13E1 STEAMPUNK POWER AMP

This is my most recent last amplifier project ever. The photo is not really my amp. I pulled this photo off the internet. I will put a picture of mine when it is completed. 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. When fully running, they have a wondrous glow, evoking a simpler, much larger carbon footprint era. I love it.

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 12.5% 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 just-now formed subsidiary of Smith Amps, lol.

The maximum anode dissipation is 90W. With 430V and 130ma on the anode, each 13E1 will drop about 56W just to operate, which is less than 70% of max Pa. 56W X 4 = 224W of delicious room-heating wasted power.

DRIVER CIRCUITS

The circuit is shown below. The input circuit is a standard long-tailed pair (LTP) made up of my favorite super-linear small-signal triode, the 6SN7. Each tube comes with a pair of triodes, and I have parelleled them. The standard cathode resistor is replaced with a constant-current source (CCS, described below) to achieve maximum linearity. The CCS is set at 16ma, which means that an absolutely constant 4ma will flow through each 6SN7. The clunky looking paralleled anode resistor works out to about 24.6K so about 196V is dropped across it (24.6*4*2). So there is 204V at the anodes of the 6SN7's (400V-196V). Plotting 204V and 4ma on the characteristic curves of the 6SN7 (see below, red dot is operating point) looks like a pretty linear region. Because the current at the anode can never change, no matter what AC voltage is there, this means that the effective anode resistance Ra is infinity (to AC voltages). So the gain of the circuit is mu, which is about 16.

The CCS's will be placed on a printed circuit board with some other circuits, discussed below.

The 1N4148 diodes are placed in the circuit to counteract temperature-induced changes in transistor Vbe's - presumably the same changes will occur in the n-p junction of the diode and cancel out those in the transistor. Is this really important? Probably not. The voltage drop across the diode is about the same as the Vbe, so we're left with Vr = Vz or about 6.2V. To get 16ma, R needs to be about 6.2/16 = about 390. The 390R resistor can be tweaked once the circuit is built, but that probably won't really be necessary. The MJE340 transistor has to be heat-sinked.

Feedback (Vfb) is applied to the grids of the lower tubes in the LTP. The feedback fraction is set by Rf and the 100R resistor. Note that the feedback signal must be in phase with the input signal. If not, positive feedback and oscillation. Rfb will be determined empirically once the amp is built. It is discussed in the feedback section later on.

EL84 DRIVER STAGE

Note the large inductance in the choke anodes. The 2 chokes are wound on the same bobbin with the same core. They are center tapped, 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.

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

13E1 POWER STAGE

Below is the output stage for a single channel. The 13E1's are arranged in a push-pull configuration in pentode mode with a regulated 250V on the screen. There is around 400V DC on the anode of each 13E1. Cathode feedback (CFB) is derived from the output transformer and applied to the cathodes of each power tube. There will be further global feedback applied after the circuit is built. At setup calibration, voltage across the 1R resistors will be measured with a voltmeter to determine Ik1 and Ik2. Vk1 and Vk2 are measured with the voltmeter. Now the total resistance from cathode to ground can be calculated as Vk / Ik = Rtotal. The Teensy is set up to measure Vk1 and Vk2, and it will use Rtotal to calculate Ik for each channel.

The circuitry to the left of the output stage provides the bias voltage to the grids. The 10K potentiometer R2 is located on the front panel and is adjusted to keep the cathode currents equal as displayed on the front panel by the large meters. The 10K potentiometer R1 is also located on the front panel and is adjusted to keep the cathode current at a given level, as displayed by the retro nixie tube display on the front panel. There are a total of 4 pots on the front panel for the 2 channels.

The maximum allowed dissipation power for each tube is 90W.  I will use 70% of that to increase longevity - so around 63W. With 450V on the anode, a reasonable anode current (Ia) = 63 / 450 = 140ma. So I'll use around 120ma to be kind to the tubes. This value is obtained by adjusting potentiometer R2, and the anode currents of each 13E1 of the pair will be equalized with potentiometer R1.

