I am the proud owner of a 70s Music Man HD-130, chassis # 2475-130. I'd like to replace the tubes and bias the amp myself, but I don't know much about the amp except it has 4 x EL-34s.

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It is an early version. It has a completely solid state pre amp and a Tube power amp design which has one 12AX7A used as a phase inverter / driver tube for the output section of four EL34 output tubes.

The bias is adjustable on early versions later versions are not (see above).