Hi.
Long time since my last post... try to make the long story short.
It happened about two years ago when I got a chance to play my buddy's 1956 GA-40... there was no coming back. The moment you finally realize that this is the tone that you've been searching for. Been trough Fenders, Marshalls, Voxes etc. but always had the feeling that something is still missing. It took 34 years of my life to realize that I'm a gibson guy.
I made a clone of the GA-40 and been playing the hell out of it. It has been my main gigging amp about for a year now.
Lately I've been having a major gas for GA-70, the 1957-1958 model.
Very few (172 amps) of these were ever built so my goal is to build a clone. It will be a totally different animal than GA-40, but I have a feeling that it will pair nice with the GA-40 in a two amp gigging setup.
The schematic is available with voltages, but there is no info about the output transformer. I have few recycled OT's lying around and I was wondering maybe I could reuse one of them. MM is making toneclone GA-70 OT but I haven't been very satisfied with MM iron lately. A bit too bright and "sterile" soundind to my ears and it's gonna be at least double the the list price to ship one overseas. I've never built an amp with 5881 power tubes before so I have no experience how it behaves with different primary loads. According to schematic plate voltage is 420V, screens are 425 volts and output power is about 35W, fixed bias. If any of you have a clue what could be the good ra-a match for 2x5881 it would be a great help. 4k? 5k2? 6k6? 8k?
Gibson GA-70 Country Western 1957-1958 model.
Moderators: pompeiisneaks, Colossal
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Dingleberry
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Gibson GA-70 Country Western 1957-1958 model.
Simplify before you amplify
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martin manning
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Re: Gibson GA-70 Country Western 1957-1958 model.
Unless you can find some specifics on the original, I would think a 4k 2x 5881 or 6L6 Fender transformer such as for a Bandmaster, Pro, or Vibrolux/Vibroverb would be appropriate. See for example Magnetic Components 40-18006.
Re: Gibson GA-70 Country Western 1957-1958 model.
With 4K Ra-a and the voltages shown in the schematic, the output power is far greater than 35W, so the Ra-a is likely to be quite a bit higher ~7k-8k... But it would be best to verify with someone that actually owns the real thing... 
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Stevem
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Re: Gibson GA-70 Country Western 1957-1958 model.
I have a 54 GA77 , also very very rare, two 6l6gc`s two 12ax7s , 6J5 and a 5V4 recto, cathode biased as they all where and 25 watts rms with 35 watts peak, 15" Jensen speaker, and I love this amp as if it was Raquel Welch in her 30`s!!
Its OT was bad when I got it and I replaced it with a OT from a 1958 Cordovox Accordion amp that used two 7591 output tubes and it sounds great and smooth with top end roll off when cranked just like you want for smooth highs!
The only difference that the Gibson OT had shielded primary wires which the Cordovox does not!
In short you do not need a or want a super MM type OT to keep the sweat nature of these amps, I have the spec of this OT written down at home and I will post it if you like Tomorrow?
also since you are building something I would make a provision to have it fixed bias, or cathode!
Its OT was bad when I got it and I replaced it with a OT from a 1958 Cordovox Accordion amp that used two 7591 output tubes and it sounds great and smooth with top end roll off when cranked just like you want for smooth highs!
The only difference that the Gibson OT had shielded primary wires which the Cordovox does not!
In short you do not need a or want a super MM type OT to keep the sweat nature of these amps, I have the spec of this OT written down at home and I will post it if you like Tomorrow?
also since you are building something I would make a provision to have it fixed bias, or cathode!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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martin manning
- Posts: 14308
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Gibson GA-70 Country Western 1957-1958 model.
Could be, depending on how you measure. Note that Fender amps from the same era (mentioned above) with similar voltages are rated ca. 35 watts.jazbo8 wrote:With 4K Ra-a and the voltages shown in the schematic, the output power is far greater than 35W, so the Ra-a is likely to be quite a bit higher ~7k-8k... But it would be best to verify with someone that actually owns the real thing... :D
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Dingleberry
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Re: Gibson GA-70 Country Western 1957-1958 model.
Thanks for replies guys.
It would be a real help if You have some specs about your OT Stevem.
I was thinking the same thing jazbo8, that the primary Z will probably be higher than the "nominal" Fender value. With those voltages, fixed bias and with approximately 4k ra-a you go easily to the 40-50W territory.
Was there a cathode bias version also of the GA-77 Stevem? Is it like the older version of GA-70, the Country AND Western?
I saw only GA-77 schematic that was fixed bias and actually identical to the GA-70 circuit just with added "tone" control in the tremolo channel.
Meanwhile I start to plan a layout to my existing chassis.
-T
It would be a real help if You have some specs about your OT Stevem.
I was thinking the same thing jazbo8, that the primary Z will probably be higher than the "nominal" Fender value. With those voltages, fixed bias and with approximately 4k ra-a you go easily to the 40-50W territory.
Was there a cathode bias version also of the GA-77 Stevem? Is it like the older version of GA-70, the Country AND Western?
I saw only GA-77 schematic that was fixed bias and actually identical to the GA-70 circuit just with added "tone" control in the tremolo channel.
Meanwhile I start to plan a layout to my existing chassis.
-T
Simplify before you amplify
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Gibson GA-70 Country Western 1957-1958 model.
My amp seems to be a cross between a GA75L as it has a recording Jack, and a GA77 Vangard, in fact the OT is stamped in the metal banding GA77.
If you go to the Gibson garage site you will see a picture ( number 4 and 5) that Rob from Buffalo amps posted of the top and back of one, mine has the same two channels, one with just a volume and tone and the other with V,B and treble, but like I posted the tube set is different and on mine the channel with just the tone control since it does not loose signal by going thru a tone stack is the screamer and the one you fall in love with, the other one is great for clean tones, of for lead with a box in front of it!
If you go to the Gibson garage site you will see a picture ( number 4 and 5) that Rob from Buffalo amps posted of the top and back of one, mine has the same two channels, one with just a volume and tone and the other with V,B and treble, but like I posted the tube set is different and on mine the channel with just the tone control since it does not loose signal by going thru a tone stack is the screamer and the one you fall in love with, the other one is great for clean tones, of for lead with a box in front of it!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
For What it's Worth
The Master Service Manual is posted on Schematics Unlimited. First entry in the Gibson list.
http://www.schematicsunlimited.com/g/gibson
Likely this is already known but just in case.
I found in the back of the manual various troubleshooting tips giving the distinct impression many of these models went out the door untested. Such as
Noisy: Lose connections prevalent in early runs of this model. Look for them everywhere...
see page 489 for other service hints...
I do know some were nice amps.
Silverfox.
http://www.schematicsunlimited.com/g/gibson
Likely this is already known but just in case.
I found in the back of the manual various troubleshooting tips giving the distinct impression many of these models went out the door untested. Such as
Noisy: Lose connections prevalent in early runs of this model. Look for them everywhere...
see page 489 for other service hints...
I do know some were nice amps.
Silverfox.