OUTPUT TRANSFORMERS

The output transformers were designed by me - with a LOT of guidance from Patrick Turner. The gory details of their design and construction can be seen on the transformers page here. The design of these transformers is old-school and they should rival the best transformers in the world, namely the multi-thousand dollar transformers from Tango, Lundahl, and others.

My output transformers are extensively interleaved and are shown in schematic fashion below.

Each of the horizontal line segments represents a layer of winding and the arrows indicate the direction of winding. The thin lines are primary windings and the thick lines are secondary windings.

The deepest secondary winding has wires coming out of the transformer labeled A and B. The next layer of secondary winding is C and D, followed by E and F. The top layer of secondary windings is divided into 3 portions each having 20 turns. If you just connect A,C,E, and G on one side and B,D,F, and L on the other side, and then connect G-H, I-J, and K-L together, you get 4 windings of 60 turns in parallel. So your secondary is equivalent to a very thick wire with 60 turns. This is the arrangement for a 4 ohm speaker

OTOH, if you hook B to G, and hook D to I, and hook F to K, you now have 3 secondaries, each with 80 turns. Now parallel the 3, and you have the equivalent of a thick wire of 80 turns. This is the arrangement for an 8 ohm speaker. At the bottom of the diagram, calculations are made. If you have secondary of 60 turns, and hook to a 4 ohm speaker, Ra-a calculates to 7.3K. Likewise, if the secondaries are connected for an 8 ohm speaker, and you connect to an 8 ohm speaker, you get Ra-a of about 8.3K. This is all close enough. Speakers are never even remotely close to whatever impedance they are rated at.

These arrangements can be accomplished with a big switch. I am so proud of this design, I feel compelled to show it again below, lol. The amplifier will need two switches, one for each channel, and they are huge, to handle the large currents.

FEEDBACK AND COMPENSATION NETWORK

In addition to the cathode feedback on the 13E1 power tubes, there will be a bit of global negative feedback (GNF). This will improve distortion and lower output impedance to give a punchier bass. Patrick Turner used only 10dB of feedback, and I will stick with that number. No magic there, 12dB or 8dB would work fine.

RIGHT CHANNEL
With the speaker set at 8 ohms, and an 8R dummy load, the open loop gain A=40. On the 4R setting A = 45.
Using my handy-dandy Excel spreadsheet, to get a GNF of 10dB, I calculate that the feedback resistor is R=1750 on the 8R setting, and R=1981 on the 4R setting. A 1.8K resistor should work fine for both.
With this 1.8K resistor, GNF = 9.8dB on the 8R setting, and GNF = 10.5 on the 4R setting. That is perfectly acceptable.

LEFT CHANNEL
On 8R setting, A= , and on 4R setting, A= .

BALANCING THE GAINS

TEENSY 4.1 MICROCONTROLLER CIRCUIT

I am using mostly analog meters customized with SmithsofWeston faceplates, and some very old-fashioned Nixie tubes. In order to sample all the voltages, currents, temperatures, etc. to display on the meters, I am using a rather more modern approach. I use a microcontroller which I have programmed to carry out all these functions.

The schematic is shown below. I am using the awesome Teensy 4.1 microcontroller, which works just like an Arduino, but is much smaller and has far more I/O ports. It is a 2 sided integrated circuit, but I will only be using the top side, because that will provide me with enough I/O ports. It will drive the Nixie tubes via the antique 7441 nixie drivers. J1 receives all the voltage and current information from the "Overvoltage/CCS board described below and sends it to the Teensy. J2 is mostly output to the meters, but it also inputs the digital output from the IR receiver. Input from the thermometers also arrives via J2, as well as 2 switches. The KILL signal to the PSU outputs through J2. The control signals to the volume potentiometer exit via J2.

