Thursday, 3 October 2013

27 October draws ever closer, so I figured I really HAD to make some inroads to the electronics side of this build this evening. The trouble with this build is the lack of space and wiggle room. There are some soldery bits of the build that can only happen right at the moment that I screw the chassis down.

With that in mind, I've been thinking about how far I can get before that point.

My usual amp builds follow a pattern that I've fallen into through force of habit. This one has been good for forcing me out of the habit.

Tonight started with simply bolting the valve bases into place.

I found some rather neat stainless steel Allen bolts and locking nuts in my bits drawer, so they got pressed into service to hold the valve bases down.

Because the valve base on the output stage will be sitting quite close to the top of the mains transformer, I splayed the connectors to gain some more clearance. And the reason the output stage will be the one sat over the transformer is because it will have less gain than the pre-amp, therefore it won't be quite as sensitive to hum.

Next on my list of things to do was to get the heater wiring in place. The output transformer has a 6.3 volt AC tap for the heaters. Being AC it's important to try to minimise electrical noise and hum by routing the heater cabling away from the sensitive parts of the amp. It also helps to twist the pair of cables quite tightly together to help cancel the hum field.

The two green wires in this pic are the (quite thick) heater cables from the transformer. Rather than attempt to get them twisted tightly together my plan is to cut them short and run them into a chocblock connector that will be bolted to the back of the box. From there I can run a twisted pair to the chassis. This will also make the final assembly a little easier (I hope). It should just be a matter of making sure I can get in there with a screwdriver.

Rather than mess around, I tied the cables to a convenient hook, then clamped the other ends in a hand drill and wound away for a few seconds.

Heater wiring in place. Hopefully you can see that I've tried to keep the twist all the way up to the valve bases. Again, because the valve base on the right is sat over the mains transformer, I've insulated the connections with a couple of heat shrink sleeves.

Next up are the four diodes that make up the bridge rectifier.

Followed by the first of the smoothing capacitors...

...and the first of the HT filter resistors.

I was going to stop and call it a night at that point. But there are only three components and half-a-dozen solder connections to make up the power stage. So I did that while the soldering iron was still hot.

The grey wire going to pin 2 is the shielded input to the stage that will be taken from the wiper of the master volume pot. The two purple wires, attached to pins 1 and 6 connect the anodes of the valve to the output transformer. The black wire from pin 7 running under the tag board to the capacitor is the ground. The other two components are a 330k resistor from pin 2 to pin 7; and a 470R resistor from pin 7 to pin 8 then its tail connected to pin 3.

Tomorrow should see the pre-amp built and the pots wired in. I may even get to the point of powering the thing up!

Watch this space.

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