We have some important news to share about HackSpace: Issue 81 was the last issue of HackSpace as a standalone magazine, and HackSpace has become part of The MagPi, the official Raspberry Pi magazine. Starting with issue 145, We’ll be adding pages to The MagPi to make room for the stories and tutorials you’ve come to expect from HackSpace.
By Thomas Hafemann. Posted
I started playing guitar in my teens, and saw DIY as an opportunity to build all the pedals that I could not afford at the time. I have built several mini amplifiers using sub-miniature tubes, but mostly they are still too loud to play in an apartment, and still require a lot of power just to heat up and power up the switched-mode power supply (SMPS) which multiplies the voltage, from 12 V to 200 V. I always wanted to build a battery-powered tube amplifier for the guitar, like some old radios, but wanted to use the current available LiPo batteries; much easier to recharge than the old 45 V batteries.
So, first I tested if the SMPS that I was currently using could work at lower input voltages. 3.7 V in this case was not enough, so I used two batteries in series, and that worked up to 150 V. The tubes I found while looking for battery tubes. There is an article about the 1J24b, stating that, with a much lower filament, it could deliver the same amplification as the DF96, making this tube the best gain/consumption tube available. For the power stage, I chose the 1J29b, from the same family of Russian tubes with lower filament currents.
The amplifier is composed of two main stages: a paraphase phase inverter, and a push-pull output stage. I used a 100 V line transformer as the output transformer, to keep the cost low, and to achieve the high primary impedance required by the output tubes. A proper transformer would be at least two times bigger, and would cost at least four times more than this one.
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