Tube cross-reference
4D21 alternatives, equivalents, and replacements
Review exact electrical variations and directional substitutions recorded for the 4D21 power - tetrode (not beam) (over 1000v 100w). Compare core specifications and verify every restriction before using a replacement.
Exact variations of 4D21
These designations belong to the same electrical variation group. Their names may differ because of international standards, manufacturing tolerances, or materials, but they are recorded as electrically equivalent—not merely substitutes.
4125A Power - Tetrode (NOT beam) (over 1000V 100W)
- Base
- Not recorded
- Socket
- METALSHELL5
- Base pins
- 5
- Sections
- 1
- Heater voltage
- 5 V
- Heater current
- 6.5 A
Where 4D21 can be used as the replacement
In these recorded relationships, 4D21 is the replacement tube. The listed original tubes are not automatically suitable as replacements for 4D21.
Power - Tetrode (NOT beam) (over 1000V 100W)
The database records 4D21 as a possible replacement for6155.
No special restrictions are recorded for this substitution.
| Specification | 4D21 replacement | Original tube |
|---|---|---|
| Tube type | Power - Tetrode (NOT beam) (over 1000V 100W) | Power - Tetrode (NOT beam) (over 1000V 100W) |
| Construction | Tetrode | Tetrode |
| Base | Not recorded | Not recorded |
| Socket | J4 | Not recorded |
| Base pins | Not recorded | Not recorded |
| Sections | 1 | 1 |
| Heater voltage | 5 V | 5 V |
| Heater current | 6.5 A | 6.5 A |
Verify before substituting
Matching a base or heater voltage is not enough to prove that two tubes are interchangeable. Check pin connections, heater current, maximum ratings, bias requirements, warm-up behavior, and the equipment manufacturer’s service documentation before making a substitution.
View the complete 4D21 specifications →