AN12: Monitoring AC heater currents in tube amplifier

(c) klausmobile 2001

This simple current monitor helped me in prototyping Mammoth - which had 12V * 12A heaters in output stage. So I had to make sure that all heaters are on. This 4-channel device (only 2 channels shown) uses DIY current transformers - 30mm ferrite pots with 2-section bobbin. One half of bobbin is filled with 0.3mm wire (secondary); primary is 3 turns of heater wire. R1 eventually decreased to 20K; 3A in primary causes 1.5-2.0V across C1. Relay RR1 disables output stage plate power. Power supply is drawn from any available DC source, in my case - tapped to the stock relay power supply ( It doesn't need reference to ground (ground symbol in the picture is for reading convenience only).

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