Saturday, 15 October 2016

Additional measurements, power supply design

In the last couple of weeks I've spent a lot of time agonizing over how on Earth I am going to make a chassis for this project. I have zero sheet metal skills and very little equipment. I vacillate between buying a ready made chassis from AliExpress and somehow cutting all the holes I need in it, to ambitiously planning a magnificent construction with corner braces and a polished aluminium top plate.

Fair to say this is not resolved yet and will be the main factor holding me back now.

However on a happier note, I've decided that the power supply circuitry will be going onto a PCB. This is necessitated designing a layout, which I have been doing completely the old way - manually. Measuring component footprints and putting the pads on manually.

Those who are more seasoned at such things will observe my technique and doubtless recoil in horror in much the same way you might when you see Grandma transcribing an email because she doesn't know about copy-and-paste. However with nobody to get me started or show me how to do this, I've resorted to the electronic version of essentially manual sketching.

So on this power supply board - which is 150mm X 150mm (and once again seasoned professionals at this type of work could probably fit all this onto a board quarter the size!) will be the rectifiers and main capacitors (4 of) plus the voltage droppers and capacitors for the 4 input valves (one each) plus the output valves bias supply. As well, there is a delay circuit consisting of a 555 timer IC and the necessary discrete electronics, to switch on the HT supply (on the AC side) after approx. 30sec warm-up time for the valves.

This circuit will be fed with a rectified and regulated 5V DC sourced from the heater supply. An LM2940 5V regulator will be used, solid state is allowed in this amplifier, just not in the signal path!

The PCB layout isn't quite finished yet as I still need to add the bias supply. Those of a professional disposition may wish to avert the eyes at this point. This is what the layout looks like so far:

As you may deduce, this is the first PCB I have ever designed

If anyone's got a genuine "Warning - if you build that, then XYZ will happen!" to contribute, please let me know. But if you just want to criticize how crap my design is, kindly please don't. As long as it works, I will be happy, and I am not mass-producing these things. 

I will post photos of this board from both sides once I've stuffed it.

Yes the main caps are in series, this is intended, yes I know that series connecting electrolytic caps is weird. There will be balancing resistors across them... it's just that the caps are 450V rated and my B+ is 560V.

Moving on, I have also sourced a signal generator that goes to 65 kHz, the previous one I was using had a max frequency of 20kHz. So I have made some more measurements of the test rig on the bench - this is the input, splitter and driver stage. The -3dB point measures at around 50kHz. With the cheerful assistance of my daughter as a lab assistant entering numbers into the spreadsheet that I called out as I made measurements, this is the response I am getting:

Which I am also reasonably happy with.

In other news
 - the KT88 output valves have arrived
 - I am still waiting for the remote volume/input switcher from AliExpress
 - and I still haven't figured out what to do about the chassis!

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