’63 Gibson Mercury-II // Repairs & Enhancements

this low-gain behemoth, and its associated 10″/15″ crossover-ed cab, came lumbering in after being worked on previously … Trem circuit didn’t work, amp had no goose, and showed no balls … after studying the schematic provided by this generous chap : http://www.backfromthesixties.co.uk/Pages/Gibson_Mercury_II.aspx I went to work … turns out this is another tube amplifier with the choke in the wrong place (same as some Fender E-series tweed models) … also, there was NO main cap in what normally is the main B+ node in the PSU … no wonder !

so, I inserted a main B+ cap (47uF/500v) and wired the choke right // inserting a proper variable bias circuit … as for the rest of the circuit, I replaced the wimpy low-gain PI circuit by that of the classic VOX AC-50 (as it also has no NFB coming off the speaker as well) // and also inserted 22uF caps in the cathode circuits of both 6EU7 preamp gain stages … finally, biased in JJ 6L6GC tubes (around 28mA with B+ @ 480 vdc) now and the amp exhibited the normal “humph” of a 6L6 duet amplifier

next came the Trem circuit … as it normally worked on a minute filament tungsten bulb I replaced the whole affair with an Silonex NSL-32 opto-coupler // in an almost identical way as my original Univibe “LED-mod” … the result was an extremely wide ranging Trem circuit, going easily from nice-round shapes to ultra-choppy

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