rock soderstrom
Tour de France
Hi guys, here is a small handy PCB project for (not only) the self-etchers among us.
You surely know this, you want to realise a small tube amp project and once again you don't have the right power transformer at hand. Many power transformers for tube circuits only come with a 6.3VAC winding, but you want to heat your tubes with a rectified and regulated 6.3VDC. This also applies to many recycled transformers from old tube equipment such as portable R2R tape recorders or radios.
What can you do? You can try to achieve this with special low voltage drop regulators and diodes. But it's not really cool and flexible.
My solution is this circuit named TPSU_2XFIL, which was designed exactly for this purpose. With this you can heat smaller tube projects with DC voltage even if there is only one 6.3VAC
winding.
TPSU_2XFIL:
The trick is a Delon bridge circuit that acts as a voltage doubler with a downstream LM317 regulator and additional (optional) double filtering stages, which is useful for certain circuits like some tube microphones that get their biasing from the heater voltage.
Other more exotic DC target voltages for the heater are also possible, such as tubes from the P/X/U series, which brings many new possibilities.
Possible applications are mic/line preamps, buffer or headamps for microphones with rather low power requirements. I would not recommend this circuit for devices like stereo compressors or similar with many tubes. Do not use this with power amplifiers!
One should always bear in mind that voltage doubling and rectification put a corresponding load on the transformer. I would multiply the required amount of current by 3, so your transformer should be on the safe side. For example, if your tubes need 400mA, then use a transformer with a min. 1200mA 6.3VAC winding and everything is fine.
The voltage doubling produces a higher voltage which is partly converted into heat by the regulator and the filtering stages. Therefore, please consider the thermal load capacity of the resistors and the regulator. A heat sink is mandatory! The board also offers an additional series resistor in front of the regulator to distribute the thermal power loss more evenly.
The B+ section is a stone-age unregulated CRCRCRC circuit with the optional possibility of Z-diode clamping similar to the NU67 of the Neumann U67.
An LED indicates the voltage present in the circuit and, together with the R10 series resistor, serves as a bleeder and also sets the predominant current of the filter circuit when working with very low load currents (e.g. a microphone) of less than 1mA.
As you can see, I have not specified many values in the schematic, as this depends entirely on the application and the corresponding transformer. Some components are also optional, you can but you don't have to install them.
The general rules are - Ohm's law is your friend. Note the current capacity of your diodes and resistors and the maximum voltage of the capacitors!
Here is a concrete dimensioning example for a tube microphone with a 5654 tube. The tube requires 175mA +-15mA heating current and is represented by a 33Ohm resistor. The flowing current at FIL+ is 192mA.
At B+ the tube is simulated by a 220k resistor, 0,78mA current is flowing.
In this example I use a transformer with the following data: 16VA, 185V/40mA, 6.3V/1.4A.
R7,R8 and R9 are each 10K, the two series resistors for the LED2 are each 100K/3W, which corresponds to a total resistance of 50K and allows 3.6mA to flow in this branch.
Since I have a following potentiometer in my application and use additional ripple filtering in the microphone itself, the 172.5V output voltage is just right. If you wanted to supply a microphone with the more common 120V without that, I would rather use 20k (or more, do the math) series resistors and install a parallel pot to R8. You will find soldering points for the external pot wiring, as well as a place for a optional clamping zener diode on the underside of the PCB.
My example works perfectly. The 6.93VAC supplied by the transformer at the input is converted very cleanly into an adjustable 6.3VDC (or more). R4 and R5 are 5.6R resistors in my example. Nothing gets excessively hot, everything is within the green range. I can't measure any residual ripple voltage with my current possibilities.
Atached are the layouts for PCB self-etching, ready for direct printing, keep it at 100% scaling.
I have atached two complete circuits on a common Euroboard 160x100mm PCB plus a single 80x100mm version.
I hope someone can use this in the future. I would appreciate feedback. Have fun!
