Previously I tested a El84 using series of 9 volt batteries to generate the high B+ voltages. While it was very simple by nature, it still fell short of the desired voltages that are required to make a useful audio amplifier. Normally the solution to this problem is to use a step up transformer and rectify the mains current. These transformers are not cheap, very heavy, and costly to ship, but above all I did not have one at the time. Another option is direct rectification of the mains AC. This technique has been used in radios of the past and is extremely dangerous! Without a fuse this circuit will draw as much current as you house will deliver. Also the ground of the circuit will be directly coupled to the neutral of your home. If you use a non polarized plug or there is a mis-wiring of your outlets, the ground of your circuit would complete a direct 120v path with the “ground” of another circuit. So for initial testing I decided to go with a boost converter.
The Boost conveterter
The operating principal of a boost converter is very simple, am inductor is charged and then released though a diode to charge a capacitor. This principal is used in switch mode power supply units. Most switching regulators are designed to work at lower voltages, say bump up a 5V supply to a 12V supply. I have a few MC34063A switching regulators I bought a while back for the exactly purpose of running 12V out of a couple AA batteries.
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This devices is only rated to 40V. So I had to add some supplementary components in order to ramp the voltage over 100V. To do this I did the switching using a IRF740 MOSFET. At first I tried driving the opamp directly from the regulator. In simulation this would work however I discovered that real MOSFETs have a fair amount of capacitance at high frequencies that will create a lot of resistance. So I had to create a MOSFET driver. For this I chose a 2n222 / 2n907 NPN/PNP pair. When working with high voltages it is important that all the components are rated appropriately. This is espcially important for the capacitor and diode following the MOSFET. The capacitor I chose was surplus disposable flash capacitor rated at 330V with a capacitance of 270uF. The diode is a simple rectifying diode, I may have had better performance with a Schottky diode however all the ones I have were not rated for voltages this high.
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Designing an Audio Amplifier
For designing the audio amplifier I went back to the circuit simulator. Falstad’s simulator has a triode and the it’s model parameters can be edited by exporting and re-importing the circuit file. I asked Paul how to change the characteristic of the tube and he replied with the model that he used to simulate the triode that is as follows: ((Vgk+Vpk/mu)^1.5)/kg1 where Vgk = gate-cathode voltage and Vpk = plate-cathode. In order for me to simulate commercially available triodes I needed to find kg1 and mu values that would get me close. For this I wrote a python program using matplotlib to allow me to play with different values and visualize the results. Below are a couple screen shots.
An 12ax7 Preamp Troide, The y axes is milliamperes and the x is volts
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A El84 Poweramp Pentode running in Triode mode
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The source code for this project can be found on my Github account under Tube Plot. This code was developed on Ubuntu using python 2.6.5 and matplotlib.
After finding values that were close to the original data sheets (at least for the voltages I intended to run) I then did some simulations.
The Preamp
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The Poweramp
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Putting it all together
Now that I ran some simulations I deiced to try building it all out. First I tried using the switching power supply and only the power amp state using a function generator as the preamp.
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While this did work, it was very stressful on the boost converter. A lot of current had to be drawn. I spent about a day or two playing with different inductors and switching frequencies to get optimal efficiency. In this time I blew a couple mosfets and diodes. Eventually a ham buddy of mine gave me a step up transformer to play with so I deiced to make a traditional power supply.
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I used a broken PSU as the case so I could recycle the power connectors and power switch. I also added a fuse along with the rectification circuit. This PSU reads arround 200v unloaded and 180v loaded, about the same as my switching supply.
So I could actually play music from my iPod, I built out the 12ax7 preamp. Both tubes heaters are using a 6V battery in this test. So far everything seems to work well. In the video blelow I used an old Bose speaker and played some music.
The song being played is “Against all Odds by” Brian Botkiller. The little pocket oscilloscope from Gabotronics is displaying the voltage ripple of the power supply.