L'il Tiger Audio Amp Kit Docs
I purchased a SWTPC L'il Tiger audio amp kit in the early 1970s. I never got the amp to work correctly. Of course at the time the only test gear I had was a VOM. I suspect oscillation and/or a power supply problem caused a blow-up. I eventually parted out the L'il Tiger. Later I got an old Eico tube amp with a gold tinted case. Working on the Fox Point rubbish crew had a few perks. The amp worked but had some hum. I paired it with a Knight stereo tuner that I got from one of my Elmers, Mr. Dobbratz:
https://wb9kzy.blogspot.com/2022/08/i-found-out-that-one-of-my-elmers-died.html
Eventually since I never really liked tube equipment I gave it away. Now it's probably worth a fortune :)
https://www.stereophile.com/content/eico-hf-81-integrated-amplifier
Anyway, back to the L'il Tiger, after discovering the mighty-mighty Worldradiohistory site I looked up the L'il Tiger in Popular Electronics. It was the cover story for December 1967:
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| The unobtanium for this amp are the heat sensing diodes in the metal can on top of the heatsinks |
see page 27: https://www.worldradiohistory.com/Archive-Poptronics/60s/68/Poptronics-1968-12.pdf
There is a better quality scan of just the L'il Tiger article on this page:
https://frankosite2020.com/SWTPC-ARK/index.html
I noticed the relatively low quiescent current of 5 to 10 mA. This is close to the LM386 spec of 4 mA. I was thinking that maybe the L'il Tiger circuit (if operated at 13.8 volts) might be a candidate for an "LM386 killer" audio amp kit. I tried prototyping a circuit with some subs and changes but was never satisfied with the performance. I thought I was being clever by substituting a pair of Darlington transistors for the output/driver transistors - also since there aren't any sources for the heat sense diodes spec'd I subbed another pair of Darlington transistors wired as pairs of diodes to work as the heat sensors for the bias circuit. The diode connected Darlingtons were bolted to the output Darlingtons to perform the heat sensing bias. Since it would be run at a much lower voltage (maybe 1 or 2 watts output maximum with an 8 ohm load) the thought was that no heat sink would be needed. Another problem was noted, the amp will never have the gain possible with an LM386 (voltage gain of 20 (or 26 dB) to a maximum voltage gain of 200 (or 46 dB). A preamp transistor would be required and this would undoubtedly bust the 4 mA quiescent current hoped for with an "LM386 killer" circuit. :(
Recently I came across the docs for the kit I bought:
Why bother posting these when we have access to the PE article ? Because SWTPC changed the circuit by adding a couple of transistors as well as emitter feedback resistors and a Zobel network. If I ever get the gumption I may try building this updated L'il Tiger - it might still make it as a kit !
BTW, the L'il Tiger sold for $11.10 in 1969 per the catalog on the same neat SWTPC page:
https://frankosite2020.com/SWTPC-ARK/index.html
Here is the semi-obligatory inflated price:
You can see why a kid without much extra money would only buy one kit :)
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm
