Do you leave CD players with tube outputs on?

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.

Sir,

Pardon my ignorance and I hope you do not mind me asking. I am not looking for a debate, I do not have sufficient knowledge on this topic to hold one, and although my questions are pretty direct, I am just trying to understand what you wrote. So, if you do not want to answer my questions, I do understand.

What if there's a thermistor on the primary winding of the tube amp, for example, what happens to the inrush current analogy? At t=0, caps (in the PSU) are shorted, does the inrush current reach the signal tubes?

I have never encountered a tube failing on preamps because of cathode stripping. Some tubes last for ages, like Telefunken for instance, are they not affected by cathode stripping? How many cycles does it take on the average before cathode stripping is really an issue? Is the decrease in gm be considered as a good measure? Is it safe to assume that cathode stripping only matters on high power tubes where bias voltages are over 1kV and not those tubes whose bias voltage is less?

Again, I apologize for being direct and I hope that you will not take my questions as a challenge to what you have posted.

regards,

Abe

Hi Abe,

Thank you for the polite nature of your questions, but I'm not able to answer them particularly knowledgeably.

Yes, I would think that a suitable high-current thermistor, suitably placed somewhere in the path between ac inputs and tube filaments, would be an effective means of inrush current limiting. I know that is commonly done in power supply designs, although I don't have a feel for how widespread their use is in audio.

I'm not sure I understand your reference to shorting of caps, but that would seem to only be relevant to designs that incorporate a dc filament supply, not to those that drive the filaments with low voltage ac directly from the transformer.

Re cathode stripping, as I indicated its significance with respect to small signal tubes is a matter of some controversy, and I have no particular feel for its degree of significance myself. But given the possibility of it being significant, it would seem to be good practice to design a tube component to bring up the B+ slowly.

Regards,
-- Al

A1,

Thanks a lot! I am glad that you accomodated my request.
Thanks for your time answering my inquiries. I learned something today!

Best regards,

Abe

Tubes reach the point of normal operation faster than semiconductors thus require shorter time to worm up (for the small signal tubes near 2min. Why bother to keep them on??

I think a lot depends on your unit. Some take longer to warm up than others, also some are a little more "blunt" with how they power up. If Telefunken 12ax7s last 10,000 hours, you do the math....
I tend to leave my system on all weekend if I'm listening a lot. But, if I'm not going to listen for a day or two, it gets turned off.