Edit: added print ready silkscreen PDF for euro PCB
You surely know this, you want to realise a small tube amp project and once again you don't have the right power transformer at hand. Many power transformers for tube circuits only come with a 6.3VAC winding, but you want to heat your tubes with a rectified and regulated 6.3VDC. This also applies to many recycled transformers from old tube equipment such as portable R2R tape recorders or radios.
What can you do? You can try to achieve this with special low voltage drop regulators and diodes. But it's not really cool and flexible.
My solution is this circuit named TPSU_2XFIL, which was designed exactly for this purpose. With this you can heat smaller tube projects with DC voltage even if there is only one 6.3VAC
winding.
TPSU_2XFIL:
The trick is a Delon bridge circuit that acts as a voltage doubler with a downstream LM317 regulator and additional (optional) double filtering stages, which is useful for certain circuits like some tube microphones that get their biasing from the heater voltage.
Other more exotic DC target voltages for the heater are also possible, such as tubes from the P/X/U series, which brings many new possibilities.
Possible applications are mic/line preamps, buffer or headamps for microphones with rather low power requirements. I would not recommend this circuit for devices like stereo compressors or similar with many tubes. Do not use this with power amplifiers!
One should always bear in mind that voltage doubling and rectification put a corresponding load on the transformer. I would multiply the required amount of current by 3, so your transformer should be on the safe side. For example, if your tubes need 400mA, then use a transformer with a min. 1200mA 6.3VAC winding and everything is fine.
The voltage doubling produces a higher voltage which is partly converted into heat by the regulator and the filtering stages. Therefore, please consider the thermal load capacity of the resistors and the regulator. A heat sink is mandatory! The board also offers an additional series resistor in front of the regulator to distribute the thermal power loss more evenly.
The B+ section is a stone-age unregulated CRCRCRC circuit with the optional possibility of Z-diode clamping similar to the NU67 of the Neumann U67.
An LED indicates the voltage present in the circuit and, together with the R10 series resistor, serves as a bleeder and also sets the predominant current of the filter circuit when working with very low load currents (e.g. a microphone) of less than 1mA.
As you can see, I have not specified many values in the schematic, as this depends entirely on the application and the corresponding transformer. Some components are also optional, you can but you don't have to install them.
The general rules are - Ohm's law is your friend. Note the current capacity of your diodes and resistors and the maximum voltage of the capacitors!
Here is a concrete dimensioning example for a tube microphone with a 5654 tube. The tube requires 175mA +-15mA heating current and is represented by a 33Ohm resistor. The flowing current at FIL+ is 192mA.
At B+ the tube is simulated by a 220k resistor, 0,78mA current is flowing.
In this example I use a transformer with the following data: 16VA, 185V/40mA, 6.3V/1.4A.
R7,R8 and R9 are each 10K, the two series resistors for the LED2 are each 100K/3W, which corresponds to a total resistance of 50K and allows 3.6mA to flow in this branch.
Since I have a following potentiometer in my application and use additional ripple filtering in the microphone itself, the 172.5V output voltage is just right. If you wanted to supply a microphone with the more common 120V without that, I would rather use 20k (or more, do the math) series resistors and install a parallel pot to R8. You will find soldering points for the external pot wiring, as well as a place for a optional clamping zener diode on the underside of the PCB.
My example works perfectly. The 6.93VAC supplied by the transformer at the input is converted very cleanly into an adjustable 6.3VDC (or more). R4 and R5 are 5.6R resistors in my example. Nothing gets excessively hot, everything is within the green range. I can't measure any residual ripple voltage with my current possibilities.
Atached are the layouts for PCB self-etching, ready for direct printing, keep it at 100% scaling.
I have atached two complete circuits on a common Euroboard 160x100mm PCB plus a single 80x100mm version.
I hope someone can use this in the future. I would appreciate feedback. Have fun!
Edit: added print ready silkscreen PDF for euro PCB
Last edited:
