An excellent writeup by Raquel, and excellent advice.
For the record, though, I'd like to cite a couple of minor points that I believe are mis-stated.
Most small signal tubes that are used in audio components, such as the 12AX7's and 6SN7's, use indirectly heated cathodes (meaning that filament and cathode are different elements). The filaments in some designs are powered with low voltage ac supplied directly from a winding on the power transformer, in which case tube vs. solid state rectification has no bearing on the in-rush current to the filaments that occurs at turn-on. Better designs often incorporate dc filament supplies, in which case the design may provide better in-rush control.
In either case, controlled ramp-up of the rectified high voltage that is applied to the plate circuits (that can be provided for in the design of a good solid state rectifier circuit, or that occurs naturally with tube rectification) prevents plate voltage from being applied before some degree of warmup of the filament (and consequently the cathode) has occurred. Application of high voltage to the plate (or screen, or other high voltage elements, if present), prior to establishment of what is called a protective "space charge" of electrons around the cathode, leads to a phenomenon called "cathode stripping," which in the case of some tubes can shorten tube life considerably (although its significance in the case of small signal tubes is controversial).
Cathode stripping occurs, I believe, as a result of stray gas molecules in the tube being positively ionized by collisions with electrons being drawn toward the positively charged plate, the collisions causing electrons in those molecules to be knocked away. The positively charged, heavy, ions are attracted by the cathode due to its low potential, where their impact would damage the cathode's emissive surface over time were it not protected by the space charge.
Again, thanks to Raquel for the excellent writeup.
Regards,
-- Al
For the record, though, I'd like to cite a couple of minor points that I believe are mis-stated.
Most small signal tubes that are used in audio components, such as the 12AX7's and 6SN7's, use indirectly heated cathodes (meaning that filament and cathode are different elements). The filaments in some designs are powered with low voltage ac supplied directly from a winding on the power transformer, in which case tube vs. solid state rectification has no bearing on the in-rush current to the filaments that occurs at turn-on. Better designs often incorporate dc filament supplies, in which case the design may provide better in-rush control.
In either case, controlled ramp-up of the rectified high voltage that is applied to the plate circuits (that can be provided for in the design of a good solid state rectifier circuit, or that occurs naturally with tube rectification) prevents plate voltage from being applied before some degree of warmup of the filament (and consequently the cathode) has occurred. Application of high voltage to the plate (or screen, or other high voltage elements, if present), prior to establishment of what is called a protective "space charge" of electrons around the cathode, leads to a phenomenon called "cathode stripping," which in the case of some tubes can shorten tube life considerably (although its significance in the case of small signal tubes is controversial).
Cathode stripping occurs, I believe, as a result of stray gas molecules in the tube being positively ionized by collisions with electrons being drawn toward the positively charged plate, the collisions causing electrons in those molecules to be knocked away. The positively charged, heavy, ions are attracted by the cathode due to its low potential, where their impact would damage the cathode's emissive surface over time were it not protected by the space charge.
Again, thanks to Raquel for the excellent writeup.
Regards,
-- Al