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Standby switches

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  • #31
    Originally posted by Beer$ View Post

    If it’s not true, I know some websites that need revision before they get some poor kid killed.
    I wonder if it has any affect on the cap's life or performance.
    Probably not if it's an amp that gets frequent use, but what if it's an amp that only gets ran a couple times a year.
    Would being drained-out after usage be bad for longer term storage?


    • #32
      It is sort of true, but not exactly truthful. They will discharge for as long as the tubes and other electronics can conduct. As soon as the tubes can no longer conduct, the current draw goes away and no more discharging takes place.......In amps that don't have a bleed resistor. Most modern amps use a bleed resistor. This resistor is a controlled leak of the B+ to ground. This will discharge the caps slowly, so that after several minutes, the amp is safe to touch.

      Now when I said not exactly......In amps that don't have a bleed resistor, the caps can be discharged quite a bit before the tubes no longer conduct, BUTTTTTTTT, caps have a memory, and even if discharged significantly, they can rebuild a charge to stunningly high voltages. Caps only discharge about 60% of their energy on the initial zap, if left alone, they will recharge a little and then discharge another 60% of that on the next zap, and so on and so on. This assumes no constant shorting of its leads.

      This is where the bleed resistor is helpful, it constantly shorts the caps out to ground when the amp is not on. When the amp is on, the " leak " to ground is minimal and insignificant.


      • #33
        Interesting interview with Fender's Stan Cotey (Vice President, R&D):

        There is a lot of debate on the origins and relevance of the standby switch on tube amplifiers. First, did Fender invent the standby switch? What was its original purpose?

        Cotey: While it's been suggested that Fender invented the standby switch, I don’t know if that's true. And whether it's a "necessity" depends on how you define the word.

        The myth is that the use of standby switches is to prevent cathode stripping before the tube is at operating temperatures (which is provided by the tube's filament). But I believe the reality is that cathode stripping does not occur at the plate voltages of typical tube amplifiers (between, say, 300-700 volts DC) and is something that is much more likely to occur in radio frequency (RF) transmitters, where the plate voltage can be in the thousands or even tens of thousands of volts.

        In fact, I think the earliest use of standby switches was with vacuum tube radio transmitters where cathode stripping would definitely have been a factor. So Leo Fender, being a radio repairman, may have borrowed the feature for his instrument amplifiers.

        Is the standby switch a necessity on amplifiers today? It has been suggested that the switch is no longer vital to amp function but has remained there because its disappearance would confuse players.

        Cotey: Standby switches are still quite useful, though, for working musicians who take breaks. The reason is that the most wear and tear on a tube’s filament comes from a cold start where the resistance is lowest and the tube, therefore, draws the most current and generates the most heat.

        I think it is better for the tubes to do as few cold starts as possible, so one cold start at the beginning of a gig with use of the standby switch on breaks is better than going through multiple cold starts from turning the amp on and off several times.

        One other thing about standby switches: If the amplifier has a tube rectifier (like a GZ34, 5AR4, 5Y3, 5U4, etc.), then it’s better for the filter capacitors to leave the standby switch in the "on" position when first turning the amp on. Tube rectifiers don’t produce output voltage until their filaments are warm, and this provides a gradual voltage rise to the capacitors rather than a rapid surge, which can cause premature aging.

        Regarding the disappearance of the switch, I think users are used to seeing it as a useful feature—and many players do believe cathode stripping does occur.

        Administrator of the SDUGF


        • #34
          Posted 2 hours ago....