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Ampeg SVT CL

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  • Ampeg SVT CL

    Have a biasing issue with an SVT.....The green LED indicator on one side goes not light up....The red one does....On the other bias indicator, both the green and red leds do their thing as you adjust the bias controls. So I swapped around the 12Au7's to see if the problem would more or stay where it was...it Stayed!! So, I took the output tubes and swapped each set of three around to see if the problem would move...it did not...then I replaced the 12AX7 tube to see if that was the problem...it was not...So.....Looking at the schematic, IC3 and IC4 are the ic's that control the biasing LEd's labeled D48...(this is the one where the green led will not light when you adjust the bias control for that respective circuit)...Ic 5 and 6 control the other bias indicator, Led38....This one is working fine. I checked the reference voltages on all the chips in both the good and bad sections and they all measure the same...Then I measured the output of IC3 at pin1 and varied the associated bias pot....there was no change. When I did the same thing with IC5, and measured the output at pin1 and varied the other bias pot, the voltage switched back and forth from a positive and negative reading depending on the direction of rotation of the bias pot...But with Ic3 this will not happen...Pin7 of IC3 will switch fine.....SO I replaced the ic,a tl074...still the same problem.Checking pin3 of Ic3 showed not signs of adjustment when you varied the bias pot for that circuit...however pin3 of IC5 did show signs of adjustment when you varied the bias bot for that circuit. I checked the values of the associated diodes and resistors there in the defective circuit but they all test good. I even checked the traces to see if any were broken..they all checked fine..Anybody willing to shed some light on this one?? I plugged in a bass guitar and the amp plays fine....it is just that this green led does not light up when that particular bias pot is adjusted...and the led light checks good...

  • #2
    Start simple, see if you have an open screen or grid resistor, pull a tube from each side and compare the voltages on the output tube sockets.

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    • #3
      Thanks..Found the problem after...There was an open resistor on the output tube board...a 220 ohm...R4.....and there was one more that was burnt slightly but still checked good so I replaced both of them...The biasing lights all work now the way they are supposed to...I learned some things about this...it was a good experience...one thing I have to ask...this amp has 6 tubes.... 3 per side...can you run this amp with only one tube per side if you had to...or any amp that uses four or more tubes in the output stage...
      Cheers,
      Bernie

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      • #4
        There is a good article on these amps at Bustedgear.com.BustedGear | Home

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        • #5
          Thanks for that info..I'll check it out....
          Cheers,
          Bernie

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