USNRET Posted July 10, 2011 Share Posted July 10, 2011 OK, so I know that I should have the power amp off for some time to protect the tubes during a cycle. If I was going to try out some different tubes in my Tercel phono stage or in my Peach pre amp how long should they be off before turning them back on. I am assumming that the KT-88s and the rectifier in my VRDs are the most likely to be damaged by a hot cycle. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted July 10, 2011 Share Posted July 10, 2011 Much depends on the power supply design and how much of a reservoir the electrolytic caps provide. In most tube equipment, a bleeder resistor is used to discharge the caps in the event power is applied with the tubes removed or burned out (in which case the electrolytics can remain charged since no current is being drawn). A safe number, which should cover virtually any piece of equipment, is 5 minutes to allow full discharge. It may be much longer than needed for some units, but better to opt on the side of safety. I'm not sure what you mean by a hot cycle. But, if you are concerned about applying B+ (i.e. high voltage) to tubes with a cold filament, there's not going to be any damage to the types used in preamps or amps (some would disagree and claim that doing so results in "cathode stripping," but that has never been documented in tubes of this variety). Quote Link to comment Share on other sites More sharing options...
USNRET Posted July 10, 2011 Author Share Posted July 10, 2011 I'm not sure what you mean by a hot cycle.I guess what I was trying to say (hot cycle) was to power down, change tubes and power right back up without a fair amount of time in between the power off and power on. I was wondering if it was as important on the preamps as it is with the power amps. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted July 11, 2011 Share Posted July 11, 2011 If the power amp is on when you shut down the preamp, pulling a tube before the p/s caps are discharged could give you a bit of a "pop" in the speakers. While this would probably not damage a tweeter (in particular), it would be best to wait until the orange glow from the tube cathode is no longer visible. At that point no appreciable current is flowing through the tube so you'll be quite safe even if the electrolytics are not fully discharged. With small signal tubes like 12AX7s, the process will be very fast. Repowering after installing the new tube can be done immediately. Quote Link to comment Share on other sites More sharing options...
sunnysal Posted July 11, 2011 Share Posted July 11, 2011 in case you also worry about current in rush you might consider a soft start circuit bringing on the voltage at turn on, thermistor based units seems to work well. tony Quote Link to comment Share on other sites More sharing options...
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