Unlike the 555, it has only one set of horizontal deflection plates and the trigger and sweep circuitry are built in to the mainframe. The vertical amplifiers of the 545A, 551, and 555 are identical. The 551 uses an L-C delay line for each vertical channel.
Like the 555, the 551 has two parts: the "indicator unit" containing the signal path electronics and CRT, and an external power supply unit. The filament current for the tubes in the indicator unit is produced by a transformer in the indicator unit, T750, Tektronix part number 120-235. T750 has seven isolated secondaries, each supplying 6.3 VAC. The indicator unit also contains the CRT circuit, which uses 5642 tubes and produces the +8650 V CRT anode voltage and the -1350 V voltage for the CRT cathode. The external power supply unit produces regulated DC voltages of -150 V, +100 V, +225 V, +350 V, and +500 V. The external power supplies of the 555 and 551 are not interchangeable. However, the cable that connects the power supply to the indicator unit is the same for the 551 and the 555. It is Tektronix part number 012-032.
A factory modification, MOD 108A, was available for increasing the CRT acceleration voltage from 10 kV to 12 kV. This improved trace visibility at high sweep rates. However, it reduced the deflection sensitivity. With MOD 108A, each division on the screen is 0.85 cm. A graticule with 0.85 cm units was supplied.
|CRT||T57 (T551) in early, T5511 in later (SN 2032+) models|
|Rise time with Type K||14 ns||12 ns|
|Acceleration Voltage||10 kV||10 kV|
|Filament Regulation||none||saturable core reactor|
| Price in 1965
| $2,650 |
|Indicator Weight||52 lb (23.6 kg)||68 lb (30.8 kg)|
|Power Supply Weight||42 lb (19 kg)||45 lb (20.4 kg)|
|Power Consumption||850 W||1050 W|