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Old 20th Dec 2019, 8:55 pm   #5
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 3,606
Default Re: Marconiphone 538

Great work there Jerry, I still think that these mid/late '30s sets represented something of a pinnacle in domestic AM radios- OK, components and valves later became smaller and more sophisticated in design and production, but power consumption and performance didn't greatly change post-war, the basic circuit design stayed much the same until transistors came along. These EMI group sets have lovely well-made cabinets and sturdy chassis and a set like this with RF stage, extensive SW coverage and potent audio capability would have been a source of some pride.

Those resistor results are instructive in just how much they can change over a long time, confirming the "usually, but not always upward!" trend. I wonder if Marconi were playing it a bit safe with the local oscillator, giving it a generous screen feed to make sure that a slightly tired valve continued to oscillate, even in the more difficult circumstance of the higher SW band? It might be interesting to check anode and screen voltage at both lowest and highest frequency on this band. Marconi/HMV seemed to like a main potential divider for screen grid etc. feeds- at least it means HT discharges quickly and stays that way for maintenance!

I wonder if the KT66 has gone a bit gassy with age and will improve with a few hours running? Judging from others' experience, particularly GrimJosef who has done a lot of work with kit using these. Is the U50 a "real" one?- some later substitutes may be indirectly-heated types with lower forward voltage drop, compounding the high transformer secondary voltage issue.

Re. highish HT voltages, I know you've relegated the idea of using a primary side resistor but one measure I've adopted with elderly valve radios is to fit an SMPSU-type inrush limiter of 220 ohms cold (25 degrees Centigrade) whenever I can- these settle when hot to a voltage drop of around 5-10V, knocking HT down a tad and giving the transformer a bit of an easier time. The main thing is that it gives a gentler start for things like scale lights and directly-heated rectifiers with fragile tape cathodes- and U50s aren't getting cheaper or more plentiful. I've a Marconi 559 here, ISTR that your set may similarly have a difficult-to-obtain long SBC scale bulb that's under-run by using primary tappings as an auto-transformer, giving it a soft-start may help prolong its life. I fitted a steel angle-bracket to the inner pair of mains transformer screws, supporting a Melamine terminal block for the inrush limiter.

I bet that cabinet came up a treat and looks great,

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