IF tube oddity: 6CB6 and 6AG5

[Penthode]

The 6CB6 and 6AG5 are similar construction tubes. The 6AG5 predates the 6CB6 and you will generally find manufacturers switched from the 6AG5 to the 6CB6 in the early 50's. The only difference I recall is the elimination of the internal link between suppressor grid and cathode in the 6CB6 which may have helped raise the transconductance.

Because the IF stages are stagger tuned, swapping even tubes of the same type can lead to a change in IF response. Certainly swapping a 6CB6 for a 6AG5 or give versa would require at least realignment or at most minor change in circuit design.