SUGDEN: P51: Anyone could shed some light on this one?

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I recently got my hand on a Sugden P51 amplifier, I already have an old integrated A21 this one landed out off the blue from someone who had no interest in it. It was apparently well cared-for, but as I wanted to have a better idea of the general condition I had to get the cover off. I am clueless as to what I have.

I expected an amplifier made using roughly the same topology and active devices as other product (late A21 SIII, A48 II ...) from this maker from that same period ie; T0-3 (2N30055) output devices.

Now this amplifier has as drivers, BC639/640 in pairs driving in final a quartet of MJE3055 & complementary (plastic TO) in parallel. That is 16 output transistors, four per heathsink.

The only reliable informations I can refer to is 2 different schematics, one using TO-3 for which I have a component list, and another one probably of later date since it has a few more transistors and include an "anti-thump" circuit that is present on the amplifier I have for which I have no component list...

I have asked on another forum in U.K. if someone would have some information on the subject, It was said that Sugden did go for plastic TO220 transistors for it was seen as the way forward.  Someone have suggested it would be a modification . I can yet resign myself to this conclusion, not that I would have a problem if it had been modified the work is flawless.

The amplifier was wired for north america main voltage (120) a label underneath states so , the output devices were install on a same heatsink as those used by Sugden ( same material, profile, and width)
using cheeshead screws that were never adopted in america... ?

Any information will be most apreciated

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