The circuit at the bottom of the schematic is a simple OR gate. It outputs a LOGIC1 if either the manual on-off switch (J2-pin1) is pushed or if the Teensy (activated by the remote) sends a turn-on signal.

To the left are the 7441 nixie drivers. The nixies are mounted on the front panel well away from the Teensy, because the teensy had to be located in a remote part of the chassis to eliminate any issues with noise near the input signals which happen to be pretty close to the nixies. So 30 wires have to be soldered to the board and run to the nixie board shown later.

Below is the PCB that I designed for the Teensy 4.1 controller circuit. The ground plane and the power plane are not shown because they make it impossible to see the traces clearly. This design is sent is the PCB Express fab shop to be made into a real board.

The nixie board will hold the nixies and be mounted to the front chassis. It will obviously be a DIY board.

The teensy board needs small voltages, less than 3.3V, so I designed a board to input the current and voltages from the amp. Note that each voltage input has a 1K resistor (resistor array U4) dropped to ground. Each of these is the second half of a voltage divider, reducing the high voltages (some > 400V) to small voltages, suitable for the teensy. The 1st half of each voltage divider is mounted in the chassis away from this board. For example for the high voltage rail voltages VbR and VbL (around 430V), the off-board resistor is 390K, so that the voltage presented to the board is:
430 * 1 / (390 + 1) = 1.1V.
This is a very safe voltage. Nevertheless, the signals are still sent through an op-amp (note that the negative voltages are sent through an inverting op amp) and then onto a protection scheme consisting of a 470R resistor and 2 schottky diodes. This should pretty much guarantee that the teensy never sees a high voltage, which would certainly fry it.

There was plenty of room left on the board, so I stuck the constant current sinks for the first stage on the same board.

And below, of course, is the PCB design.

OVERCURRENT PROTECTION, METER BUFFERS, MOTOR DRIVERS

The Overcurrent Protection 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 J5 (KILL2). If Teensy detects any cathode current greater than, say, 200ma, it sends a LOGIC1 to the the C106 SCR U3, which then trips and pulls pin1 of J5 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 (taken from the cathodes of the 13E1's) 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.

The meter buffers are simple op amps with an extremely low output impedance, so perfect for driving analog meters. The Teensy receives input from the temperature sensor and calculates the temperature. It then divides the temperature by 50 to convert it to a voltage suitable for output. e.g. if T=25C, .5V is outputted; if T=75C, 1.5V is outputted. The meters are wired so that a 2V input causes full deflection of the needle (100C). Unfortunately, the Teensy cannot output an analog signal, so instead it outputs a pulse-wave modulated (PWM) signal. An example of this is below left. This signal might be OK to send to the analog meters, but I have buffered it with a simple RC buffer. There are 4 such buffers made up of R12-15, C14-17 (see Teensy schematic above). The buffered output is shown below right. This is sent to the meters. When T=25C, V=.25V and the needle is deflected 1/4 full-deflection. When T=75, V=1.5V, and the needle is deflected 3/4 full deflection.

The current difference (deltaI) signals are a little more complicated. The deltaI meters need to show negative values, as well as positive values. The Teensy obviously cannot output a negative voltage, so the following Smithlabs innovation (lol) is used: The Arduino software is adjusted so that when the deltaI = 10ma, 3V (after buffering) is outputted, and when deltaI = -10ma, 1V is outputted. This is sent through the 2X amplifying buffers shown above, so that when deltaI is 10ma, the meter gets 6V and when the deltaI = -10ma, the meter gets 2V. If you put 4V on the ground pin of the meter, the meter thinks it is seeing +2V and -2V, respectively. The meter is adjusted so that 6V (the meter thinks it is 2V) causes full deflection to the right, and 2V (the meter thinks it is -2V) causes full deflection to the left. The 4V regulator is shown just above the LM324 op amps above.

