Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Stevewdewitt on October 13, 2018, 06:29:07 pm
-
I have a Gibson GA-9 on my bench. It is in great shape. I think it is a 1948 model. Could be a 58. I have a few questions about the circuit. The schematic is attached. There are 2 10uf 450Vdc caps at the first node after the rectifier. Can these be replaced with 1 20uf 450Vdc cap? The model I have is a little different than the schematic. It has a 20/10/10 450Vdc cap rather than a 10/10/10 450Vdc. Can the 10uf Caps be replaced with 16uf 450Vdc caps? This is being restored for a gigging musician. It needs to be solid.
This thing is in great shape. The tweed is laquered and beautiful. I will attach some pics.
TIA,
Steve
-
more pics
-
here's Doug's copy;
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga9.pdf
if it still has the 5Y3, the 1st filter cap shouldn't be bigger than 40uF IIRC
with patience you can probably "gut" the old cap, glue in a modern one and still keep the aesthetic factor
-
There are 2 10uf 450Vdc caps at the first node after the rectifier. Can these be replaced with 1 20uf 450Vdc cap?
Can the 10uf Caps be replaced with 16uf 450Vdc caps?
Yes and yes.
-
here's Doug's copy;
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga9.pdf (https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga9.pdf)
The link you posted, ^^^^^^^^^, is a different amp, parallel single ended. His is PP.
-
here's Doug's copy;
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga9.pdf (https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga9.pdf)
The link you posted, ^^^^^^^^^, is a different amp, parallel single ended. His is PP.
The OP's schematic looks || SE to me.
-
I have a 20/20/10/10 @475Vdc that I am going to use. It will fit right in there. I think I should replace the bumblebee caps as well. I will see if they are leaky. I would also replace the 20uf/25V caps as well. One is cathode bypass and the other seems to be the cathode bias cap.
thanks,
Steve
-
here's Doug's copy;
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga9.pdf (https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga9.pdf)
The link you posted, ^^^^^^^^^, is a different amp, parallel single ended. His is PP.
The OP's schematic looks || SE to me.
:BangHead: My mistake. You guys are correct.
-
Most likely a 58 (unless speaker replaced) as the 56 and earlier had the TV screen fronts. Regardless, I don't think they made the 9 in the 40's? Can you see date codes on pots?
Jim
-
It is a 1958
-
here's Doug's copy;
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga9.pdf (https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga9.pdf)
The link you posted, ^^^^^^^^^, is a different amp, parallel single ended. His is PP.
The OP's schematic looks || SE to me.
:BangHead: My mistake. You guys are correct.
Now you know how I feel! :laugh:
-
one of my early builds was a GA-8 PSE 6L6, fortunately Sluckey helped me figure out it wasn't PP :think1: :laugh:
That build spawned my affinity for SE :icon_biggrin:
-
Thanks guys. :icon_biggrin:
When I looked at the 1st drawing, I didn't look at the bottom half of the OT primary. I saw the power tubes and they are shown like PP, stacked, not side by side. :dontknow: