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| The Schedule 40 GA-5 amp. |
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Click on the images below to see a larger version. |
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I was walking through Lowes home center one day and I spied
this cool gray Schedule 40 PVC electrical box for $10. I have seen amps built into lunch pails and mailboxes so I
decided to give this box a go. It's built very solid, the walls are approx. 3/16th inch thick. The bottom plate has
screws on all four corners and I have mounted 4-rubber feet to the bottom plate.
The front controls are left to right. Input jack, Volume control and LED pilot light. The bottom switch is a
bright switch that switches a 250P cap across the volume control. The 250P cap sounds great with single coils but was kind
of buzzy with humbuckers so I made it switchable.
Hopefully heat will not be a problem. Schedule 40 PVC is very tough stuff and I have tested the amp for hours at a time with
no heat problems so far. I may have to vent the insides since there are a few heat producing resistors inside. I'll vent
the case if it becomes necessary. |
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| The rear panel from left to right: Upper row - Speaker jack,
On-Off switch. Lower row - Fuse holder and power cord. |
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| The top view: Upper left is a 5V4 rectifier tube. Upper
right is a 125P1B power transformer as used in Champ's and Princeton's. Lower left is a 12AX7EH. Lower right is a EL84
power tube. The four tabs that you can see on the left and right are for mounting this box to a wall. I think I am going
to bend some aluminum grating and make a cage that fits over the top and screws down to those four tabs. |
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Inside View: This amp is variation of the Gibson GA-5 circuit. There are
so few parts to a Champ/Gibson GA-5 type amp that I decided to just use two terminal strip assemblies to mount all the
parts. I opted to use a 125C3A fender choke (upper right) instead of a power resistor. The output tranny is a 022905
Fender champ output (Upper left).
Note that since this is a PVC case, all components are isolated and not grounded. You have to ground everything that
needs to be grounded with a wire. In the photo above I have added a black ground wire/line on the pre-amp tube in the
lower right corner. This is so that the shield on the socket is actually grounded and acts like a shield. Use a #4 ring
terminal to bolt the wire down to the tube socket mounting nut. Solder the other end to a ground point. |
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Comments: The Gibson GA-5 circuit is a great variation of a Champ type
single ended amp. Gibson GA-5's always sounded better to me than Fender champs. I have made several mods to the amp. I
am using a EL84 power tube instead of a 6V6 and I replaced a resistor with a choke. There are several other small parts
changes here and there that can be seen on the schematic below.
This amp sounds very good with single coil pickups and it growls very hard when driven with humbuckers. It is an
awesome little recording amp. |
