Gibson GA55V schemo

[Willabe]

Does anyone have a copy they can post?

I can't find it anywhere.

Brad

[Phil_S]

Never seen this model number. Tell us about this amp, tube line up, number as size of speakers. Can you post a photo of the front and back?

[martin manning]

GA-55 with Vibrato. Scroll down a bit here for the details on the GA-55: http://www.superiormusic.com/page199.htm

[didit]

And here is the schematic for GA-55 -- http://www.gibson.com/Files/schematics/GA-55.pdf

GA-55/55V were successors to GA-50/50T. It's doubtless similar but with tremolo. Curious to know what flavour it is. One reference suggests the extra tube is 6C5. If so single triode oscillator probably AM after the PI.

Best .. Ian

[Willabe]

GA-55 with Vibrato.

Yep.

I ~think the vibrato (true pitch shift) is a mono version of Gibson's GA83S. The early Gibson stand alone vibrato unit is GAV-1. (These are what Vox copied for their AC15 and AC30, and/or the GA55V.)

I've read/have been told that Gibson had a flood in the basement of 1 of their factories and lost a lot of their schematics.

I own a copy of the Gibson amp book by Wallace Marx Jr. and it's not in there. (The schematic.) I tried emailing him, never heard back.

I'd sure like too see a copy of this circuit.

Brad

[Willabe]

Never seen this model number. Tell us about this amp, tube line up, number as size of speakers. Can you post a photo of the front and back?

I don't own one, so no pictures to post.

From the W. Marx Jr. book on Gibson amps, p. 138;

GA-55V, 1954-1958, 473 total built, 2 channels with 4 channel input jacks, 5 knobs, Ch. 1 volume, Ch. 2 volume, bass, treble, power switch, and a remote vibrato switch/control that plugged directly into the guitars output jack.

2x12AX7, 1x6SC7, 2x6L6, 1x5V4

2x12" Jensen

18w.

But the books tube count is wrong! And you can see in the photo in his book there's 9 tubes.

See below;

Gibson Forums, Jan. 29, 1913;

Also, I'm finding conflicting information with the number of tubes. One resource says it has 9 tubes (which it does), but only states the two power tubes, the rectifier tube, the inverter, and two preamp tubes-which by my count is only 6. There are three more "metal" tubes that are (what I'm going to call) spring-loaded. I have to push them towards their sockets and slightly twist them to remove them. I believe they are 6j5 tubes, I'm just trying to figure out what exactly these little guys do because even the schematic for the ga-55 from Gibson omits them.

So, then I found this, 2x12AY7, 6SC7, 6C5, 6SL7, 6SN7, 2x6L6, 5V4.

And this tube list, 2x12AY7, 6J5, 6SN7, 6SL7, 6SC7, 2x6L6, 5V4.

Could easily be 2 different versions of the same GA-55V.

Gibson is well noted for changing their amp circuits and not always releasing a schematic with those changes.

Brad

[didit]

I did a bit more reading. Vibrato similar to Wurlitzer makes much more sense this morning. There's this thread from TGP about six months ago -- http://www.thegearpage.net/board/showth ... p=18573631. Confirms nine tubes and could easily be that line up. Perhaps these amp owners could be persuaded to post enough good close-up gut shots and notes to allow recreation of the full schematic.

Best .. Ian

[ampdoc1]

I found the correct schematic in my Electric Guitar Amplifier Handbook. fourth edition by Jack Darr, on page 213. The 1st edition was the first amp book I bought, circa 1968.

Unfortunately I don't have a scanner. Perhaps you can find this shown on the internet. If not,..buy the fourth edition. Lots of good info in here including a lot more amp schematics than were in the 1st edition.


a'doc

[Willabe]

Wow! Thanks!

I have that book and didn't think to look in it for the schematic.

I'll try and get a clean scan of it and post it here.


Brad

[Willabe]

Ok, lets see if I can post the scan?

Big THANK YOU to ampdoc1!!!!!!!

Brad

[Willabe]

I've been going over this schemo and there seems to be a couple of flaws in the drawing and with Gibson schemo drawing there might be more.

1, is the B+ string looks like it's missing a 'dot'? (Red circle.)

2, is the wire connecting from the junction of the vibrato channels volume pot wiper and the modulator/mixer hi pass filter output to the junction of the PI's plate and the 'pull' 6L6's coupling cap. I think it's wrong. (Blue circle.)

3, does anyone see anything wrong with the 6SN7 modulator circuit? (That's my only interest in this amp.)


Thanks, Brad

[ampdoc1]

I wouldn't be surprised if there are variations on the schematics. I did repairs for 15 years, and still work on friends' amps. I don't remember if I never saw a Gibson schematic match the the amp, but generally not.

Others have postulated that Gibson seemed to build a lot of amps differently than the schematics, probably due to parts availability (or lack of), and used what was available at any given time.

[Long Distance Call]

This might be some useful information about the original Wurlitzer vibrato. I'm not sure where I got it, but it has a pretty good description of how the vibrato works and a couple of schematics.

I hope this is helpful,

LDC

[Willabe]

Yes, thank you.

Brad

[didit]

I've been going over this schemo and there seems to be a couple of flaws in the drawing and with Gibson schemo drawing there might be more.

I think you're correct on both. B+ for the 6SN7 should tie in where you've circled red and the segment circled in blue should be erased.

Best .. Ian