13E1 STEAMPUNK POWER AMP

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6 March 2020

I bought 4 fabulous 13E1 tubes, mainly because they are beautiful, brightly glowing masterpieces of tube technology. I think they were originally intended as transmitting tubes, but are now used primarily as audio amplifying tubes.

This will be another push-pull amplifier, with the tubes (which are tetrodes) strapped in triode mode. This will put a little more than the maximum rated power through the screen grids, but I don't think that will be a huge problem. I have learned to design and wind my own transformers and chokes so this amp will have all custom iron designed and built par moi.

TEENSY 3.2 MICROCONTROLLER CIRCUIT

As with my previous amps, I will use the awesome Teensy3.2 microcontroller, which works just like an Arduino, but is much smaller and has far more I/O ports. It is a 2 sided board, but I will only be using the top side, because that will provide me with enough I/O ports. It's main function will be to drive the Nixie tubes and handle the IR (infrared) remote functions, but is will also monitor voltages and currents in the amp, and emergently shut off the amp when necessary.

Below is the circuit. J1 receives the cathode currents; J2 receives the signal from the IR receiver, from the temperature sensor, as well as the anode voltage (reduced about 200-fold), and the input from the DPDT switch. J3 has outputs to the volume controller, and to the power supply board where it handles On-off functions. The circuit at lower left is a simple OR gate. It outputs a LOGIC1 if either the manual on-off switch is pushed or if the Teensy (activated by the remote) sends turn-on signal.

To the right are the nixie drivers and the nixie tubes all mounted on the same board.

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PROTECTION CIRCUIT

The circuit below will shut down the entire amp if any of the cathode currents exceed about 200mA. The first line of defense is from the Teensy, which is monitoring the cathode voltages (currents) - it is connected to the SCR via pin 4 of J3 (KILL2). If Teensy detects any cathode current greater than, say, 190ma, it sends a LOGIC1 to the SCR, which then trips and pulls pin1 of J3 to ground. This activates the KILL relay on the PSU board and shuts down the transformer supplying the power tubes.

The backup, if the Teensy fails, is the circuitry shown at the top of the diagram. The maximum of K1R,K2R,K1L,K2L is delivered to the base of Q1. When one of the cathode voltages reaches about 2V (200ma cathode current), Q1 is sufficiently turned on to trip the SCR.

The SCR stays permanently closed until all power is shut off and it resets.

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METER CIRCUITS

Unlike previous amps where I measured and displayed the cathode currents for each power tube, in this amp I will measure and display the average cathode current for each channel (top circuits below) and also measure and display the difference in cathode currents 1 and 2 for each channel (lower circuits). The differences will be displayed on 2 center-zero meters that go positive and negative. See photo of amp at top.

Below are 2 circuits - both channels are shown. The top circuits calculate the average of K1 and K2 for both channels. The output of the 2 220k resistors is the average of the inputs. This inputs into the non-inverting op amp, which doubles the resulting voltage. Since there is only one Nixie display displaying the 2 results, the outputs are led to a DPDT switch on the front panel which then sends the selected result to the meter. The Teensy processes the cathode voltages and drives the Nixie display.

The bottom circuits calculate the differences between K1 and K2 for left and right channel. The difference should be zero and there will be potentiometers to adjust this; they are shown on the main amplifier schematic. These are simple difference amplifiers. The differences are multiplied by a factor of 10 to make a better input into the analog meters.

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Temp measurement is straightforward. A TMP36 temperature sensor is placed on the circuit board. Its output is in millivolts. To convert to centigrade use the following formula: °C = (V - 500)/10 where V is in mV. So a difference amplifier is needed to subtract the .5V from the sensor voltage. 5V is reduced to .5V by the voltage divider and fed into the (-) input of the op amp. The output of the op amp is 2*(V - .5) volts. This is sent to a meter through a 35R resistor to give max deflection of the meter at 100°C. Note that the raw output of the sensor is sent to the Nixie, which can easily handle the little calculation.

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