Gibson Country Western Hum! Update: Fixed/

[plexi50]

I have a small amount of hum left after doing a complete re-wiring of this amp. I could not believe what i saw while in there. The GA70 schematic follows almost nothing in the way this amp was wired.
I still have a 8" yellow wire coming from the PT and unsure where it goes. It was grounded when it came in. In another pic it is not shown at all. It has 160A/C on it. The power amp section hum is completely gone.
The grids of the power tubes were connected to the standby switch with 2 separate wires. I'm at a complete loss/
Now i have Preamp hum. I have re-soldered all the tube pins. I have checked all cathode voltages and they are in the fender ball park. 1-1.4VDC. The hum does increase with the instruments channel's volume pot rotation.
The CT IS ON PIN (1) of the second power tube and grounded through the cap can power supply cap. I can shunt the volume pot and kill the hum 98%. I have been on this for 4 days now and had to step away from it last night. I'm back on it today and have to get away again for a while. I re-soldered the entire board and have replace coupling caps and mica 100 pf  snubber cap.I have DC resistance of 56K on pin 7 of the PI where the 56K resistor is. 

[plexi50]

This is how the PT which is right / and the Standby switch was wired with wires going to the grid of each power tube. 

[sluckey]

Shorting the grids of the output tubes together is a popular method to mute the amp. Silvertone 1484 did it. Others too. The out of phase signals on the grids cancel each other when you turn/flip the switch to the standby position.

[shooter]

i have Preamp hum.

fwiw, the last hum I tracked down in a GA5, went away with a new V1 socket, the 3rd or 4th tube I rolled in and a metal plate over the bottom.  Not sure which if any fixed it, it was just fixed

[plexi50]

Shorting the grids of the output tubes together is a popular method to mute the amp. Silvertone 1484 did it. Others too. The out of phase signals on the grids cancel each other when you turn/flip the switch to the standby position.

I'll be damed! I learned something today. So the standby switch in this amp is not your conventional standby switch where you are applying B+ to the PS.

[plexi50]

i have Preamp hum.

fwiw, the last hum I tracked down in a GA5, went away with a new V1 socket, the 3rd or 4th tube I rolled in and a metal plate over the bottom.  Not sure which if any fixed it, it was just fixed

These amps have a lot of small problems that appear over decades. I had a lot of cold solder joints.

[shooter]

I've done 3 GA5's n 2 Kalamazoo's, I grew up about 10min bike ride from the Gibson plant, we always had a great Pipe-line being 1/2 way tween Det & Chicago, pretty sure we're seeing what got built after lunch 

[plexi50]

I had to pull the board to look underneath, I found a corroded ground wire to the 1 meg input jacks that fell as off soon as i pulled the board. I removed the input jacks to get the board out. Cleaned and soldered the input jack ground wire and presto. A great sounding amp!
The orange drop coupling cap values were 503K. Now .022 preamp & PI .1 Coupling grid. Cant even imagine what it would have sounded like before without the bad preamp ground grid issue.
 

[plexi50]

Removed Parts/

[bmccowan]

Congrats on finding the offending issues. I have worked on a lot of Gibsons, and often think that its a damn good thing I do not do this for a living, because it seems to take me so long to find the issues. But, it is so satisfying once you get there, eh?
Did you figure out what the extra yellow wire from the PT was all about? I do think I have seen that in a Gibson amp, but I have not had a GA-70 in the mix. I imagine they used a PT that was also used in a different application?
And Shooter, I also used to live in Kalamazoo when I was 7-9 years old. I'm in my mid 60s, so it was a while ago. Rocky Calavito; Norm Cash; Tony the Tiger. We used to ride our bikes to the rail yard and ride through in silence as local legend had it that the rail workers cut kids tongues out if they were spoken to. Poor guys, we would ride through and they would be waving and shouting hello and we would stare straight ahead in silence. What idiots we were.

[shooter]

What idiots we were.

I'm 61, so we existed at the same time 
I lived on Lovers lane in Portage, then next to PNHS, but due North was Kzoo, used to hall my golf clubs on bike to the city golf park was like a par 27 putter course.  That's where I learned ppl pay money when you fish ball outta the river.  can't even fathom the toxic waste I waded through for a quarter 
I moved up to husl'n at the Country club, they paid way better!! 

[bmccowan]

Hah - well greetings. I remember a lot from those days. We also used to race the freight trains on our bikes from crossing to crossing. To get the glory, you had to cross in front of the train. Like I said, we were idiots. We were near a small lake or large pond, but do not remember the name. We moved there from Chagrin Falls, Ohio - another place with a cool name. Damn shame what's been happening with Gibson. I feel a certain strange connection every time I work on a Gibson amp, and I have an old J-50 acoustic and an old Kalamazoo built Epi FT-30 that both sound so good!

[Big chief]

A BIG thanks to Plexi50 for finding and fixing my old amps. Also thanks for the el34 world where we can go get
Information from so many good people.  I’m very lucky that plexi50 was recommended to me from this forum as he really is great.

[plexi50]

Thank PRR,Sluckey,Hotblueplates,Shooter and many others. I'm starting to get the Hum of this.

[shooter]

Chagrin Falls

we're following each other!
I was in Solon OH for about a year going to tech schools for Technicare CT & MRI.  loved wandering the Cuyahoga area after class

[Willabe]

I'm starting to get the Hum of this.