Tag: HT power supply

Flexible HT supply

The return of the Jedi

Time has come when breadboarding and HT is not longer an option. With a baby around, I was forced to remove most of the valve equipment. However, the return of the Jedi is closer than you expect. With very little time, albeit at continuous pace when I get a free 30min here or there every weekend, I keep very focused on my objective. The 814 SE monster amp is close to retirement (probably before Christmas) and although I have a fantastic LME amp already working, I need to get the valves back on the scene providing I can prove I’ve complied with “heath and safety” – as my wife well put it.

Well, I had a nice attempt of a wooden frame made out of pine wood. Yes, I’m really bad at woodworking but I’m getting better. The top plates are 4mm thick. I have added a nice set of 2 100mA meters at the front of the frame. A 300VA custom-wounded multi-tap HT transformer from JMS which gives me full flexibility from 100 to 500V in 25V steps. The supply is choke input (LL1638) and filtered with a set of nice 50uF oil caps and 20H chokes. Damper valves used with some nice SiC to form a hybrid rectifier.

Top plates are protected with paper before drilling!

This looks to be quite neat in my view!

The idea is to use this supply for my new 4P1L PSE, 300B and DHT Schade experiments with 6P21S/47 and 1624 valves. A lot planned and so little time for this unfortunately.

Cheers, Ale

 

 

 

 

Transformer inrush protection circuit

Introduction

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”

4-65A SE Amp: A2 grid current supply

Finished today the penultimate power supply of the 4-65a SE amplifier. This one is the A2 grid current one and doesn’t need a lot of filtering as the 46 driver gyrator will do most of that job. The DC stacked design of this amplifier will allow an independent loop of the A2 current between this supply and 4-65a grid-cathode.

Just only one more supply to go and I’m done!!!

40uF and 8uF oil caps
Mains transformer and bridge recitfier
Main switch and bridge rectifier
Choke and output connector