5551A
5551A has no equivalent designation confirmed by the archive. Completed sales put the typical price at $10.62 per tube.
Description
The 5551-A is a water-cooled, steel-jacketed, mercury-pool-cathode tube provided with a mounting plate for thermostatic control. It is intended for resistance-welding control service.
Construction
The anode terminal is a flexible lead. The cathode terminal is a bar located opposite the anode terminal. The ignitor terminal is within the jacket skirt at the cathode end.
Cooling Requirements
The 5551-A requires a minimum inlet water temperature of 10 °C, a maximum outlet water temperature of 40 °C, and a minimum water flow of 1 gpm. The maximum water-temperature rise is 4 °C, and the maximum pressure drop is 2.5 psi.
Thermostatic Control
The tube is equipped for mounting a thermostatic control, with a mounting plate calibrated either for controlling the flow of cooling water through the water jacket or for protection of the ignitron against overheating.
Ratings
At zero phase-control angle and frequencies from 50 to 60 cps, the maximum peak anode voltage is 500 volts forward and 500 volts inverse. The RCA datasheet for the 5551-A includes rating charts for frequency-changer-welder service and resistance-welding-control service.
| Measure | Per tube |
|---|---|
| Median | $10.62 |
| Typical range (25th–75th) | $4.25 – $20.00 |
| Most sales (10th–90th) | $3.95 – $29.99 |
| Extremes incl. outliers | $1.43 – $52.53 |
| Completed sales | 14 |
| Brand | Median | Range | Sales |
|---|---|---|---|
| RCA | $10.00 | $4.25 – $11.95 | 5 |
| Condition | Median | Sales |
|---|---|---|
| used | $10.62 | 14 |
Per-tube prices from completed sales, 2026-03-07 to 2026-09-14. Matched pairs and quads are divided out. See the price index for how these move over time.
Original manufacturer datasheets for 5551A, hosted by Frank Philipse's Electron Tube Data sheets.
| Manufacturer | Designation | Datasheet |
|---|---|---|
| RCA (HB3) | 5551A | PDF (370 KB) |
Documents held here and used to write this page.
- 5551-A
5 further designations appear alongside 5551A in the literature but below the evidence threshold used here, so they are not listed as equivalent.