The little circuit on the bottom right is to drive the motor on the motorized Alps potentiometer (volume control). They require separate voltage regulators to provide +5V and -5V. My favorite little COTO PCB relays are perfect for this and have been used in all my other amps.

This little circuit is just perfect for a handy-dandy DIY printed circuit board. It saves me the $85 to have 3 boards manufactured by the fabrication house. Look here for how these homemade PCB's are made right here at Smiths of Weston HOT labs. HOT = "hopelessly outdated technology." The PCB design is shown below. Everything black will be exposed copper. The yellow silkscreen layer will not be included.

POWER SUPPLY

The essential elements of the power supply are shown below. There are multiple outputs.
1. 430V DC to the left and right rails.
2. 250V DC (regulated) for the screens of both channels. This comes from 430V off the left transformer.
3. 170V for the nixie tubes. This comes from 430V off the right transformer which is reduced to 170V with a simple zener regulator.
4. -100V DC for the negative rail.
5. 6.3V for all the 6SN7 and EL84 filaments
6. 26V AC to the 13E1 filaments.

AC N comes directly from the wall plug, while AC L comes from a relay circuit, described below in the soft-start schematic.

The power transformers were designed and wound by me and details are shown here. Originally, I used a choke for smoothing and designed and built 2 beautiful chokes. They are shown below, but will not be used.

The power supply worked beautifully, but it weighed a TON, so I abandoned the chokes and went with capacitor multipliers, as I have done with previous amplifiers. The layout of the power supply is shown below:

The capacitor multipliers work as follows: the raw voltage from the large capacitors is sent to the drain of the N-channel MOSFET. It also passes through a voltage divider consisting of 5.6K and 470K, which reduces the voltage slightly. This reduced voltage is sent into the gate of the MOSFET through a 100K stopper resistor. The current into the gate is negligible (picoAmps) so there is almost no ripple across the 11uF (22uF X 2 in series) capacitor. Because the MOSFET is in common source configuration, the source will attempt mightily to follow the gate, so since there is almost no ripple going into the gate, there is almost no ripple emerging from the source. The 5.6K resistor in the voltage divider is chosen so that it drops a voltage at least as high as the peak ripple voltage going into the gate. This works extremely well. There is a capacitor multiplier for each channel.

Each channel has its own negative supply, approximately -90V fully loaded.

Each channel has a 250V regulator to provide voltage to the screens. It is a very simple, classic 2-transistor regulator. The voltage across the 8.2K resistor = the sum of the voltage reference voltage and Vbe of the MJE340. This is in the neighborhood of 8.6V. So, there will always be around 1.1ma through the 8.2K resistor. So the voltage at the top of the 220K resistor = 1.1 * (220 + 8.2) = 251V.

The right channel 250V regulator also provides power to the nixies. Each nixie anode gets an anode resistor. The datasheet gives recommended anode resistance for a number of input voltages. It recommends 33K resistor for a 250V supply.

The panel meter circuit seen in the middle of the page will show Vb L and Vb R when the panel switch is in the left-most position; Vscreen L and Vscreen R in the middle position, and Vneg L and Vneg R in the rightmost position. The meters are large custom Smiths of Weston meters located on the front panel of the amplifier.

Soft-start function

Because of huge currents flowing into the PSU capacitors, a soft-start is required. This is accomplished with the setup shown below. Upon startup, relay1 (REL1) is energized - note that the coil in relay 1 is energized by RelP, not by a fixed 5V. This will be explained later. With REL1 energized, line voltage flows through it and through the 12R resistor before going to the transformers. This slows the charging of the capacitors.

After a few seconds, REL1 is turned off and Rel2 is turned on, allowing the full line voltage to flow into the transformers.

The line voltage must pass through the kill relay (KILL REL) before passing on to the transformers. KILL REL is normally closed when not energized so allows current to flow. If it is ever turned ON (by a signal from the Teensy), the relay will open and kill the line voltage, shutting off the transformers.

