Friday, February 27, 2009

Ampeg Curiousites Part 5

Here's the EQ circuit card removed from an Ampeg B25B. If you look in the lower right you'll see a big rectangular component that looks sort of like a big ceramic capacitor:

If you look a bit closer you'll notice that it has seven leads - definitely not a simple capacitor or resistor. So what the heck is it? The answer can be found in the schematic.

Here's the preamp side of the schematic:

Have a look at the EQ part of the circuit - that's what we're looking at in the first picture above. The EQ section is highlighted below:

Zooming in on the lower of these you'll see the resistor / capacitor network that makes up the EQ section. There's a dotted line around the EQ section which I've highlighted in red. You'll notice that unlike R20, which is outside the dotted line, none of the capacitors and resistors within the dotted line have component numbers. That's because all of these components are contained within that little rectangular component in the first photo.

I've also highlighted the numbers that follow the dotted line. The numbers are 1-7 and correspond to the seven leads on the circuit card in the original picture.

These same little seven lead components are found in the tone circuits of a number of Ampegs. When these parts fail, the circuit has to be rebuilt with discrete components - it's an easy enough job if you just follow the schematic.

1 comment:

Anonymous said...

"When these parts fail, the circuit has to be rebuilt with discrete components"

Ok, but what should be the voltage rating of the capacitors ? Do they have to support high voltage ? Also, what type would you recommend: Polyester or polypropylene ? Metallized or Film & Foil ? For the 470pF value would it be acceptable to use a ceramic cap ?

Thanks man !