6 March 2020

I bought 4 fabulous 13E1 tubes, mainly because they are beautiful. 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 used in tetrode mode, with a regulated 250V on the screen grid. There will be around 20% cathode feedback derived from the output transformer, which will have to be fairly complex - I have learned to design and wind my own transformers and chokes so I will be custom building the output transformers; the agonizing process can be found in the transformers section of this website. I will also design and wind my own power transformers and chokes. These will be built by Smith Iron, a subsidiary of Smith Amps, LOL.

The datasheets for the 13E1 are particularly lame, so I did some curves of my own using my uTracer3.  They are shown below with 250V on the screen. Unfortunately, the uTracer only goes up to 400V, but that's OK, I can still get plenty of information; I'll be putting about 430V on the anode in the amp.

The maximum anode dissipation is 90W. With 430V and 130ma on the anode, the 13E1 has to drop about 56W, which is less than 70% of max Pa.


As usual, I will be using the very linear, very available 6SN7 for my differential input circuit. The two triodes in each package will be paralleled which will means 2 tubes per channel, just as in my bigger KT90 amps. The circuit is shown below.

The anode current for each triode will be 4ma, so 8ma through each tube. This will be set by the CCS shown on the cathode. The 22K resistors drop 22*8 = 176V each, so there will be about 214V on the 6SN7 anode. This sets the cathode voltage at about 7.9V, which agrees nicely with the 6SN7 characteristic curves. Amplification from anode-to-anode is approximately mu, so around 15.


Patrick Turner of turneraudio.co.au is a big proponent of the EL84, particularly in circuits that use cathode feedback. This is because cathode feedback raises the RMS Vk of the power tube significantly, so to get sufficient Vgk to drive the power tubes, you need a significantly higher Vg, and this means you need big driver tubes to provide those high voltages to the power tube grids. He has found that the EL84 is a particularly suitable choice. In order to get higher voltages without a ridiculously high rail voltage, he likes to choke load the EL84 anodes. This is an old school approach, and the amp would probably be fine without choke loading, but I just learned to design and wind transformers and chokes, so what the hell. A choke allows the anode voltage to swing higher than the rail voltage, which is a useful feature. Another advantage is that these chokes can survive a hydrogen bomb, as opposed to somewhat frail CCS circuits.


Note the large inductance in the choke anodes. These are center tapped chokes so half of the DC current goes one direction, the other half goes the other direction. If the two currents are equal, the core sees a DC current of zero, and will not saturate, so theoretically no gap is needed. This requires serious tube matching, so I went to my favorite supplier, Jim McShane of McShane Design and he provided me with a closely matched quad of EL84's.

To confirm the match, I used my favorite tool, the uTracer3 and generated the following curves. They are indistinguishable from one another! With 250V on the anode and Vg = -10, each tube produces exactly 20ma of anode current. So, there should be minimal DC current through the anode chokes if I set everything up correctly.

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I am not going to gap the chokes. The choke construction can be viewed here. At 1kHz, the impedance presented by each choke is at least 2*pi*1000*100 = 628K, and at higher frequencies, in the megaohms.


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 it 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.



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, 200ma, 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.



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.


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.



The very busy circuit is shown below. The main power transformer will supply 430V DC to the rail and to the 250V regulator, and -100V DC for the negative rail. There are two 12V secondaries - I'll use one of them for -12V to power the negative side of the op amps. The design and construction of the main power transformer is shown here.

The filament transformer supplies 2 6.3VAC lines for the 6SN7's and the EL84's, and 2 26V lines for the 13E1. It is a heavy draw and requires a heavy transformer. It is shown here. The 5V circuitry needs to be on at all times, to effect the ON-OFF functions, so it will be supplied by a separate "El Cheapo" transformer which will always be powered.

The filtering for the 430V rail will be a CLC filter, with C=470uF, and L=5H. I use 2 470uF 450V caps in series, paralleled with 2 other 470uF caps in series to give a total C = 470uF, 900V. This leads into a 5H choke and finally a second 470uF, 900V capacitor array. The choke is designed and built by yours truly and its construction is shown here

SOFT START: in order to prevent a massive initial current into the capacitors upon startup, a soft-start feature will be used. This involves 2 relays, REL1 and REL2. See the soft-start circuit. The input into the Soft-start circuit is from the OR gate on the Teensy board. When the amp is OFF, input is +5V.

When amp is OFF, the input to the soft-start circuit is 1 (+5V). So Q3 if OFF, and Relay Power = 0V. Q1 is turned on so the REL1 coil is pulled to ground, but nothing happens because Relay Power = 0.

When the amp is turned on, input goes to 0V, and suddenly Relay Power = 5V, so REL1 turns ON and line current begins piling into the transformer through the 12 ohm resistor. At the same time, the output of inverter1 goes to +5V, and begins charging C1 (through R1). At this point, the input to inverter2 is still low, so Q1 is still ON, and Q2 is off. After about 5 seconds, the capacitor has charged enough that inverter suddenly puts a 0 on its output, which turns off Q1 (and REL1) and turns on Q2 (and REL2). The amp is officially ON.

The 250V regulator is a simple affair. At first I used a 2-transistor regulator, but after the TIP50 transistor blew a second time, I decided to dumb it down with a simple zener regulator which works fine, as long as at least 10ma is flowing through the zeners. The zener shown in the diagram is actually 25 10V zeners arranged in series. The screens will pull about 30-32ma from the regulator. The 4.7K resistor in parallel with the 47K resistor => 4.3K, so the current through this resistor is (430-250)/4.3K = 42ma. This means that the zeners are getting their 10ma so will perform satisfactorily.