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It is currently 27 Oct 2020, 13:55

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PostPosted: 05 Mar 2020, 17:17 

Joined: 01 Mar 2020, 20:37
Posts: 1
Hi All. I am refitting an old Magnavox turntable, installing a new turntable, and restoring the old amp, model AMP147AA, in the process. It is in great shape, generally speaking. I offer pics and a home-did, terribly rendered, partial schematic. I'll redraw a better version when my grid paper arrives and post it here for anyone who needs it - it's not available on Sam's etc. This one is incomplete - I didn't include the Loudness section, which has a a few tricks up its sleeve, but that shouldn't fundamentally effect the way the preamp behaves. I hope:).

Because of size constraints, the new turntable is limited to the smallest (decent quality) brand I could find online, so it will probably be an Orbit Basic. I have a choice whether to buy a built-in preamp installed in the turntable for about another $50 - it puts out a line level signal, making it easy to match up. But the more I thought about it, I figured the old turntables were putting out a phono-level signal - much lower - so maybe I should simply re-use the configuration the amp already has, rather than adapt the old system to accept a line-level signal.

When considering the power amp, it's not much different from many 6V6 amps I've seen, but there is only one 12ax7 dividing the duties of preamp and inverter. I don't have room to fit another preamp tube, so I'd like to work with what I've got. I was wondering if anyone had any experience with this sort of thing, and wanted to offer any input regarding whether the old 50's turntables had the same phono-level output of today's turntables - or close enough to it that I can simply install a new turntable in place of the old. Still, it may be simpler to re-design the pre-amp to accept a line-level signal. This turntable rebuild is a first for me, so I don't have the experience to make the best call. Plus, I'm not sure how to design a line-level input in place of the current preamp - although I could start guessing, which is what I usually do:).

I'm also interested in the capacitors used in the original - they seem almost ceramic, and they test well, even after all these years. So I'd like to reuse them. Finally, there's a little red capacitor going from pin1 of V1 to ground. I haven't seen too many of those, so I'm guessing it's a 22nF? It reads 35nF, but that's when still connected to the circuit.

Thanks for any help offered - much appreciated.

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