Gibson G40 & G70 - Two Projects - and a mystery speaker

Discussion in 'Amp Tech Center' started by jvin248, Sep 17, 2015.

  1. jvin248

    jvin248 Poster Extraordinaire

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    I picked up a pair of Gibson non-working amps 1972 G-40 and 1971 G-70. Yeah. I think I overpaid and they don't work, which I knew they didn't work when I got them. I'm hoping the problem is 'simple' but wanted some advice before I get carried away.

    All they do is Hum. Guy I bought them from thought they needed re-capping and I thought the same but could be other things too, a gamble.

    I opened the cases up this afternoon and see that the power capacitors all have bulges on the end(s) and discoloration on the bulges. Candidates for replacement. There is corrosion on many of the steel fasteners (since steel screws and such were used to attach to aluminum which makes steel corrode faster - probably some bad grounds in there somewhere).

    Before I try the "local" electronics parts store for new caps that will perform for years ahead .. do I need to worry about seeking out vintage NOS caps (that are probably pricy)? Or just tie a bag of the original caps inside the case for any future owner?

    The G-40 has a mystery speaker that I attached a picture of. An obvious replacement due to some wire soldering at the speaker. Might be a 'blackback Celestion' but guessing since pictures I found on the net don't have the frame stamped rib darts. The G-70 has the factory Gibson speakers with labels.

    Bad appearing caps on the G40 are the three large blue 400uF-400V on the lower left of the PC board as pictured.

    Bad appearing caps on the G70 are the two large blue caps also on the lower left of the PC board as pictured. The two large vertically mounted caps seem ok from appearance.

    There are a couple of bad pots on the control panel but I'll worry about those if the rest will work again. Same with the frayed power cord on the G40, planning a 3-prong plug on both amps for secure case grounds.

    Thanks in advance for any notes/thoughts/ideas as I get into this.
     

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  2. flyswatter

    flyswatter Friend of Leo's

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    I think your best bet for cap replacements would be to aim for the middle ground between electronic store cheapos and NOS. Write down the uF and voltage data and find comparable replacements online, somewhere like Amplified Parts or Antique Electronic Supply.

    I played through a functioning G70 a couple of times and came very near to owning one. These are seriously loud, punchy SS amps that put out something like 120 watts and sound great. If you've got most of the original parts and speakers, it could be well worth your while to restore this rare, unusual classic.

    The G70/ 40 were sold by Gibson only in 1972, so they are rare birds indeed.

    Here's a thread I participated in some time ago on Everything SG. You'll find a schematic there...
    http://www.everythingsg.com/threads/anyone-owned-tried-gibson-amps-from-the-70s.24528/
     
  3. uriah1

    uriah1 Doctor of Teleocity Ad Free Member

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    Dont think Celestion speakers were labeled like that. Hard to read the 1st 3 digits
    of speaker code... Could be Magnavox or Dukane...

    wish you luck
     
  4. Wally

    Wally Telefied Ad Free Member

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    NOS caps.....there is every reason NOT to buy NOS electrolytic caps. Electrolytics go bad with age...even if they have never been put in a circuit. They are related to dry cell batteries in this respect...among others.
    Speakers....the EIA code is 575 there?.unknown to me...and were built in the 12th week of 1972.
     
  5. Bill Hicklin

    Bill Hicklin TDPRI Member

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    575 was the EIA code for Heppner. Heppner speakers are usually associated with 50s-era organ cabs like Rola, pretty light-duty, but that magnet and basket in your amp are good and beefy- that's because after 1967 Heppners were made by Cerwin-Vega, principally for high-power hifi.
     
    Last edited: Sep 18, 2015
  6. uriah1

    uriah1 Doctor of Teleocity Ad Free Member

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    Hmm

    I thought speaker code was ??2-33

    The 575212 is an inventory id, not speaker MFG id...I think..but, I could be wrong..
     
  7. Wally

    Wally Telefied Ad Free Member

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    Uriah, that is the EIA code there....575212.
    Good luck with that project, jvn248. Those are not my bag of tea. YMMV Lots of electrolytics strewn across there in those two chassi, right?
     
  8. Bill Hicklin

    Bill Hicklin TDPRI Member

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    (duplicate)
     
  9. Bill Hicklin

    Bill Hicklin TDPRI Member

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    575-2-12: Heppner, 12th week of 1972 (ie March). The other one isn't a Gibson part #, so it's probably a mfr's part or model #
     
  10. uriah1

    uriah1 Doctor of Teleocity Ad Free Member

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    hmm thx
     
  11. Wally

    Wally Telefied Ad Free Member

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    hmm.....thank Bill, too....hmmmm??
     
  12. jvin248

    jvin248 Poster Extraordinaire

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    Other than the replaced speaker in the G40, both amps seem fairly original. That's why I wanted to post something here. Before I went too far and got a comment like "wish you hadn't done that, everyone knows not to clean the patina off those things!" and find I accidentally took a collectable item down to partscaster status.

    When I did a little research before buying them I found some owners/players loved the clean channel on these. Only sold for one year due to low demand for SS in a Tube world maybe? I also saw that at the time, Fender and Marshall amps were so popular that Gibson didn't move many of their amps, including these.
     
