Vintage Newcomb G-12 amp

Discussion in 'Amp Central Station' started by pchilson, Jan 1, 2009.

  1. pchilson

    pchilson Friend of Leo's

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    The amp had installed a couple RCA 6V6GTs. I don't know how to date these tubes as I see no code other than the "KX" shown in post # 11. How do you tell if the RCAs are "black plate" or "gray plate"? Are there tell tale characteristics? The glass is kinda smoked so its hard to tell for sure the color if thats the obvious way to tell. If they are black plates what are there value on the open market?

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  2. muchxs

    muchxs Doctor of Teleocity

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    They're black plates. Value in the open market? Hard to say right now, a lot of good stuff is going really cheap. Tubes seem to be holding up well, though. Let's say $75.00 for the pair if they're exactly matched. $20.00-$25.00 each if they're not.
     
  3. pchilson

    pchilson Friend of Leo's

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    I don't want to sell them. I just like to be aware of what I have. I don't know all of how you test for "matched pair" but the local electronics store has one of the old ETA tube testers like you used to see in the drug store. Thats what I use to test my tubes. On that machine these two 6V6s tested within one point of the same value. Don't know if thats matched... I will give these a go in my DRRI and see how that sounds.
     
  4. twangcaster1

    twangcaster1 Friend of Leo's

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    Looks like a cool one to me, man.
     
  5. muchxs

    muchxs Doctor of Teleocity

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    Same "KX" codes, black plates removed from the same amp. They're matched. That old Newcomb is cathode biased anyway, cathode bias is tolerant of a slight mismatch.

    I'll bet the engineers at Newcomb never thought this would be a rippin' guitar amp 50 years later. :lol:
     
  6. pchilson

    pchilson Friend of Leo's

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    As far as dating the amp the only thing I can use is the volume and tone pots. I can't find any informative code on the PT that I can use for manufacturer or date, only the "TR 88" shown in post # 12 and that doesn't align with any manufacture code as far as I can tell. The OT is not original so that doesn't tell any stories. The volume pot code is "134943" made by CentraLab dated Oct. 1959 and the tone pot is "134008" made by CentraLab dated Feb. 1960. So the amp dates at least to 1960.

    I don't know if I mentioned yet but the schematic was pasted to the chassis bottom cover plate, it states this on the chassis itself but that cover was not with the amp. I cannot find nary a mention of this amp anywhere. I've seen mention of being able to go to the library and somtimes being able to find old service manuals with schematics in them. Anyone have ever done that? Any tips on how I might track down a schematic?
     
  7. muchxs

    muchxs Doctor of Teleocity

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    Codes tell the story, I'd peg that amp at early '60. That makes sense considering the style of other components in the amp.

    Sams Photofacts probably had a schematic for it. On the other hand most of these things were designed straight from RCA's tube data sheets or one of the major manufacturer's schematics and circuit suggestions. There are no mysteries in the power supply and the output stage, I'd expect them to be similar to any number of '50s amps using 6V6s in the PA with a 5Y3 rectifier in the power supply. The rest is easy enough to trace out, look up data sheets for 6SC7 and 6SF5 tubes.

    http://www.triodeel.com/tubedata.htm

    The tube pinout will be there.

    http://scottbecker.net/tube/sheets/049/6/6SF5.pdf

    Find the filaments, that leaves a single triode (three pins) for the 6SF5 and two triodes (six pins) for the 6SC7. Identify the individual elements, plate, cathode, grid and sketch it out. Drawing a schematic may even prove to be easier than finding one. Another advantage of tracing it out is that manufacturers sometimes changed circuits without notice, the schematic may not match what you have. Your own drawing will accurately reflect what's actually there.
     
  8. pchilson

    pchilson Friend of Leo's

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    With a little help from muchxs I am contemplating on an OT purchase for the amp. With that I would like to also take advantage of the 15% discount being offered by Weber on their speakers. Now with this amp being a quasi tweed deluxe type which of the Weber speakers would be a good and suitable choice for the amp. I also have my DRRI that has the stock Jensen C-12K in it and could a good speaker choice for the Newcomb also be a good fit for the DRRI? What does everybody suggest I consider here?

    Thanks
     
  9. pchilson

    pchilson Friend of Leo's

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    My order from tubedepot.com came in today which consisted of all the replacement caps and a NOS 6SC7GT. I have changed out the cap can giving me fresh power supply caps and the cathode bypass cap. I have not change out the others yet. I want to give the amp a spin with changing out component sections one at a time to see what clears up what. So far the power supply caps and the new 6CS7GT have eliminated the hissy/fizzy from the higher volumes so I guess the tube at least was bad. I haven't had a chance to really work it out yet to get a real feel for it. Should the rest of the caps be likely to be on the "not so good" side or is that just a try it and see thing?

