teaser quote: ‘[…] unless the builder can identify the choke by manufacturer and part number, his only recourse is to go to a catalogue and buy a choke that will meet his requirements. However, by setting up a relatively simple circuit […] those “junk-box” chokes can be readily identified as to their electrical characteristics and, hence, can be salvaged.’
a gift from Joe Roberts, SP editor: from the sound practices CD, here is the original article.
my take
I have said it before: getting the iron (transformers, chokes) together for building tube designs is one of the biggest hurdles. That’s why I like the attitude of this article, which is: fully check out the stuff you can scrounge, in order to put it to proper use. I also like that Sherman goes a lot further than just getting the basic numbers (inductance, current, DCR); getting that graph gives real insight into how a piece of iron ticks. This is also a useful exercise on any store-bought piece of iron.
A couple of notes:
- .32 Amps; that rheostat has a 100 Watt rating.
- It says at the top of the second page that ‘f in a full-wave rectifier circuit is 120.’ In about 78% of countries (quick calculation using info from this page) the AC is 50Hz and f=100.
- Quite a bit of talk is about a swinging choke. Not something that you hear that often in the context of the class A designs we build. It is more for designs with vastly changing current draw (class AB/B/C). I would say that predictable inductance under varying steady state conditions (biasing, design changes) is more important to us; that means the left halve of the curve in the article.
Now go and read the article, see you next week.
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