Gibson GA8 vs GA9

The 6SJ7 would probably sound pretty cool methinks. Converting the output stage in the GA-9 to a conventional permanent magnet speaker is pretty easy. Given you're looking at B+ = 340V, then the optimum load resistance for one 6V6 would be Zout = 340/(12/340) = 9,633R. So for 6V6||6V6 at 340V, that's 9,633/2 = 4,816R (~5k) OT primary. Bias them to 100% (Start with a shared 250R cathode resistor, and see where it lands, might need to go to 220R or 270R, depending on where your power supply voltage lands, but 340V is an ideal plate voltage for SE 6V6 in Class A. Use a 20-25W OT for better power handling.

 

2 different beasts...each unique. My personal favorite is the tweed GA-8/Discoverer amp made in the late 50's and early 60's. I have heard it described as a dual champ since it is a single ended, parallel output twin 6v6 amp. It sounds glorious at low, mid and high volume. The design is deceptively simple.

 

http://www.rru.com/~meo/Guitar/Amps/Gibson/GA8T/

 

Can't speak for the GA9 but my 59 GA8 has the most sweetly righteous tone at the break up point of any amp I've had.

It's my main gigging amp for pubs and clubs and at 8watts is perfect for hitting the sweet spot at the right Db levels.

 

That is definitely the schematic of my Gibsonette. I've only been able to find a notated version online, so thanks for posting a clean one.

I recently blueprinted mine with the intent of building a copy with a few mods. It's a remarkably simple circuit, very similar to a 5F2 with an extra 6V6 and no negative feedback. And yes, it sounds glorious with the volume past noon.

I'm curious if you would wire it on a board or true point to point like the original.

 

It's a little weak in the bass so I'd go for a 12" speaker and bump up the first two filter caps to 22uf. Despite the schematic, it had a 20+20 firecracker for those when I brought it home so I don't think another 10% is a stretch. Maybe an NFB loop & switch to clean things up every now and then. Then an extension speaker jack, and that should do it.

 

This thread points out the need to discriminate between different amps that happen to carry the same model number. The GA-8 is a different amp from the GA8T just as these two amps are different from that GA9 shown above. The GA8T has a push/pull output with a cathodyne phase inverter.

 

I have used an Allen amps output transformer as a replacement for a tweed GA-8. It is the TO-11C output transformer. The two 6v6's are running in parallel, single-ended.

 

Mongo, to keep things straight....you are thinking about building that GA9,right? Interestingly, I have one of these in for work right now. It will end interesting to see if it adheres to the schematic, or if it veers off the track as do many Gibson amps.
For my money, I would opt for a different amp...but that is your call. The GA-8 schematic shown in your post is closer to being a bigger 5F1si play do to the fact that the preamp and driver is more akin to the Champ...more modern than the grid leak biased GA-9. Frankly, model names don’t keep things straight for me with Gibson amps, either. If you are thinking about building the GA-9 with the octal preamp/driver, then tubeswell has you on track, I would think. In fact, if you are going for the first GA8 schematic there, his advice holds there as well.
That GA9 is a different amp from anything you have encountered, I am thinking...unless you have played a 5C1 or earlier Champ.

https://schematicheaven.net/fenderamps/champ_5c1_schem.pdf

 

i have a 58 tweed ga9 that uses one 12ax7, two 6v6 and a 5y3, with a 10" jensen. sort of a bright amp that plays better with a gibson humbucker guitar than a fender single coil. i usually combine it with something like the ga8t i've shown below.

https://muscmp.wordpress.com/2010/11/10/356/

i also have the ga8t discoverer that is a great amp but nothing like the earlier ones. it is a later black tolex version from 65 with a great tremolo.

https://muscmp.wordpress.com/2010/10/27/gibson-ga8t-discoverer-tremolo-amplifier-2/

play music!

 

Mongo, I have my schematic noted with the pin locations of all the P2P connections. If that would be helpful to you let me know and I can upload it.

 

Cool. Promise us you’ll post a build thread!!!

 

Here you go. Keep in mind there are no turret boards or strips in this amp, it's all done on the tube sockets. I annotated this with a mind to build one the same way.