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RE: PX-25 or PX-4

I would steer well clear of Osram globe PX4 , early versions such as these have lower dissipation than ST shaped PX4 . Genuine PX4 and PX25 have a B4 base so I don't know how these will fit a 300B amp which uses a UX4 socket . PX25 also requires a much higher impedence output transformer than 300B . Both PX4 and PX25 also have 4 volt filaments . It seems unlikely your amp can accept either unless a major rebuild/rebias is carried out or these modern 'PX4' and 'PX25' are derived from a 300B and fitted with a UX4 base and different filament .

Al


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