Featured VT: 6CB6

 

The 6CB6 was originally designed for high frequency work, especially as an IF amp for TV sets operating at 45MHz. You can see here that the plate consists of parallel planes on opposite sides of a wide, flat cathode. The plates are spaced far from the grids in order to minimize internal device capacitance. The type has a fairly high g_m, though not as high as some other types used for high frequency and/or wideband work. It also features an unusually high V_HK spec that can make it useful for other purposes, such as DC error amps, or DC coupled LTP's. As for audio performance, this loadline looks quite good.

 

Who Says Pents Have to Make High Order Harmonics

 

This particular loadline shows that h₃ and h₄ are equal to 0: this produces h₂ only. If you don't require the full output swing, the distortion decreases proportionately. If used as an LTP with active tail loading, this would be a clean gain stage indeed. The excellent audio loadlines have lead to the use of the 6CB6 in both professional (one example is given in the RCA Receiving Tube Manual) and DiY designs. It isn't exactly an audio "sleeper", but not audiophool expensive either. It also comes in some odd heater voltages: the 3CB6 (3.15V/0.6A) and the 4CB6 (4.2V/0.45A) the latter types intended for use with TV sets that didn't include a PTX and daisy chained the heaters across the AC mains. This should keep them available.