-38 Control-Indicator
CN-244/ APS-38 Volt-Reg.
F-208/ APS-38 CRT Filter
IP-224A/ APS-38
IP-225/ APS-38B Az-Range Ind.
MD-60C/APS-31 APS-38 Modulator
MT-447A/APS-31 APS-38 Mount for SN-275/
MT-1198/ APS-38 Mount for IP-225/
MX-1466/ APS-38 Scope Visor
PP-1050/ APS-38 PS
RE-142/ APS-38B Relay Unit
SN-275/ APS-38 Synch.
ref.:
http://jproc.ca/rrp/rrp3/tracker_aps38.html