Friday, 16 December 2016

Uh-oh... first teething problem

So after finishing the build and moving the amplifier to the living room to use, things were going very well. One reliability issue came up fairly early... a low-level but consistent crackling, ticking through the speakers. At first I thought this was due to power supply interference, but it persisted after I installed a mains filter.

Then I started gently tapping the output valves. Bingo! Second KT88 on the left channel, slight wiggle in its seat and the problem immediately stopped.

I remember during initial power-on that two of the valves were drawing no current whatsoever, which I rectified by replacing the contacts in the valve seats from some donor ones.

This particular valve seat didn't receive donor parts, so it's going to be worth keeping an eye on it in future. 

For now, though, the problem has gone away. But what goes away by itself can come back by itself too. I can see valve seat contact replacement being necessary in the future.

Second problem was a little more dramatic. After the first run, in which I had the amplifier running for a couple of hours, I was happy with the reliability. The next day I ran it for about six hours, and apart from getting rather warm (to use a pleasant euphemism) there were no other issues and the bias voltages remained stable. 

Then I turned it off and left it to cool for a couple of hours, before lifting it off the cabinet and opening the bottom, to take a look at whether anything was showing any signs of thermal distress. No evidence to be found, so yet again I was happy.

Then I plugged it back in again and powered it up. After the 30sec delay the HT came on, to be accompanied by a loud hum from the right speaker and the mains transformer, and a very unhealthy blue glow from one of the output valves on the right side.


Having a fairly good idea what caused this, I quickly removed it from service and had the bottom off again, and started probing with the meter, long story short my hunch was correct and the valve in question was for some reason not getting its bias voltage any more.

C'est un Catastrophe!

So a bit of probing with the multimeter quickly revealed the problem. Three days ago, when I replaced the resistors in the bias section on the power supply board, I'd made a poor solder joint. The repeated thermal expansion and contraction had stressed it and it had let go.

So I quickly removed the board, took it to the workbench, re-soldered it and tested, then replaced it - the work of five minutes. Re-assembled and tested and working happily once more.

Hopefully the one and only fireworks incident!

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