Hammarlund used three 6C4’s in their SP-600 radio receiver. 2 Heathkit 6C4 Vacuum Tubes. 1959 Power Triodes. Hammond used a pair of 6C4’s in models: A-100, B-3, C-3, D-100, M-3, M-100 and RT-3. Collins used three 6C4’s in their R-390 and R-390A radio receivers.