I designed a circuit board to handle the soft-start functions, the low-voltage regulators, and the humdingers. It is a 3.8 X 2.5 inch board, so is very compact.

SOFT START: this involves the 2 relays above, REL1 and REL2. When the amp is off, pin5 of the inverter U3 is HI => pin 6 and pin1 are LO. Pin2 is HI so Q1 is on, which would energize REL1 coil, but REL1 coil does not have power yet, since it is powered by RelP. The red LED on the front panel is powered through R3 and R4, which are arranged as a voltage divider to send 4.3V into pin 5 of U3 when INPUT is not LOW (i.e. amp is off). The zener D8 protects the inverter from high voltage if, say, the LED burns out and becomes an open circuit

When INPUT (J1 pin 10) goes LOW, the soft-start begins. INPUT is from the Teensy board in the amplifier chassis. First, the red LED is turned off, because its input is shorted. At the same time, Q3 is turned ON, so RelP = 5V, which energizes coil of REL1, turning it on. This partially turns on transformers because the line voltage is going through the large 12 ohm resistor shown on the relay diagram above. This is the soft-start. Pin 5 of U3 inverter is LO => pin 6 HI. C1 begins charging through R5. The time constant is 27K * 100u = 2.7sec, so somewhere around 2 seconds or so, the capacitor is sufficiently charged to make U3 pin 1 HI. So, pin1 is HI => pin 2 is LO, so Q1 turns OFF, turning REL1 off, but Q2 turns ON, turning REL2 ON and allowing the full line voltage to reach the transformers. Q2 also energizes the COTO 4007 relay on this board, and lights the meter lights.

LOW-VOLTAGE REGULATORS: these are standard regulators. The 5V regulator is a high-current device and is heat-sinked. It is protected by an overvoltage crowbar circuit comprised of a C106 SCR, D6, R15, and R17. If the output voltage of the regulator goes above around 5.7V, .6V is presented to the SCR which turns it on. This shorts the input to ground and blows the fuse. I have lost many sensitive 5V parts in the past, and I hope this helps prevent some of those losses in the future.

HUMDINGERS - these are simply 500R trimpots placed between each pair of AC filament windings. They have worked extremely well in the past to make my amps dead quiet.

Below is the PCB board design.

PSU CHASSIS ANALOG METER BUFFERS: the analog meters are delicate D'Arsonval movement devices. Just as in the amp chassis, the high voltages need to be knocked down by a voltage divider. A 470K:1.9K divider gets the voltages down by a factor of about 250. The voltages selected by the switch pass to the LM358 op amps in U1 and then on the the meters. The negative voltages are similarly reduced by a voltage divider and are passed to the op amps in U2 where the polarity is reversed and they are doubled. These signals then pass off-board to the switch and pass through the switch just like the other positive voltages. R13-16 are selected to perfectly calibrate each meter.

Below is the PCB board design. It is relatively simple, so I'll save $70 and DIY it.

CONSTRUCTION STEPS

The chassis was manufactured by Landfall Systems in Austin. A custom chassis like this is outrageously expensive - way more than most amplifiers on the market. I had a separate chassis made for the amp itself and for the power supply

The filament (heater) wiring is done first. 18ga red and black wire is used throughout and twisted as tightly as possible. This reduces (hopefully eliminates) the irritating hum that accompanies many tube amps. Note the layout around the edges. This is to keep it away from the input signal which is low voltage and very susceptible to picking up the AC hum.

I had already wired the screens on the power tubes when I took this picture.

The next step is to wire the power tubes, the 6SN7 input tubes, and the EL84 driver tubes. The RCA jacks are wired and the electric ALPs potentiometer is installed and wired. The speaker jacks are wired also. The coupling capacitors are the red rectangular components. The wiring has not been organized yet, so is a bit messy appearing.

The next step is to build and install the output transformers.