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SUNN 300T bassamplifier green/red LED behaves strange

Discussion in 'Repairing Electronics' started by Stewen, Nov 23, 2017.

  1. Stewen

    Stewen New Member

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    Nov 23, 2017
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    Hallo ! This is my first post here . This amp has six 6550 powertubes. Each pair of these have a control , bicolour LED in its screencircuit . At start
    each of the three LEDS lights red and when turning on Stand By , high voltage reach anodes and screens , and the LEDs turn green . The problem is
    that after 3-5 minutes , one LED turns back to red . That is meant to tell that a pair of outputtubes has lost its highvoltage to anodes and screens and that
    only 4 powertubes or two pair are still working properly . The issue here is that all six tubes work OK and nothing serious have happened ! I have measured
    the voltages and all three LEDs show about 2,2 VDC at the "greenanode " when they at first are light up green . Then , suddenly , after 3-4 minutes ,
    voltage at the anode of the "problemled" slowly start to raise up to 3,6 VDC and the red LED in this bicolourLED turns on . The greenside of these three bicoloured LEDs are driven by a 2N4401 transistor . Please look at the schematic of the SUNN 300T bassamp . All three transistors are fine .
    The only thing I can think of is that the internal resistance of this "problemled" is increasing with temperature and shuts off the greenled .??? The base of
    each transistor is feeded from 330 VDC ( screenvoltage) , through 220K+220K , base , and finally 4,7K to ground . That gives about 3,6VDC if the base-
    current is near zero ...... This is a Mystery ........ I´d be very happy if someone would like to look at the schematic . Best regards from an old tubetech...
     

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