Transistor choice for cathode follower. Does it matter?

[Fender56]

Hi!

Does the type of transistor has some effects on the tone when used as a cathode follower? J210? MPF102? 2N3904 etc? Any noticeable differences?

Just wondering!

[Ben N]

Assuming your purpose here is to have a low output impedance in order to drive a current-hungry load, like a passive tone control, JFET devices, like the J210 and MPF102, have relatively high output impedances. Your generally better bet is with bipolars (like the 2N3904 you mentioned, or 2N5809, or MPSA18 or any other low-noise small signal BJT--I guess that gain is irrelevant in a unity gain application) or mosfets (eg BS170), although there are certainly applications where the difference in output impedance does not matter. In any event, you cannot always substitute different transistor types in the same design will-nilly. Biasing in particular is different for fets than for bjts.

Read the buffers article at AMZ, and also check out designs that do this, like the ROG English Channel (jfet source follower, analogous to triode cathode follower driving Vox tone stack) and the like; also note that the term "cathode follower" applies to tubes, as they have cathodes; what you are referring to is technically an emitter follower (BJT) or a source follower (FET). Using the right term will help you get correct responses.

Ben

[petemoore]

 Generically you want the output impedance to be lower than the input impedance on any two seriesed circuits. A factor of 10x is generalically always good, easest to install is when you already have a Vref handy.