• New Sponsor: ShipNerd, Ship Your Gear with Us... for less! Click Here.

Gibson GA40 schematic versions and output transformer?

ES2nl

Member
Messages
19
I've been looking into building a GA40 reproduction from scratch, partly because vintage gibson amps are quite rare in my area (Holland) and partly because even if I find a vintage one usually many parts have been, or should have been replaced by now.

I've decided on the later ones with the 5879 preamp tubes, for which I found 2 different schematics (most of the difference is in the preamp section).
One I believe is from the second edition (1955-57) also used in a very inspiring build by leonc on this forum: http://www.chalnick.org/LeonsGear/GibsonGA-40/GA-40_Schematic.jpg
And the other schematic seems accurate for most of the 3rd and 4th version photos I've seen (1957-61): http://theguitarpodcast.net/ga40/gibson-ga40-les-paul-amplifier-schematic.JPG

I was hoping some of the experienced minds on tgp could confirm my ideas about the version of the schematics :)

Also I'm struggling to find the 'right' output transformer. The only source I've found who measured the primary impedance states it's 10.2k for a 1958 GA40. Leonc build his clone using a deluxe reverb OT: 20watt, 6.6k ohm primary (https://www.thegearpage.net/board/index.php?threads/new-build-gibson-ga-40-type-amp-long.1006701/). And based on the data sheets of 6V6GT tubes both 8k and 10k ohm are recommended for a AB pushpull design, though with different plate and screen voltages (8k: 285V, 10k: 250V) resulting in 14 or 10 watt power output respectively.

I then decided to ask an Italian transformer maker (inMADout) for their recommendation, as they don't hesitate to share the specifications on their site, contrary to Mercury. However, to complicate things more, they recommended their 5E3 OT which is rated at 25 watts with a primary impedance of 8k. To me this seems like putting a deluxe reverb ot in a princeton reverb; sounds very good but is definitely different from the original.

What would you recommend for a close reproduction of the vintage tone? Anyone who owns a late 50's GA40 who can chime in? Or am I over-complicating things :rolleyes:
 

ES2nl

Member
Messages
19
Yes I've seen these, very cool amps! Should be quite similar to the original as well. But at €2700 + shipping and import fees it is not exactly in my budget I'm afraid. I figured I can build one for around a 3rd of the price myself, especially since I already have a reconed 59 jensen P12P sitting on the shelf. Plus I love the building challenge, much more satisfying than buying something ready made.
 

Steppin' Wolfe

Silver Supporting Member
Messages
5,788
I don't think you are over-complicating things. IF you want what Gibson built, then you need the specs for the transformers that Gibson used. I am of the opinion that Mercury Magnetics knows those things. IF someone else has blueprinted those transformers while tearing them down, they too will have those specs. that experience is valuable, right? Wire size, windings...everything about the transformers. Maybe that is what one pays for when one buys MM ToneClones?
Otherwise, you take your chances....choose your parts...do the build...and enjoy what you have at the end of the line.
Looking at those two schemes I now see why my Ga-40 has two quite different channels. One of them is much more eager to overdrive. Those input resistors might explain that????
 

ES2nl

Member
Messages
19
Steppin' Wolfe, yes I agree. The knowledge put into the toneclones is worth paying for, and I would probably have ordered those already if I was living in the U.S., but with all the taxes, fees, shipping costs and more taxes the price is almost double by the time it gets here.

Maybe I should have been a bit clearer :D. I like to build it as close to the vintage tone (not vintage correct per se) as I can, but also preferably within budget. That's why asked for specs, so I could then find the best match or local custom winder myself, and enjoy a reasonable
approximation (which I can always replace by a MM in the future).

Maybe it's easier if I give some OTs I was considering:
  • Classictone musicmaster bass 40-18093 (9k @ 12watt)
  • Classictone princeton (8.5k @ 15 watt
  • Fender DRRI (6.6k @ 25watt)
  • Mojotone/TAD 5E3 (8k @ 20watt)
  • Hammond 1750E (8k @ 15watt)
  • Hammond 1609 hifi (10k @ 10watt).
Love to hear some opinions!
 

ES2nl

Member
Messages
19
For anyone else looking to build or repair a GA40:
The first schematic I posted I'm now convinced is indeed from the second edition GA40 (1955-1957) with the carpet print tolex.
It differs only for a couple of resistors in the input and preamp section, but as you can hear in this extensive doug and pat show it makes a difference (together with another speaker and I think a different ot).
 

Mattbedrock

Gold Supporting Member
Messages
4,807
If I was doing it here in the states, I'd go with Classictone, likely a Princeton or Deluxe OT. They are a great value and I've always had good results. Not sure what they'd cost over in Europe.
 

ES2nl

Member
Messages
19
If I was doing it here in the states, I'd go with Classictone, likely a Princeton or Deluxe OT. They are a great value and I've always had good results. Not sure what they'd cost over in Europe.
Thanks, good to know those are decent despite their low prices. Importing a $35 ot would cost about another $70 on shipping and fees and so on, but together that is not too much more than the transformers from TAD or other quality european transformer brands.

I also have an option on a unused marvel transformer (princeton type 12 watt, 8k primary) made in 2004. Anyone know if these comparable to the classictone transformers in quality?
 

Leonc

Wild Gear Herder
Platinum Supporting Member
Messages
18,372
Marvel = Magnetic Components = Classictone ;)
 

Leonc

Wild Gear Herder
Platinum Supporting Member
Messages
18,372
FWIW, they make very good transformers. They've been around forever; they are one of the suppliers that Valco used back in the 50s and early 60s. You'll find many Valcos/Supros/Nationals, etc. with EIA 166 transformers...
 

stillyer

Member
Messages
1
Hi all. Long time since I've been on this site and my first post.
I'd like to comment on this Gibson GA40 transformers thread.
This might have been stated before but......
I own a 1959 Gibson GA50 tweed amp that I have just restored electronically. I've had this amp for quite a few years, but it was in rough shape electronically. Sounded very weak.
I have just finished a complete restoration of the electronics and the amp is an absolute monster now! A fantastic sounding amp. It was quite the chore to restore, due to the double sided eyelet board and general layout, but well worth the effort.
The EIA (Electronic Industries Association) code 166 is stamped on all 3 transformers (power, output, choke), along with the date they were made (1959) and the amp model GA40 as well stamped on the transformers.
The 166 EIA code is Magnetic Components Co, a division of Marvel Electric and the maker of today's Classic Tone Transformers. I have seen many vintage Gibson's with the 166 EIA mfr code stamped on the transformers. I'm convinced Magnetic Components made these great sounding transformers for Gibson amps.
If anyone is interested, I can measure the primary impedance of the output, if you need.
 






Trending Topics

Top Bottom