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Sparks fly in a 5f6-a clone

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  • #16
    Originally posted by Steve Conner View Post
    Just to be a pain in the ass, I put the standby switch in series with the choke in a C-L-C filter.

    I then use a catch diode between the switch-to-choke node and ground, to catch the kickback that would otherwise happen when you flipped the standby off, interupting the choke current.

    I use a DPST switch with both poles in series, to up the DC breaking capacity.

    Finally I put a RC snubber across the standby switch (100 ohms 2 watt resistor and 0.1uF cap in series)

    The result is a more or less silent standby switch that doesn't crackle, pop or spark at all. Leo never did this because he didn't have high voltage silicon diodes for a few pennies each.

    About the flashing tubes: The white flash in the preamp tube is a piece of exposed heater that warms up before the rest, and then cools down again as the resistance of the remaining heater wire increases.

    The flashing in the power tubes is probably just fluorescence of the glass. Some amps put a big surge of plate current through the power tubes when you take them off standby. Stray electrons hit the glass envelope and make it light up blue. If it's not that, it would be something really bad. :-o
    Thanks for the description of your standby switch - I think I understand what your doing, any chance you could show it on a cct diagram, because I might just give it a go, if there's no crackle or pop. I knew my standby switch might cause a pop, and had heard of a number of different methods from this web site, but none of them described why they were better, so I stuck with the first method instead.

    Thanks also for the ideas about the flashing - I'll have to keep looking at the power tubes, and hopefully I'll eventually see that it's a blue, rather than white flash, then I'll be happy , I hate it to be 'something really bad'. Last night, after a few hours of playing, I toggled the standby switch, to see if even when 'hot' the flash occurred - it did.

    I've still got a few more hours of playing before the transformers and speakers are worn in, but at the moment, I must confess I like the sound.



    • #17

      Just to let you know I replaced the GT SAG-AX7_MPI with a standard Mullard 12AX7 and no longer see the flash at warm up. I've also taken to switching on the amp with the standby cct in the on mode, so I don't get the flash across the bass of the power tubes, as there appears to be a lot of people saying you don't need standby switches anyway, and my guess is the warm up from the rectifier will do less damage than the switch that causes the flash.

      So thanks for your help guys.