STEP UP 12VDC-B+ TUBE AMP PSU KIT

The use of switch-mode power supplies in tube amplifiers is still rare. There is no functional issue with switch-mode power supplies as long as they are well designed, use quality components, and ensure that individual components can be replaced by service technicians, as entire modules are virtually never available as spare parts years later. Ready-made Chinese switch-mode boards have performed variably, some better and some worse, but none have been convincing enough for us to consider using them in our kits. So, we set out to design our simple step-up converter.

 


  1. STEP UP 12VDC in B+ out - hakkurivirtalähde rakennussarja putkivahvistimille
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    Hakkurivirtalähde, jolla voi nostaa 12VDC hehkujännitteen maksimissaan 300VDC B+ jännitteeksi. Taajuus ja takaisinkytkentäsäädöt mahdollistavat hyvän jännitteen säädön. Idea on käyttää 12VDC ulkoista virtalähdettä, joka viedään suoraan hehkuille (6,3V putket aina 2 hehku sarjassa) ja tälle hakkuri step-up virtalähteelle, joka tuottaa anodeille tarvittavan korkeajännitteen. Learn More


The idea is to feed the filaments directly with an external transformer at 6V or 12V. 12V is preferable as it halves the current draw. 12.6V would be more accurate, but 12V is close enough, and power supplies are easy to find. From the 12VDC input, we get the voltage going to the switch-mode power supply and "switch" it to a level suitable for the tubes, typically between 200-400V, using a high-frequency oscillator. A simple step-up converter works well when the current demand is not high (up to about 100mA). For a stereo EL34 amplifier, we ended up using two step-up modules, halving the current demand per module.

A 4x DIP switch was added to the circuit, which changes the value of the capacitor that determines the frequency of the oscillator, allowing the oscillator frequency to be adjusted. A trimmer adjusts the feedback and thus limits the maximum voltage. With the trimmer fully open, the output voltage can be too high at startup for the circuit's capacitors (if they are rated at a maximum of 500V), but adjusting the trimmer lower, limits the maximum voltage, correcting this problem.

Voltage adjustment is done by both changing the frequency (DIP switch) and adjusting the trimmer. It's advisable to set the trimmer slightly over halfway and then adjust to an approximate voltage by trying different DIP switch combinations. The trimmer can lower the voltage, but at some point, interference increases, so it's best to use the trimmer to lower the voltage only slightly (which also corrects the startup spike problem).

The circuit uses a high-quality FET and switch-mode coil. This is essential to maintain low thermal losses and good efficiency. The components are chosen through both calculation and testing. The challenge is finding values that work well for different voltages and currents.

The module's connections are DC in, ground, and DC out. Note that if the switch-mode power supply raises the 12V to 300V and draws 100mA (300V x 0.1A = 30W), it will draw 2.5A from the 12V input (30W / 12V = 2.5A) plus any losses. In practice, a 12V 4A transformer would be safe in this case. It's always wise to leave some headroom. In the stereo EL34 SE amplifier, there are two of these switch-mode supplies. The filament and B+ power demands are met by a 12V 7A power supply (84W).

Assembly

First, check the contents of the package and ensure that all parts are included. The components might look slightly different from the images in the instructions. The important thing is that the values are correct.

This device operates with over 300VDC voltage, so be especially careful to ensure that all components are in the correct places and oriented correctly. Make solder joints carefully and inspect each one visually before moving on to the next.


First, install the lower components according to the image, parts list, and PCB designators (C1, C2, R1, R2...). In the image, R1 differs from what is included in the kit: always check the correct values from the parts list, not from the photos.

Continue installing more components. Note the polarity of the electrolytic capacitor: install the + and - terminals in the correct directions. The transistor and diode must also be installed correctly, as they have a specific orientation.

The TO-220-cased transistor and diode should be installed as shown in the image. You can install them on the other side if it suits the application better, but carefully ensure the pin layout and orientation.

Lastly, install the switch-mode coil. Before using it, secure it to the PCB with a zip tie or glue.

Before powering on, check that all components are correctly installed and oriented.

Testing

Set the trimmer to halfway. Set DIP switches 1, 2, and 3 to the ON position, and 4 to the OFF position. If you have a variable power supply, connect it to the DC IN - GND terminals and a multimeter to the DC OUT - GND terminals. Power it on and read the multimeter. It should show 200-400VDC. Adjust the trimmer and see if the voltage drops. The trimmer starts to have an effect about halfway. Then try different DIP switch combinations and adjust the trimmer to get a feel for how the frequency and trimmer affect the voltage.

If everything doesn't work as it should, don't worry. The fault can be found. Check the placement of components and solder joints, find the error, and correct it. If everything works: Congratulations - you've built a working switch-mode power supply!

Applications

This type of Step-Up switch-mode power supply works best in small tube amplifiers: preamps or perhaps single-ended amplifiers. Or even in a small push-pull amplifier. The maximum current is about 100mA, but this depends on the output voltage, ambient temperature, cooling of the transistor and diode, frequency... We don't recommend pushing them to their limits. If you need more power, use another application or two boards. In a stereo amplifier, it’s convenient to use a separate switch-mode power supply for each channel. This reduces the load per supply and separates the channels from each other.

With lower currents we get more voltage boost, meaning it can rise over 400V. However, preamps rarely need such high voltages.

The switch-mode power supply's output is very clean. In the All Octal SE amp, the noise level is so low that it's inaudible to the ear.

The circuit has only a 1uF filter capacitor by default, so often a 10-100uF filter capacitor is added to the B+ line going to the output transformer. It's recommended to use high-quality electrolytic capacitors as the first filter capacitor after the switch-mode power supply.