Dating silverface vibrolux
In addition, the bias circuit will be converted to a standard blackface type that allows for precise adjustment without pulling the amp chassis.
This saves time and money on future output tube replacements.
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For most Silverface amps not built in the above time period, you still get the bias adjustment mod but the other changes will be a bit subtler.
Its in the middle of the 6 preamp tubes with the removable shields. For a black face, this transformer would have been made in 1964 during the 23 week (—423) The trick to this is they don’t renumber the decades.
Selected wires are re-routed and/or shielded or small capacitors are added to stabilize the amp so that it will not become unstable after changing the components.
These wiring changes are often necessary because some SF amps become somewhat unstable after converting to blackface spec.
(.00 additional at that time) Always pull on the volume controls to see if they pull out-these were pull boosts, not labeled on 70’s faceplates.
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These circuit changes fixed the instability but adversely affected the tone of the amp.