• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

SUNN 300T bassamplifier green/red LED behaves strange

Thread starter #1
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...
 

Latest threads

EE World Online Articles

Loading

 
Top