mad dog said:
... The GA40 has a dark, hot sound, swirly and alive. West coast hot, jazzy too. Not much clean ... The amp itself is compression rich, so I can do without early alnico breakup. The clean you do get is so rich, dense, and the overdriven tone is not easy to describe. VG called it "chocolate thunder" ... funny but somehow appropriate.
Based on Mad Dog's moving description of the tone of a GA40 and the relationship between the GA40 and GA20T Ranger circuits, I bought one
(threaded here) , and then another, GA20T Ranger. I couldn't be happier with the first one but wanted one based more closely on the GA40's circuit ... and got one by dumpster diving on eBay. Some compare/contrast on the two circuits.
The preamp of the GA40 Les Paul is on the left, the GA20 on the right and, as a practical matter, the only difference is that the GA40 has a 5879 (green circle) in Channel 1 where the GA20 has a 12AY7 or a 6EU7 (probably later models; my other GA20T has a 6EU7 and a higher serial #). With minor exceptions, both amps have the same wiring for Channel 2 including an array of resistors and capacitors (red boxes) that cause a 'fluttery' or sort of foamy quality; similar to tremolo but without speed or depth controls and not as obvious. Both amps use a 6SQ7 as the tremolo oscillator (blue boxes) to make the volume go up and down and is controlled with speed/depth pots and a footswitch. Although not identified on the schematic, some/many/all? GA40s were fitted with 15" Jensens; the GA20Ts have 12" P12Rs.
The tubes used in these two amps are; the 12A_7 and functionally-equivalent 6EU7, the 6SQ7 - an odd-ball combination of a simple triode section on the left (1/2 of a 12A_7) and a dual diode in the other half (neither amp uses the dual-diode 1/2, its pins are left unconnected), and the 5879 on the lower left which has more in common with other output tubes like the EL84 on the right than it has with conventional preamp tubes like the 12A_7s.
So ............ ? It's the 5879s in both channels that sets the GA40 apart from its Gibson relations including the GA20T and all of ampdom back in the day and now. Since it's a single-section or single-stage tube, engineering departments might have frowned on its use; after all, twin-section 6EU7s/12AX7s did twice as much work in the same footprint/tube socket; why waste the space?
Why waste the space? Listen here:
Nocaster bridge pup through the Gibson GA40 amp (Channel 2: 5879)
http://www.ce.chalmers.se/~janjo/Vintag ... 40_Ch2.mp3
Nocaster bridge pup through the Gibson GA40 amp (Channel 1: 12AX7)
*
http://www.ce.chalmers.se/~janjo/Vintag ... 40_Ch1.mp3
* This is a typo or a mistake; the GA40 has a 5879 in Channel 1, see above.
Vintage applications:
Kay model 720 amplifier
http://www.regiscoyne.com/kay720/
Vintage Ampex mic and preamps using the 5879
http://fixthatmix.com/ampex601ad_stuff/ampex601_ad.html
Current 5879 applications:
Snake River Amplifiers
http://www.treblebooster.com/snake_rive ... _works.htm
Divided by 13 Amplifiers; Model FTR 37
http://www.dividedby13.com/ftr37.html
Youtube:
http://www.youtube.com/watch?v=YFlmMNnw ... 1&index=14
Abraxa Amplifiers; high end stereo
http://home.att.net/~abraxasaudio/proje ... t-pp1.html
Winfield Cyclone w/ EF86, similar to a 5879 in the preamp
http://www.winfieldamps.com/
http://www.radiomuseum.org/tubes/tube_5879.html
.... and the topic of this thread, the Victoria Electro King GA40;
http://www.svvintageamps.com/catalog/pr ... 5ec5f7d238
Gibson experimented with using a power tube as a gain stage and Victoria Amps, Divided by 13, and others have decided that's okay with them; it's ok with Mad Dog ... and it's ok with me too; a remarkable tube probably stepped over because of manufacturing economics ... and not because it lacked tone or didn't work well.