All about electronic valves and hi-fi
Finally after many years, I managed to document this great PCB. If you already have this PCB and want the build guide and BOM please let me know.
Here is an extract of the build guide for the ones who are interested in this circuit:
Continue reading “Cap Multiplier PCB: build guide and BOM”Surely you’re as tired as I’m with COVID-19. One of the best things I can do to distract my mind is to keep myself away from social media. Every stone you turn, there is COVID or a statement about it. I won’t moan as I have a job for now and a healthy family. Some members of my family were infected but nothing major. I can only say is that the world has change. And so my day to day life looking after the young family whilst working is a real challenge. Starting my fifth week of lockdown, I have to distract somehow my mind at times, otherwise will go mad.
As time is very limited these days, I’m focused in continuing my modular building approach in LEGO style. I have developed several PCB modules which are flexible to be used in multiple amplifier and pre-amplifier designs. Now, I used the power of PCBs to build some additional supporting modules to speed up my breadboarding over the IKEA boards. Not the most elegant approach, but building becomes a very fast process this way.
You will see what I’m saying when you see a few of the following additions:
After few tests and revisions (Rev 04) I updated the capacitor multiplier board I developed time ago
I’ve been working on a few PCBs lately with great results. I started with an HT power supply PCB which allows the use of full silicon rectifier bridge, hybrid rectifier (e.g. for damper valves) and any combination of either CRCRC or CLCLC stages with external chokes. Last capacitor is bulky WIMA DC-Link film instead of electrolytic. That worked really well and I’m using it now in a +50/-300V supply for the fixed bias and source follower drivers in the amps.
Continue reading “DHT LT power supply PCB tests”
This year I entered the shootout competition and will bring my DHT system to the European Triode Festival in France. It’s comprised of the ER801a stage plus the 01a (if extra gain is needed) and the 4P1L PSE output stage
I will have to swap out the amorphous OPT for the Monolith Magnetic ones as the speaker load is 5R.
It’s going to be interesting!
I managed to find the time this weekend and do a quick test on the Cap Multiplier PCB. It turned out that I missed a thermal pad on the PCB so had to add a short jumper. Nothing major, but the PCB needs adjustment.
Below is the diagram of one of the ways the PCB can be used. This is the most complex circuit, a basic one can be wired instead. The CCS (M1 and M3) provides better PSRR as well as regulation. A stable current is fed to R4 and P1 to set the voltage. C2 is the cap multiplier and the M5 used can be any suitable MOSFET. T4 provides current surge protection to the MOSFET as well as short circuit. R7 sets the current limit.
The boards fits various film caps, I have some WIMA DC Link which are great and fit perfectly:
I received many requests throughout this year of folks building the 01a preamp (Gen2) asking for advice on the HT supply design. Well, I have my own incarnation which serves multiple purposes as is a shared supply, therefore isn’t useful to anyone.
My friend Andy Evans came up with a supply using available components. This is exactly what you are looking for the HT supply. I made some tweaks to Andy’s design, so all credit should go to him.
The HT supply design is very simple. It’s mainly a choke-input valve rectifier supply. It has an additional LC smoothing stage. Here is the high-level circuit, see some notes
The transformer is a 250-0-250V / 50VA with an output current of 60mA. It has two windings for 6.3V AC heaters, but you only need one. Here is a great choice from a recommended seller.
The valve rectifier is a double-diode damper. Of course you can use some other options, but I like the sound of them. Here are some you might want to consider in your build:
For the indirectly heated diodes, it will be better to connect one end of the filaments to the cathode.
The chokes are commonly available. These are from Hammond (155J) and have 15H @ 30mA. The downside is the high resistance, over 1kΩ. This isn’t an issue here as the current consumption is low so the voltage drop is minimal. With a choke input supply you need a minimum current to operate. In this case is about 15-16mA so a bleeder resistor is needed (29kΩ 5W wire wound) .
C1 serves to equalise the output voltage. It pushes the supply to operate a bit more like cap-input (hybrid) by increasing the output voltage. I use it to tune the output voltage to 200V.
C2 and C3 are classic motor run capacitors. I personally use ASC Oil ones 450VAC rated. You can choose what you can get hold. You can use any good quality film capacitors. I like the WIMA DC-LINK ones, they are great.
The output ripple is about 7mV. This isn’t a problem as the gyrator load has a very high supply rejection (PSRR) so no need to go crazy on this. If you
The design is so simple that anyone should be able to build this easily.
Hope it works for you.
Merry Christmas!
A common challenge we all face when building HT supplies for our valve amplifiers is the inrush current at start up produced when the filtering capacitors charge up and blow the fuses. There are several workaround, albeit most of them are not effective. Increasing the value of the primary fuse seems like an easy solution, but is pretty dangerous. The fuse will not blow at start up, however, what is worse, it will not blow at all before any other damaged is already produced in the supply in case of a short circuit or any other issue. If we add some resistance to the secondary, this will drop volts, waste energy and increase the supply output resistance. If we add resistance in the primary, like an NTC, is a much better approach, however we want to bypass this NTC to increase efficiency and performance.
A nice solution is to bypass the NTC (or a resistor) after initial in-rush. A simple circuit is possible to implement using a timer and a relay. The same circuit is used also to apply a longer delay (e.g. 2 min) to turn on the HT supply automatically if you wish. Continue reading “Transformer inrush protection circuit”
I’m a heavy user of fixed-bias output stages. Yes, I do prefer them despite the additional complexity. However, I’m not looking to open a can of worms around this subject. On the contrary, I wanted to report a fantastic product developed by Situbes.
Here is the brief description from the website. I suggest you take a look at the datasheet as well:
The SiTubes DPM is a digital “panel” meter packaged in a standard octal tube envelope.
It measures a DC input voltage from 0 to 2V full-scale. Several selectable legends (V, mA, A, etc.) can be selected by an external programming resistor. It is particularly useful to measure supply voltages (such as plate voltage) and tube bias levels (normally plate current) in tube amplifiers.
The DPM can be powered from an AC or DC voltage. The power input is isolated from the measurement input, so floating measurements can be made up to 1500V above or below the power input. This allows high-side current measurements – for example, sensing plate current directly at the tube plate – or the ability to be powered from a supply that is not referred to ground.
Looking at their construction you will realise instantly the high-quality of this product. Impressive finish and presentation. The tube is made of glass and fits very tightly to the 8-pin plastic base.
I did a simple and basic test on my work bench to test this device and its accuracy. In 5 minutes I wired it on my curve tracer to access the pins easily without soldering a test rig. With a 1Ω 1% resistor I configured the device to current mode and placed my 5½ digit bench meter in series for reference. The refresh cycle is very good, more than what you’d need in normal operation. Accuracy with the reference resistor was great. It’s calibrated as provided by the seller and error was below 1mA up about 200mA which is the planned use case for me. The OLED display has the right brightness for day operation. It’s just great.
You can use them easily to measure anode/cathode current, grid bias or supply voltages in multiple configurations.
They are pricey, but worth every penny. A top quality product which I’m keen to use shortly in one of my next builds.
