Monday, 19 September 2016

Listening tests

Having got the initial stages of the amplifier working to my satisfaction on the test bench, I felt rather impatient to actually put something through it other than test signals to look at on the oscilloscope. With the output from the 12AU7 gain stage being able to reach around 110v peak-to-peak before clipping sets in, I got to thinking if there was some way I could reduce this signal down somehow to a line-level amplitude, such that I could feed it into an Aux-in somewhere.

And so... with an appearance befitting the haste with which it was constructed, I introduce the leading contender for the title of world's ugliest potential divider:


That's a 1M resistor reducing the signal and a 33K load resistor that I'm taking the signal off.

I did say it was ugly. But it served the purpose perfectly and I was able to put a signal through it and listen through a portable speaker with an Aux-in.

What music did I choose for this auspicious occasion? That honour went to Bob Seger and the Silver Bullet band: "Still the same"

It sounded clean and clear and gave me great satisfaction to hear some signal coming through this unholy contraption.

The rest of the session was spent tuning up the bias to get the 12AU7 operating within its most linear range, as tested by increasing the drive and watching the output go into clipping on the oscilloscope. Once the top and bottom of the output sinewave start clipping at the same point, I'm happy.

In other news: All the way from Canada, my output transformers arrived today. They look a lot less ugly than my power transformer



Now "all" I need is:
  • Chassis
  • KT88 output valves
  • Some better preamp valves than the $10-a-pop Chinese ones I'm using for testing
  • Remote control motor-drive volume control and input switcher

My next step is going to be assembling a circuit board containing the delayed turn-on circuit for the HT. The intention is to wait until the valves are hot and the bias supply is stable before switching on the B+ power – this will protect the output valves (expensive) and the supply capacitors in the gain stages which are rated at 400V but at initial turn-on are shooting up to full B+ potential (currently 570V) before the valves warm up and start drawing current. 

This will be a small circuit board the size of a credit card containing a 5V regulator, 555 IC and a couple of capacitors and resistors to set the IC's parameters at a 20sec delay, plus a couple of SPST relays (one for each arm of the HT) as we'll be switching the AC going into the diodes.

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