sound practices reading club /36

We touched upon hard-wiring tube circuits last time, but now we re going to do it, properly. We are reading Hardwire Technique for Tube Circuits by Ron Sawyer, from sound practices issue 11. In it, Ron first shows an example of hardwire hell and contrasts it to the superior Tektronix style. Then he shows step by step how to achieve such a fine construction. A bonus line amp schematic is part of the example.

teaser quote: ‘Hardwire circuit assembly techniques offer several advantages for the Do-It-Yourselfer who is constructing a one or two of a kind vacuum tube audio project. […] hardwire assembly can result in superior ventilation of components and also excellent serviceability, including the ability to make radical modifications.’

a gift from Joe Roberts, SP editor: from the sound practices CD, here is the original article.

practical note: in print and on the CD (and the pdf), Photo 6 is wrong (it is Photo 2, repeated). This was only ever corrected in printed issue 12. The real Photo 6 is at the top of this blog post.

my take
This is a cool article, not only because it shows how to apply superior old construction methods to your projects, but also because it shows how to do this with modern components. So no lusting/hunting for vintage ceramic terminal strips required. If you can only scrounge from modern electro-junk or order from mainstream catalogs, then you can still do this. But the article is not limited to modern materials. If you can find old-style fibre terminal strips, or even the ceramic ones, then it is clear how to use them to best effect. I also like the stress on modifications and that removing parts is just as important as solid construction.

What I have also taken from this article is the rule for derating resistors, by a factor of four. I do this a little more practical than Ron presents it. If the (derated by 4) calculation says 2.2 Watt, then a 2W resistor will do, especially if the next available choice is 5W or more. The derating is still generous enough.

Watch out for equating neat layouts with better working circuits. For example, the closely bundled, parallel running wiring shown at the bottom of Photo 2 could spell trouble. Parallel wires couple magnetically. If a sensitive input line is bundled with a high current output line or AC heater wiring, then instability or hum can be the result. Run line pairs (it is always a pair, through which runs the current loop) individually and cross at right angles with other pairs.

Wiring that is bent in sharp 90 degree corners may look very neat, serviceable and satisfying, but is bad news. It stresses the conductor, corners are radiators of interference and it could maximise the area of a conductor loop—if the pairs of wires are not close together. The latter two arguments also matter for printed circuit boards.

bonus tracks
See another superior construction in SP issue 9, page 25 (Mary Anne, Mary Anne).

Now go and read the article, see you next week.