Wednesday, 7 December 2016

Construction, Part 3

A good amount of progress has been made since the last update:

  • The output transformers have been mounted
  • Output transformers secondaries wired up to speaker terminals
  • The volume / input selector / remote power-on switch has been wired up to the standby transformer
  • The USB power supply has been wired up to the standby transformer
  • The Bias pots have been mounted to the chassis
  • The driver valves (2 x 12AU7) have been wired up and tested


The books I've been reading (mainly by Morgan Jones) advocate a big-bang approach to first switch-on - complete with an unintentionally hilarious guide to the smells you may encounter if something goes wrong:



Contrary to this advice, I've been powering up the circuit in stages, as each is built, I'll add dummy loads to sub out the following unbuilt stages, then apply power and check voltages and signal levels on the oscilloscope.

So far there has been no drama - for which I credit the prep work I did in building the test rig. When I powered up the circuit today to check the 12AU7s, everything just worked... voltages, signal levels - all exactly as per my test rig.

The only real difference, other than there being twice as many valves (two channels... this little thing called STEREO...) is that I'm trying – as best I can – to make the wiring tidier than the haphazard mess that was the test rig.

So - today's photos:

Preamp section is complete, the coupling capacitors can be easily seen

Note component leads bent at right-angles wherever possible, and heatshrink to avoid accidental shorting

Next step will be to build the output stages - wire up the bias supply and the KT88 sockets, then power it up and see what sort of current we're pulling through them. That will be an exciting day because – unless the amp goes into crazy oscillation (always a risk with the Williamson topology) that will be the day I could theoretically put a signal through it and connect it to a pair of speakers.

I've decided I'll get a set of cheap disposable speakers from the auction site for the first actual test, I don't want to risk this thing going unstable and pummelling my beloved KEFs with the best efforts of an angry pair of KT88s driven at 560V plate voltage.

Stay tuned... 

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