Sunday 13 March 2016

ECL86 Amplifier Test

Every once in a while, life in general and my shack in particular gets into such a mess that I can hardly breathe. At times like these my long-suffering XYL comes to my rescue and helps me to tidy things up. We had a bit of a tidy today and I found some goodies - including the old ECL86 AF amp prototype and a dummy load for AF amplifier testing.

After the tidy up was completed, I treated myself to a little gentle play at the bench, by hooking up this little tube amp to the 'new' HT supply. First (after fixing all the broken wires) I just listened to it, then I connected it to the dummy load...

Just as with RF experiments, cheapskate audio engineers find it difficult to get hold of perfect dummy loads. In the case of AF, there isn't a problem with inductance (as the frequencies are so low that wire-wound power resistors can be used for amplifier testing to first order of approximation, at least). However, exactly as we can't find 50 Ohm resistors straight out of the E series for RF, there similarly aren't 8 or 16 Ohm resistors to emulate ordinary nominal speaker loads.

Fortunately, 10 Ohms is close enough (and we don't get standing wave patterns on speaker cables, HI HI), so that's what I've always used to model an 8 Ohm speaker...

Note that as well as the ordinary 4mm binding posts, my 10 Ohm dummy load has a BNC socket, which allows me to read the voltage across the load. This allows me to simply measure - for example - the voltage gain of an amplifier, which I took the opportunity of doing for my little toy triode/pentode amp...

It offers about 32dB of gain over a pretty narrow bandwidth, which is fine for the voice applications for which it was conceived.

Now I have to tidy away this little amp - which is making the place look a mess and getting in the way of any real work. Then I have to go and say 'thanks' to the XYL.

...-.- de m0xpd

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