1957 GA-8 GIBSONETTE PT WIRING QUESTION

[t7mackie]

Hi all,

I'm rebuilding the circuit on a 1957 GA-8 Gibsonette from scratch because there's a lot of previous work done to it and it needs to be un-screwed.

There's a wire on the power transformer that I don't know what its function is. The PT is labeled, "Gibson KEA 166" and it should be original to the amp. The, "bottom" side of the power transformer has the 2 yellow rectifier tube wires and the 2 green heater wires (no CT). The "top" side has the 2 black mains wires, 2 red HT wires with a red/yellow CT as well as a mystery yellow wire. It does not have any continuity with any of the other wires nor to the body of the PT. I was hoping it might be the CT for the heaters but that doesn't appear to be the case. It was connected to a lug on the rectifier tube socket (on the outside, not to any of the tube pin lugs).

It looks like there were a couple other grounds connected to this same lug but since the wiring is not original I don't trust any of it. The yellow rectifier wires (as well as the 6.3's) have a plastic outer heat shrink on them and the mystery wire, as well as all other wires, are just plain yellow cloth.

What's this mystery wire all about?

Thanks!!

[brewdude]

I wonder if it is a “shield” to reduce noise?

[martin manning]

...as well as a mystery yellow wire. It does not have any continuity with any of the other wires nor to the body of the PT. I was hoping it might be the CT for the heaters but that doesn't appear to be the case. It was connected to a lug on the rectifier tube socket (on the outside, not to any of the tube pin lugs)

An electrostatic shield would be my conclusion. Sounds like it was grounded via the rectifier socket flange.

[Stevem]

It’s odd!
The ga-8t schematic shows a center tap for the filaments, yet the straight up 8 schematic does not show such!

My thoughts are that the 8 is like the fender champ where one leg of the filaments are landed on the chassis for the economy of saving on some wire in exchange for more 60 hz noise.

I would rewire the amp with no leg grounded , use twisted wire to feed the filaments and make a noise reducing artificial ground with the use of 2 100ohm 1 watt resistors Y’ed to ground at the center tap landing point of the HT winding.

I do this to every Champ I work on, or any amp made that cheap way!

[sluckey]

Look at this schematic. The PT has 10 wires. The mystery wire is connected to a shield just as Martin suspected.

https://el34world.com/charts/Schematics ... _8_6v6.pdf

[t7mackie]

Thank you, Stephen. You answered one of my follow up questions - should I make a VCT for the heaters. Is this still necessary with a field coil speaker? Lotta new stuff for me here. I thought this was gonna be an easy Sunday afternoon project.

I have several drawings for the GA-8 though none are an exact match. The closest drawing for the power section is Robrob's, "Gibsonette Early" and on that particular drawing it appears there is a ground to the core maybe?

[Scumback Speakers]

Is this the GA-8 with dual 6V6 tubes, or the GA-8T with the EL84/BQ5 circuit. They use different PT and OT, since the dual 6V6 is single ended Class A, and the GA-8T is push/pull (IIRC).

I have both schematics from Gibson, email me if you need them.

Jim

[Stevem]

Yes, as the field coil is on the DC side of the power supply, not the A/C side.

Gibson was famous back then for almost never doing something the same twice!
If they got a good deal on tube stock of whatever tubes it seems they then had the engineers design a amp around them!

When a got my 54 GA-77 recording amp which was made for only a few short months with the tube compliment it had, it took me near a year of searching to find the right schematic since Gibson made a good few different models of this amp!

There was also a GA-8T that used 6BM8 tubes it seems.

[Phil_S]

I own two old Gibson amps, one from c. 1963 and the other c. 1965. I've worked on both and can attest that the filament supply is not twisted on either one. The wires just lay there on the chassis. If you are not experiencing 60Hz hum, I wouldn't touch them. While I wouldn't be surprised to see only one wire to the socket and the other side to chassis for ground, I wouldn't expect to see that. I've seen pictures of several of the amps from this era.

Regarding Gibson's amp du jour approach to amp making, I will confirm the truth of that. One of my amps is a GA20-RVT (c. 1965.) It came with a schematic pasted to the cab. The amp, surprisingly, conforms to the schematic. However, this particular schematic is not to be found in the usual places on the internet, and that includes the Gibson MSM, a tome of a PDF. (I have the MSM and if anyone wants it, I'll figure out how to post it somewhere.) There are two other schematics that can commonly be found on the internet. Mine makes #3. All this for an amp that was made for only 2 or 3 years. Go figure.

Along with others here, I'd concur the mystery wire is the shell ground. It's simple enough to check for continuity between the wire and the lams, though you may need to press hard on the probe to penetrate to the metal on the outside of the transformer. If there is no obvious appropriate ground point, go ahead and drill a hole in the chassis for a #8 or #10 screw. Put a ring terminal on the end of the wire and ground it on that screw with a couple of star washers. I think there is nothing unholy about good practices prevailing over any notion that such a thing shouldn't be done. And please, don't use a transformer bolt.

Good luck with your GA8!!

[R.G.]

If you're using the original transformer, consider using a 12Vac bucking transformer as described in the article "Vintage Voltage Adapter" at geofex.com. It will reduce the very common 120 to 125V wall socket voltage down closer to the 110Vac the transformer was designed for.