5B/255M

Beam Tetrode — Loctal

STC

Known Equivalents & Substitutes

CV3915B/254M5B/254G5B/252MCV80285B/251M

Similar Tubes (Not Drop-In Compatible)

5B/256M

12V/0.47A heater variant (CV499) — not pin-compatible due to different heater ratings

5B/257M

12V/0.3A heater variant (CV2220) — not pin-compatible due to different heater ratings

5B/258M

19V/0.3A heater variant (CV2347) — not pin-compatible due to different heater ratings

Electrical Specifications

Section 1 — Beam Tetrode

Absolute Maximum

Anode Voltage (max)Anode Voltage (max)600V

Screen Voltage (max)Screen Voltage (max)300V

Anode Dissipation (max)Anode Dissipation (max)25W

Screen Dissipation (max)Screen Dissipation (max)4.5W

Notes

Characteristics identical to 5B/254M per STC datasheet. The 5B/254G has flexible leads instead of a base for direct wiring.
The 5B/256M (CV499), 5B/257M (CV2220), and 5B/258M (CV2347) are the same valve with 12V/0.47A, 12V/0.3A, and 19V/0.3A heaters respectively — not pin-compatible substitutes due to different heater ratings.
Anode is brought out to a top cap as well as to pin 2 — double-ended construction for improved HF insulation. For high-altitude or anode-modulated operation, the double-ended versions (5B/254G, 5B/254M, 5B/258M) are preferred due to base insulation.
DC grid circuit resistance must not exceed 30 kOhm at maximum ratings. At less than maximum ratings the value may be increased to 100 kOhm.
Cathode bias is not recommended per STC datasheet; fixed bias source or a combination of methods should be used.
Bulb temperature must not exceed 250 °C under any operating condition. Adequate ventilation must be provided.
Prototype designation: LS650.

Pin Layout — Loctal

1Heater

2Anode

3Screen Grid

4

References