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If it lacks current limiting resistors in the output section, power tubes will fail easily.

Posted by Ralph (M) on May 14, 2020 at 09:46:57

In Reply to: What is the value of the current limiting resistors in the output section? posted by Ralph on May 13, 2020 at 11:54:58:

The best place to place the resistors is in the cathode circuit of each section. We use 5 ohm 5 watt devices and what might sound weird here is that they will actually lower the output impedance of the amplifier because they will cause the power tubes to work more harmoniously together. If a power tube is hogging current it will limit the output power and it will be more likely to fail.

So this is a common sense improvement to the original circuit.

The other thing I would do is pre-condition the power tubes. 6080s and 6082s need about 4 days and 4 nights of pre-conditioning prior to use. You can't precondition a used tube- this only applies to new tubes or NOS. Pre-conditioning is essentially running the filament circuit and no B+ whatsoever during the process. It can double the life of the tube and vastly reduce arcing due to flaking of the cathode coating.

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