    I also placed an order today with Weber. I ordered an Alnico Sig12A-S and a Ceramic Sig12F. Took advantage of the 15% off sale before it ends on Saturday. I also ordered an output transformer for it so I won't have a mismatch when the speakers arrive. I'll follow up when these items arrive.
     
  10. pchilson

    pchilson Friend of Leo's

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    My order from Weber arrived today. I picked up a BR-2 Bias-Rite two lead, an alnico Sig12A-S, a ceramic Sig12F and a W041318 output tranny. The OT and the Sig12A-S were for the Newcomb.

    And thanks to muchxs for lots of help and advice on this.

    All is installed now. Sounds much better, cleaner, clearer, tighter. I have a fffiiizzztttt in the cab somewhere that now I have to trace and dampen.

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  11. krrraaannnggg

    krrraaannnggg NEW MEMBER!

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    How did it work out?

    Hey i'm fixing up a Newcomb like yours for a friend. Thanks for posting all the pictures and comments. Would you mind, if you still have the list, providing the parts info for all the stuff you ordered in the rehab? You'll save me a pile of time learning bumblebee codes, at minimum! And -- now that you've had it running for a month, what's the verdict on tone? Alnico or ceramic Weber? 4 or 8 ohm? Is it a warm tweed deluxe or what?
     
  12. pchilson

    pchilson Friend of Leo's

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    krrraaannnggg, sent you a PM.

    I found the buzzing in the amp cab. It was the steel mesh grille. Replaced it with some old grille cloth I had laying around. This amp has become my present favorite even steals time from the DRRI...
    Here is how it looks at present.

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  13. pchilson

    pchilson Friend of Leo's

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    The cabinet on the Newcomb didn't have a removable back so it was very difficult when putting the amp chassis back in to be able to reach in and connect the speaker. I cut the back out and fastened some cleats and used 4 screws with finish washers to make the back removable allowing eaiser access to the inside.

    A couple more shots.

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  14. SUPROficial

    SUPROficial TDPRI Member

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    Hello, I have a Newcomb G-12, apparently from around 1953. It is a bigger amp than you would guess by looking at the pictures, the cabinet is 18.5" by 15". It looks similar to the one in the photos in this thread, with the exception of the knobs (huge round white plastic things which appear to be original) and the Newcomb logo being positioned much lower. It has a great clean sound and a surprising amount of headroom, but turned up past halfway it gets very angry and sloppy sounding and does the "I'm about to explode" thing really nicely. It compares quite favorably to a tv front Deluxe but has its own sound.

    Visible through one of the louver openings in the chassis is the inscription "circuit diagram on bottom plate" and I'm sure more than a few G-12 owners searched in bewilderment for this "bottom plate", which is actually a metal cover for the bottom of the chassis held in place by seven screws. Mine came with a couple of screws missing and emitted some rather annoying rattles. The chassis cover or "bottom plate" is frequently missing on these amps, perhaps people remove it to try and eliminate buzzing (which is usually coming from that damn metal grille) and eventually lose it.

    If anyone needs the schematic, here it is:

    http://img354.imageshack.us/img354/8108/newcombg12.gif

    I hope that's clear enough, the paper it is printed on is maybe 3"x4" and I don't have the world's greatest camera! Hope this helps anyone who needs it.
     
  15. Nocab28

    Nocab28 NEW MEMBER!

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    Hey guys, I just got one of these amps as a gift. Mine appears to be all original with the exception of the power cord, which is now grounded. I think the amp sounds killer, especially with a tele! I believe this amp may be slept on as a great vintage amp...
     

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  16. Muzicdepot

    Muzicdepot TDPRI Member

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  17. Muzicdepot

    Muzicdepot TDPRI Member

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    I picked a G12 its very nice looks all orignal very clean. Sounds awsome might be selling it
     

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  18. Muzicdepot

    Muzicdepot TDPRI Member

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    I have a Newcomb G 12 all orignal sounds great nice condition .Im looking to sell it if you know anybody looking thanks. Mike .401 556 6399
     
  19. Keep It Simple

    Keep It Simple NEW MEMBER!

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    Can you tell me what speaker is the best for a Newcomb G12? Seems like there aren’t a lot of options that would fit in the shallow cab.


     
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