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.

1J29b tubes

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.

Two main stages

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.