12K5 Flea-Power Amp
On one of my business trips to USA I acquired a couple of 12K5 space charge power tetrodes as I was buying anything that is remotely interesting to me and cannot be found back home. I like the idea of very small power amp so these looked interesting with their minuscule 35mW output. Some time after that I saw a cute little output transformer in Radio Shack with 1K:8ohm ratio and max power handling of 50mW that could be used with 12K5.
When it comes to space charge tubes, one of the best sites to get the info is http://www.sophtamps.ca. Power amp is based on 12V all-tube amp found there. I didn’t have any 12U7 tubes, so I decided to make it a hybrid amp and do a JFET preamp. For preamp I used AMZ Mini Booster with few components changed.
Since it’s a tiny power amp it’s appropriate to squeeze it into a proportionally tiny box. I got a nice aluminum sheet box in a local store together with anodized blue cover. Preamp and power supply are fitted on separate boards. I used Measure Explorer 1″ square proto boards I got on eBay during one of one those trips to USA. These are the best quality perfboards I’ve ever tried. Tru-hole plating and each hole has two smaller holes next to it that share the same plating which makes connections simpler to do. Really nice for small circuits like this.
You will notice a couple of nice Dale resistors on the power supply board. Those are four 100ohm resistors wired in parallel to give (close enough to) 22ohm cathode resistor which I didn’t have at the moment. Amp is powered using 12VAC@0.83A wallwart which gives slightly over 13VAC under appox. 0.5A load so I added a 1.7ohm 1W resistor in series with the heaters. B+ is around 16-17V when rectified.
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You must be wondering how loud (or quiet) is 35mW. I can’t say it’ll make your ears bleed, but it’s comfortable level for room and especially for night practice. Using my 12″ Greenback, volume is a tad louder than typical conversation between few people without anyone getting really angry 🙂 . Also, it’s has some bluesy overdrive vibe to it, not very clean when volume is over half, but it never gets harsh. I was a bit concerned about it’s 300Hz-10kHz frequency response but it’s actually fine. It doesn’t seem to lack bass.