NAPD: New Amp Project Day - 1973 GMT 300B

Discussion in 'Amps and Cabs [BG]' started by funkytoe, Mar 1, 2014.

  1. funkytoe

    funkytoe Supporting Member

    Jan 17, 2008
    Northern California
    Picked up these two pieces over the last few months on separate local craigslist sales. After a little effort, they are up and running strong. Its a 1973 GMT 300B head and matching 4x12 bass reflex cab. The cab sounds massive, and the original Cerwin Vega drivers are still in it sounding great. Seriously, its only 150 watts but this head and cab can crank it out. Also picked up an original owners manual. Now, I just need to find the GMT footswitch. Anyone got one?

    I know, no photos = no amp. So, enjoy:




    mindwell likes this.
  2. funkytoe

    funkytoe Supporting Member

    Jan 17, 2008
    Northern California
    Now the manual:



  3. Sweet, never knew GK existed back then! And a crazy 4x12 to boot. Congrats.
  4. SirMjac28

    SirMjac28 Patiently Waiting For The Next British Invasion

    Aug 25, 2010
    The Great Midwest
    That is so cool I love the design of both the cab and head congrats.
  5. funkytoe

    funkytoe Supporting Member

    Jan 17, 2008
    Northern California
    Thanks guys. Gotta say this is right up at the top of my all time favorite solid state rigs. People tend to dismiss reflex cab design in favor of modern two and three-way direct cabs, but this sounds so massive with just 150 watts. Goes deep too. Realistic gig volume with no discernable drop off down to around 40 Hz. I'm loving it. Best part, I'm in it for less than $200 total. There are some great deals out there if you are willing to take on a project...
  6. P Town

    P Town

    Dec 7, 2011

    The only brands I knew about for amps back then was Ampeg, Fender, Vox, and Kustom.

    Never heard of GMT.

    My back hurts just looking at the cab, (says a guy with a fridge).
  7. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    Wow! Someone was selling one of those cabs local to me a while ago but it wasn't a deal like that!
  8. funkytoe

    funkytoe Supporting Member

    Jan 17, 2008
    Northern California
    It's heavy... But not Fridge heavy. About 90 pounds and comparable in size to an average 6x10 cab. Definitely manageable.
  9. funkytoe

    funkytoe Supporting Member

    Jan 17, 2008
    Northern California
  10. rogypop


    Jan 10, 2009
  11. funkytoe

    funkytoe Supporting Member

    Jan 17, 2008
    Northern California
    Gigged the head and cab together for the first time this weekend. Came through loud and proud even with a full band and five-piece horn section. No P.A. support required. And, I heard myself as well on stage as I ever have. Not sure why people think these reflex cabs are so quiet on stage and so loud 25 feet out in the audience. I certainly did not find that to be true. Bandmates loved it and thought it had "mojo" to spare. I have to agree. She's a keeper.
  12. sanfordandsonny


    Nov 15, 2006
    El Presidente of Sanford and Sonny Audio Weapons (Bluebeard Fuzz)
    And here I almost shelved my bass reflex 212 idea this is awesome.
    I assumed when you said gmt 412 it was a GK folded 412 w 210
    GMT is sexy now I have to look up the old big aluminum GK head I used to have...
  13. oerk


    Oct 16, 2009
    That's a nice rig! I've never seen anything like it, and certainly didn't know GK was around then.

    Can you elaborate how the cab works? Around here, the term "bass reflex" is used for a simple ported design, but this looks more like a short front loaded horn to me.
  14. funkytoe

    funkytoe Supporting Member

    Jan 17, 2008
    Northern California
    I wish I could explain it better! I would agree with you. Based on my experience, I would have called it a short folded horn cab with 2 12 inch speakers at the top and 2 12 inch speakers at the bottom. But, as you can see from the manual description above, GMT listed it as a "bass reflex" design. Sorry, I cannot say why that description would apply to this type of cab.

    I can say that it is an excellent sounding cab, with great clarity and surprising ability to reproduce fundamental frequencies down to 40 Hz. These 12 inch Vega drivers are really excellent and very efficient in this design. It can get REALLY loud with just 200 watts.

    Currently, its only getting 150 watts from the 300B head into 4 ohms. But, last week I picked up a 600B head that will give me 200 watts into this cab and 300 watts into 2 ohms. Really loving this old gear!

    Here is a photo of one of my bass heroes - Carl Radle - playing a similar GMT rig in the early/mid-1970's.
    Like I said, they get plenty loud!

    Last edited: Jul 29, 2015
  15. bottomend!


    Oct 23, 2007
    Just bought it. The thing just looked too weird/alien to pass up... and it was located close to me too! Any recommendations as to what speakers would go well in this thing are welcome.

    I dont know why it's been referred to as a 'relex' cab because it's just a pair of sealed 2-12" speakers connected to a horn throat as seen in the pics. There are no ports anywhere to be found on this weirdo.
  16. 1n3


    Sep 13, 2007

    Bob Gallien is in an elite group of guys who've founded AND maintained control of a major MI manufacturing company, along with Jim Marshall and Hartley Peavey. Props to them.

    GK packaging has often been innovative... can't remember another company that used aluminum castings for the sides like this. Cool piece.

    The cab is definitely a folded horn design, not reflex.
  17. funkytoe

    funkytoe Supporting Member

    Jan 17, 2008
    Northern California
    I really like the OEM Cerwin Vega speakers in my cab. Given the design, I don't think I would spend a lot of money on replacement speakers. Might be nice to reduce the weight some. I would probably start with Eminence Beta 12A or BassLite S2012 speakers if I were to replace all four.
  18. bottomend!


    Oct 23, 2007
    I just looked at the charts for both those drivers and while I proudly confess to not being nearly well enough equipped to actually understand all things T/S, it looks to me like the Beta 12a wouldn't be a good match while the Basslite would be good but... it's out of my price range for sure.

    I'd love to find a vintage set of CV's but I bet be hunting for the next couple years or so....
    I dont have much experience with infinite baffle speakers and what they want from their respective cabinets so I'm still scratching my head.

    I'm wondering if this might be an interesting application for a car audio speaker?
  19. funkytoe

    funkytoe Supporting Member

    Jan 17, 2008
    Northern California
    The other speakers I find interesting for that cab at a lower price range are the Jensen Mod 12-70 /16 ohm drivers. You could wire all 4 of them for a vintage sounding 4 ohm cab rated at 280 watts. Would be a good match for a GMT 300b / 400b or a GK 400rb.
  20. bottomend!


    Oct 23, 2007
    The MOD looks good on paper from how I'm interpreting the #s although I pretty much exclusively use old Acoustic gear and these might shred under the weight. The next model up in the Mod chain ( 12-110) looks good too but size and $$ rise accordingly. Whats up with the dip at 400hz and 1.3 k though? thats kinda where alot of the tone I like lives. Do these speakers have a 'scooped' sound?