  13. jvin248

    jvin248 Poster Extraordinaire

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    I found an image of a Heppner 10 inch, marked "5757249". No frame darts (probably only a need for the 12 inch+ size) but the magnet construction and the font stamp plus types of numbers seem very similar.

    Found a second Heppner 10 inch "575..." with frame darts and both have pre-punched rim holes for locating the wire connection plate on any window.
     

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  14. jvin248

    jvin248 Poster Extraordinaire

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    That's what I think too for NOS as being sketchy, but I also don't want to throw random caps in there if there is a benefit to targeting something specific.

    ... and yes there are more electrolytic capacitors than disk types on there.
     
  15. jvin248

    jvin248 Poster Extraordinaire

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    Thanks! I see you tried one of these out:
    "After work I went and retried the G70. Yep, it's a winner.
    This is a LOUD hybrid, .... Has a proper Fender-style reverb tank that works perfectly. In fact, it's a very Fender-style amp in appearance and configuration, although the tone is more Vox than Fender."

    Did that one you tried have visible tubes? The G70 I have only has the big cannister Mallory Caps behind the transformer (in the picture above the ends with the screws can be seen. All SS in the one I have.
     
  16. Wally

    Wally Telefied Ad Free Member

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    jvin248 wrote: "This is a LOUD hybrid, .... "

    jvn248, that amp is a pure solid state amp...nothing 'hybrid' about it...if by 'hybrid' one is meaning a mixture of tube and SS technology. That is the accepted meaning for 'hybrid' in our amp world.
    There is a reason why Gibson didn't sell many of these amps at that time. IN that link to the everythingsg.com site, one poster refers to the Lab Series. Those amps were not Gibson amps. They were designed and built by the Moog synthesizer folks...who understand intimately what makes a musical not....for Norlin, Inc., who also owned Gibson at that time. The Lab Series are the best solid state guitar amps I have ever heard. I still own the one I bought new in 1978. The Labs still don't supplant a good tube amp for me....but they are better than some tube amps I have played through, for sure. These Gibson 'G' series amps have no relationship to the Lab Series amps....and sound nothing like them. And...yes, I have experienced the G series in the form of the G-80, which is the same amp as the G-70 but outfitted with 4x10.
    Fwiw, I am a big Gibson amp fan. My first amp was a little Skylark from the early '60's.....and this was in 1963 when I got my first electric...a white Gibson Les Paul Jr. I wish I still had both of those.
    I have never heard a Gibson from 1960 on back that didn't have something working for it in my ear.
     
  17. jvin248

    jvin248 Poster Extraordinaire

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    Hybrid was an extract from the other forum quote that I snipped back ... I don't see any tubes in either of the ones I have. The original quote was after (flyswatter) tested the G-70 in a store and must have had a 'tube amp' tone to it.

    I saw a post where someone claimed the Lab Series and GXX shared similar circuits, looks like you've experienced and owned both and they are not. More curious for me to hear what these things sound like when they are running again.
     
  18. Wally

    Wally Telefied Ad Free Member

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    jvn248, I still have all of the paperwork..including the full schematic, that came with my Lab L-5, and I can tell you with no uncertainty that the Lab Series L-5/7/9 schematic makes that G-70/80 schematic look simple. when unfolded, the L5/7/9 schematic the better part of a square yard. I don't know who said that there was similarity. They are both fully solid state, so there must beg similarities, but the Lab's Multifilter section in and of itself sets the Lab apart from any other SS amp, ime. The Lab also has a parametric EQ for the mids and active tone controls for the treble and bass. The compression circuit is one of the best I have experienced. So...imho...there is not a lot of similarity.
    I have not owned a G-70 or G-80. One came in for repair, and a young EE Masters student worked on it. I didn't care to mess with it, and I didn't like how it sounded. Picayune I am when it comes to amplification. IF it had been for sale at the right price, I might have bought it for the speakers. Others' mileage varies, I realize.
    When I showed that young Master EE grad the schematic for the Lab Series L-5, he fell out when he came to the Multifilter section......and admitted that he didn't understand exactly what was going on there. Those Moog folks knew what they were doing. IF you know how to run a Lab Series L-5/7/9 amp, you can fool some people into thinking it is a tube amp. You can get that loose, compressed OD of a Tweed Champ or the bark of a big Marshall type of amp....or mimic a Twin Reverb. IT has massive overdrive...if wanted. OR...it will sing like a bird with clean sustain. IF one doesn't know how to run it, it can also sound like the worst SS amp one has ever heard. B.B. King knew how to run a Lab L5.
     
  19. flyswatter

    flyswatter Friend of Leo's

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    The tube thing was just mistake I made upon glancing briefly into the back of the amp after the clerk told me it had a preamp tube. I'm fairly sure it does not... more likely a cap can.
     
  20. uriah1

    uriah1 Doctor of Teleocity Ad Free Member

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    My hmm was, I thought they (speaker codes) were always split
    into two pairs xxx-xxxx...maybe just some..been awhile, and you see
    them all the time...old and new
     
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