I added these PCB for sale. I made a small batch for my own use (you can see them in the 300B amplifier here) but am selling the rest of them.
Tag: HT supply
Flexible HT Power Supply (Part V)
I’m now back in business. Building a new 4P1L PSE output stage so will reuse the 300B/4P1L Flexible PSU. I never managed to post an update on the troubleshooting I had to do to get this HT PSU to its optimal state.
The output voltage was lower than expected and the 50Hz component extremely high. Something was wrong. So I traced the issue down. I found a bad solder in one of the rectifier’s cathode. The supply was operating in half-wave mode.
The supply is choke-input with 6AU7 rectifiers (hybrid bridge with FRED rectifiers). The transformer is custom made and has multi-taps for 300-400 and 500V. The tuning capacitor for the choke input is 470μF, then choke is 2.5H into 50uF oil cap. The filtering stage per channel is 20H + 100μF Oil caps;
Here’s a test of the supply at 330V/60mA per channel. It’s very rewarding now to see no 50Hz component and that the ripple at 100Hz is just 4mV (ignore the mA typo on the image):
Flexible HT Power Supply (Part III)
After some further efforts, I managed to complete the power supply. And it works, which is always rewarding when you get it right first time:
Flexible HT Power Supply (Part II)
Here are some picture of my slow progress on this flexible power supply. I started with the layout of the parts before any marks were made on the top plates:
Once I was happy with the layout, I submitted myself to the mercy of the most dreadful job on earth which is metalwork!
300B Amp: the journey starts here
I can’t get away from my big HT power supplies. This time the HT +400V for the 300B amp is here. Lots of Lundahl iron and a lovely hybrid bridge made of mercury rectifiers and damper valves. Yes, I’m in love with the blueish colour of the 816s. Despite they are mercury rectifiers, I know.
I promised the wife I’d get a smaller amp, that was my excuse for the 300B reference system. Still this is not going to be small, however, it will be compared to the 814 SE Amp!
HT Power Supply Noise Measurement
I posted recently a great idea to measure noise levels from our power supplies. Yesterday I managed to put together this small interface circuit. I used a remainder piece of double layer PCB big enough to fit the bulky capacitor, the transformer and the output BNC connector. The input is a just a simple set of copper turrets. Special care is taken in laying out the ground planes to avoid ground loops. Also the transformer is grounded at one side only of the case. A finished interface looks like this:
4-65a SE Amp: HV3 supply
Building the HV3 (+330V Supply)
I could easily say that by now I’m tired of building power supplies. Yes, I’m and fundamentally can’t see the day when I get to fire up this amplifier.
Filament supplies and three stacked power supplies is the price I’m paying to get a completely cap-less and DC coupled A2 amplifier. I guess that my analysis once completed will be made with full perspective of every single implication of this amp: iron, heat and weight. Yes sir, this is a heavy-weight challenger.
I guess that this specific HT power supply design is quite flexible as could be easily reused in many of the projects I have in mind, which unfortunately keep growing.
Good thing is though, I can make these supplies pretty quickly, but don’t do this at home ok? There is serious HT involved. I don’t have pets or kids (but do have a wife) and these supplies should be hidden and away from any poking curious finger.
The final design is similar as the one used before. Could be adapted to choke input, but with the components I had at hand, this supply is very well filtered. It provides only 15mV ripple noise which at 330V is a lovely noise floor around -87dB:
The mains transformers are Weiss (excellent quality) which has screened windings. Instead of having a full Graetz valve rectifier and waste more heat (and use more damper valves), the rectifier is hybrid using a pair of UF4007. Capacitors are oil and polyester ones for the output HF decoupling on each rail. Each channel will feed a Salas SSHV2 shunt regulator which will provide the stable DC reference for the 46 driver stage and bias point for the output 4-65a stage.
Bad news is that, there is one supply left to be built before I can test at least one channel!
New HT bench power supply (+600V)
Here is my first test of modified bench supply providing 600Vdc from existing 250V transformer via a voltage multiplier circuit, gyrator filter and passive MOSFET regulator. This circuit was built with components I had around and supply provides 0-600V with less than 5mV ripple thanks to the 53dB attenuation. Maximum current is 100mA continuous but passive regulator has a 200mA limiter built in.
Above is a classic voltage-doubler configuration. I had a 250V secondary, so had to add one of this to get HT to 600V at least. Resistor equalisers have been added to leverage whatever electrolytic I had around and could use here. The raw supply should provide a low ripple of about 1.3Vpp at full load (100mA).
A classic gyrator to simulate an inductor in an LC stage. Attenuation of this stage and the passive regulator which is configured as a capacitor multiplier can provide at least 53dB @ 100Hz. This will bring down the ripple to less than 5mV!
Finally the passive regulator. It has a current limiter set to 200mA. M2 will dissipate large amounts of heat. Just do the maths: dropping circa 600V to whatever voltage is set as the output by the 1M pot. This will be bolted to the chassis C shaped aluminium frame. Probably needs a bigger heatsink as could melt down when delivering large currents at low output voltage. This is a situation we want to avoid. Output will have a slow fuse of around 100mA, but the circuit has a 200mA current protection to avoid killing the MOSFET whilst charging a capacitor,etc.
4-65a HT +300V supply
First breadboard of the 4-65a SE amplifier. Will start this long project with tuning the HT supplies. Will need a +600V, +300V and +100-150V supplies to be stacked.
Here is a simple breadboard using metal rails over an old wooden frame I had around. Oil caps will be glued temporarily and damper tubes will be fit later:
A snapshot of first test….
