This Ampeg SVT-CL kept shutting itself off soon after being powered on.
By monitoring the DC voltages on the pins of the six output tubes during shutdown, the problem was traced to an open cathode resistor.
The bad resistor kept one of the
output tubes from conducting current, causing its normally low cathode
voltage to rise.
In this photo, the six cathode resistors are located within the red circle.
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Here, a replacement resistor (circled in red) has been installed.
We
needed a 10 Ohm, 2 Watt, 1% tolerance part. At the time, we could
only find suitable 2% tolerance resistors so we selected one of them that was
within 1%
of its rated, 10-ohm value.
Now, we have the 1% cathode resistors for the Ampeg SVT-CL. They're
available for sale on our Resistor Page:
10Ω, 2W, 1%.
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The SVT-CL fault-detection circuitry (right) monitors the voltages across each of
the six cathode
resistors (outlined in green).
If a tube were to short out, the increased current through its cathode
resistor would cause the voltage across it to rise enough to trip a voltage
comparator. The comparator's output voltage would then open an
electro-mechanical switch, shutting off the amp's main ac power.
In this SVT, no tube shorted out. Instead, a cathode resistor
stopped conducting. Without a source of electrons, the tube's
cathode voltage floated up toward that of its plate. That rising voltage shut down the amp and caused the Standby-Fault LED to flash.
Enlarge Schematic Image
Ampeg SVT-CL Schematics
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