Notice with regard to the use of KT150 tubes for TU-8340

June 1, 2017

TU-8340 Push-pull Vacuum Tube Amp Kit accepts various types of output tubes including KT150. However, we found that some KT150 display overcurrent error (red LED flickering), indicating that the tube is out of adjustable range of the semi-automatic bias adjustment system mounted in TU-8340.

This is due to the individual difference of KT150 tubes. The error occurs when the cathode current goes over the specified value, approx. 55mA, when the bias voltage is at -48V, the limit voltage that the semi-automataic bias adjustment system of TU-8340 can adjust.

Below are the countermeasures for this error.

For those who purchase KT150 from now

Since TU-8340 is a push-pull amp, you will need matched pairs of KT150. Matched pairs always have its ip value on each tube. Please tell the supplier of the tubes to provide you with ones with small ip.

For those who already have KT150 and they displays overcurrent error

As mentioned above, TU-8340 cannot generate the bias voltage higher than -48V. Therefore, the cathode current when no signal shall be set a little bigger.

To do so, the values of cathode current detecting resistors, R120, R121, R220, and R221 (all 12ohm) need to be lowered a little. However, exchanging these resistors is a big task. So leave them as they are now. Instead, add a resistor which value is around 100ohm in parallel to each said location. As a consequence, the cathode current of each tube when no signal becomes approximately 8% bigger. However, under this new setting, not enough allowable power is secured for EL34. Therefore, it is recommended to remove those added 100ohm resistors when EL34 tubes are used.

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