The Fine Things are Always Hand Made
Masterpiece / Super XII overlap? transitional?
Replies to This Discussion
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Permalink Reply by George
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Screen to Cathode bypass cap in a 6K7 first audio? That sounds very unusual since 6K7 tubes are variable-Mu and not generally suitable for audio only (which typically use straight-Mu gain tubes).
If it's a dual cap there usually is a common negative terminal, but you've described this as seeming to have a pair of 10uF caps which have completely independent connections in all places. Can you please confirm this about the capacitor?
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Permalink Reply by Kent King
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I agree with George - do the solder joints appear original? Is the component a period unit? If it was replaced, I would not trust it being factory spec.
As to the chicken and egg: The Super XII comes first, produced in 1938 and up through July 1939. The Masterpiece was brought out to replace the SuperXII after it was discontinued as the "low end" set in 1939.
Kent
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Permalink Reply by David C. Poland
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Scott used the 6K7-G an Audio stage tube, with the caution in the Phantom Tech Service Manual, page 5, that the tube needs a spiral filament heater to minimize hum issues.
If a hum issue, try a Sylvania 6K7-G, probably with the green print on the base rather than an earlier hot branded Sylvania tube, in the 1st audio stage socket. Also, pull the 2nd AF tube and if hum still present work to remove that hum first, trying different drivers and output tubes.
I regard the Scott Masterpiece as a hopped up Super 12 with a revised audio with push-pull driver stage. Mine sounded great with a CD player into the phono input.
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Permalink Reply by Randy Rago
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Ill reply to George and Kent first. Yes this cap can is a period piece, original cloth wiring and all. Maybe my description was wacky, but the sections share a common neg.
BTW, gotta love the cap symbols, looked like rectifiers on the schem. lol
David, all indications are that its a masterpiece, but that oddball schem. had me guessing. Im chasing audio loss (total) between the 1st audio 6K7 and the inverter 6J5. If i inject at the P-P 2nd audio grid she's happy. So far only one resistor out of spec. Not bad for a 90 yr old gal.
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Randy, and this is a general rule I’ve worked by in the past in my employment, all passive components are suspect and should be inspected one by one. I used to work at a military contractor many years ago and it was this same approach we used when inspecting some bit of kit. It takes quite a while.
If in doubt replace everything with new.
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Me to, George. All caps done. Working through resistors now
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Tale to share. Some years ago I would work on tube radios over the winter. One of the more involved sets was a Hallicrafters SX28A which I did for someone.
The radio has never seen a malicious soldering iron in it’s life but due to the radio’s perpetual deafness someone in the recent past added a solid state RF preamp encapsulated in rubber heat shrink.
The radio carried two assembly line defects, simply put two wrong resistor values in a couple of places one of which would kill receiver sensitivity. These were all factory original parts BTW. So when the radio failed QC testing back in the 40s it was simply set aside then later when the war ended it was sold as defective war surplus.
The overhaul effort discovered the two factory mistakes and once repaired the radio sprang into life.
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Randy - A Scott Masterpiece is identified (compared to a Super 12):
- total of 14 tubes on the 2 chassis: 11 including the eye tube on the receiver and 3 on the power supply..
-absence of out put 6V6's on the receiver chassis - they were moved to a 3 tube power supply using a 5U4 rectifier.
- Stradavarius black dial had a added 5 th Ultra High SW band (for Television audio).
- Phono switch moved from the band switch to the lower left SELECTIVITY control switch.
- single Scott badged 12 inch flange mount speaker with octal plug on speaker cable, with two field coils.
(Same speaker used for the earlier Scott model 16 and the 19 tube Phantom.)
- the presence of 6 control shafts is not conclusive, because the late production Super 12 model also had 6 control shafts but some with different purposes - and had a small single rectifier for a 1 tube power supply.
-FYI - one FM/AM Masterpiece has shown up and it had 2 eye tubes but unlikely what you have.
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