Ampeg SVT Classic 300w all tube bass head came in …
Ampeg SVT Classic 300w all tube bass head came in the other day for a noise issue presenting as an intermittent low level crackle but any vibration of the chassis results in a low hum which is totally unaffected by any front panel controls. This thing weighs an absolute ton which is not helpful when trying to dismantle for diagnostic. The preamp chassis is independent of the power amplifier chassis & has to be disconnected from all its interconnects & removed before the power section can be extracted. The sextet of 6550WE Tubes are mounted on a removable plate exposing the main board for service /diagnostic. Eight screws should release it but in this case the factory applied hot melt glue over everything had glued the tube base cables to the main board preventing the access plate lifting off.
At this moment everything is pointing to a faulty tube. Hum is induced after power up by a light tap to one of the six. It is suspected that this is also the root of the crackling.
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