Saturday, 5 October 2013

So what have we here? Yep. It's wired up and ready for testing!

Today's "ah bollocks" moment came when I tried to dry assemble the chassis to the box with all the parts in place. The input socket snags on the dropper resistors that I had to relocate! There are a couple of options. I can move the resistors to a completely separate board mounted in the bottom of the box, or I can move the input socket and fit an on/off switch in the hole it leaves. Or I can put the whole thing in a bigger box...

Anyhoo. First job was to make sure everything was electrically safe. I laid the amp out in a line on the bench. The output transformer is floating free at the moment. I didn't fancy sticking it in the coffee tin until I knew it was all working.

First power up is always done without the valves in place. And the first check I did was to make sure I'd got 6.3VAC across pins 4 and 5 of each of the valve bases. The next check was to make sure I'd got HT in all the places it should be...

Yep. That's 378 volts of DC. More than enough to seriously ruin your day if you touch it! That's why I keep harping on a bit about safety.

Once the valves are in place, that will drop to the 320 volts or so that I'm expecting to see on the output transformer. It will be a little lower on the pre-amp stages.

Power off. Then wait! Those decoupling caps in the power supply take a while to discharge! There are ways of discharging them more quickly, but I was prepared to let it take its time. I could fit some discharge resistors across the caps to let them come down more quickly, but space is a bit tight already. What I have (somewhere) in my toolbox is a lead with a couple of crocodile clips and a 20K 5 watt resistor soldered into it. This lets the capacitors bleed down gently but quicker than leaving the amp standing.

During the power-on test, nothing caught fire or even got hot, so it was time to drop the valves in place.

Power back on, with the volumes turned all the way down! And it lives! The valve heaters glow nicely - and there's an awful hum from the speaker! But I've got sound. The next thing is to connect a guitar up and see whether it will actually amplify an input signal. And it does!

The hum is down to a couple or three things. I need to add a couple of resistors to create an artificial centre-tap and reference the heater circuit to ground. I need to connect the chassis ground to the signal ground. And I need to shield that output transformer inside its coffee tin.

But I did manage to record a fragment of video on my camera. I think the CBA sounds quite growly and well suited to a bluesy CBG...

CBA clip

Next jobs (but not tonight) are to make those additions and alterations I mentioned, then start on the speaker cab. Tonight is eating chocolate and watching a movie night.

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