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Voltages too high on a Jim Kelley amp.. what to do?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by PSaulino, Jan 22, 2004.

  1. PSaulino

    PSaulino Silver Supporting Member

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    Media, PA
    Hi,

    Have a Jim Kelley amp that is running *really* hot (even for a Jim Kelley) at 520 volts on the plates, roughly same on the screens of the 6V6s. Design is for like 477, already over spec, but at 520 there are not many 6V6s that can take it for very long. Anyone know what options are there to "correct" this problem, bring the amp more into spec with what it is supposed to be?

    Note that a previous tech installed a power resistor, and it worked for a while, things were very hot inside there, and the resistor eventually fried.

    Oh yeah, I would like to use 6V6s, not 6L6s..
    Thanks!
    P.
     
  2. KLB

    KLB Member

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    Stick it on a Variac. Reduce AC output to 95-100v. Check filament voltage to see if remains close 6v. Plate volts should be nearer to your target now.
     
  3. PSaulino

    PSaulino Silver Supporting Member

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    Hi KLB,,

    Yes, that would work, but is not a likely solution to enable me to use the amp live, unless I carried a variac around with me!

    P.
     
  4. John Phillips

    John Phillips Member

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    Fit a larger-power-rating resistor, preferably a chassis-mounted heatsinked type. You can get anything up to 200W if there's room inside the amp.

    The other tech should have calculated the power dissipation and made sure the resistor was big enough - you need to allow a fair safety margin (I'd go for a factor of two over the max power in any condition) since the inside of the amp does get hot, which reduces cooling. If you use the chassis as the heatsink and things are still getting too hot, you can then fan-cool the whole thing from the outside if you have to.

    Remember to get one with high enough insulation voltage rating too...
     
  5. Heiko

    Heiko Member

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    The best way I've used for reducing B+ voltage is by running a string of zener diodes in series from the power tranny secondary B+ center tap to ground. I usually use a string of 5 or so 10 volt 5 watt zeners for this to drop the voltage down by 50 volts. You could go with a larger wattage single zener too. People use this technique all the time over at Ampage www.firebottle.com/ampage.

    Heiko
     
  6. Carl Zwengel

    Carl Zwengel Guest

    The amp always been running this hot or did it just pop up that high after a tube change? If the latter is the case then either your wall voltage changed or you might have something funky going on with your power transformer. If its just a hot wound tranny follow the suggestions some of the other guys have submitted, otherwise take it to a GOOD tech for a thorough shakedown.
     
  7. csanford

    csanford Supporting Member

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    My question as well. How much current are those 6v6s pulling? Sounds like they may be biased way cool, and thus the PS volatage isn't dropping very much. A cool bias point, combined with high line voltage could account for what you are seeing.

    BTW, although JKs do run the output tubes hard, the screen resistor setup allows them to live longer than one might expect.

    HTH,

    Chris
     
  8. LarryN

    LarryN Member

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    I've successfully used a reverse polarity zener diode on the center tap to ground on a Twin Reverb. The procedure is outlined in Dan Torres' "Inside Tube Amps" on pgs. 264-265.
    I used NTE #5271AK, although #5275AK would drop about 50 volts. (max recommended) He says the power resistor method is not very consistent at different output levels.
     

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