Monday, 30 September 2013

Not much work done on the amp today. Being back in the office and still feeling under the weather has put a bit of a crimp in the proceedings.

However, I sat down with a pencil and paper and roughed out the circuit diagrams for the various stages: power supply, pre-amp and power amp.

Don't worry. I will tidy these up and transfer them to a drawing package. I'll also make them available as PDFs for downloading.

So here's the pre-amp. It uses a Russian 6N2P dual triode. There's no magic, just a fairly conventional pair of gain stages. The grid stopper values have been set at 68k based on what worked well in the original T'Watt. Also the coupling cap values have been chosen based on the ones used in the T'Watt. However, when it comes to voicing the amp for use with a cigar box guitar, these are the components that will get tweaked first.

The pic below is a rough draft of how I see this layout working on the tag board.

It's not the clearest picture in the world, but as I say, I'll turn these drawings into graphics then PDF them.

I find it VERY useful to sketch out the actual layout of components before I heat up the soldering iron. I started doing this because I found myself having to undo and move stuff quite a lot in the early days. Doing a layout drawing also helps me see where I might have trouble if I solder components in place in the "wrong" order. It's remarkably easy to leave yourself no access to a joint that needs soldering because you've stuck a whacking big capacitor in the way!

Here's the power supply.

I've shown the PSU (power supply unit) decoupling caps in the diagram here. However, these caps actually appear in the layout drawing for the pre-amp boards. The reason is to keep noise down. It's better construction practice to have the cap physically close to the node it's supplying. I'll go into the details and the "why" in a later instalment.

And here's the power amp. It's a Russian 6N1P set up as a self-split push-pull. And don't worry if that means nothing. As with the PSU, I'll be explaining a bit more about the theory in a later instalment.

The power stage only has a handful of components, and they can all be soldered directly to the valve base, rather than having to be mounted on the tag board and connected by wires. The fewer the number of connections, the better.

I'm afraid that's it for today. This cold is wearing me down.

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