Converting Kit/Kat6550 and Kit/Kat34 Rectifier from 5U4 to 6D22S

[Richard]

* Safety Notice *

Be careful. 6D22S uses a top cap cathode connector. If the top connector is removed with the power on, the top connector and lead will still be live from the power supply capacitors AND the exposed metal cap on top of the valve will also still be live, as the heaters will still be working, so the HT will still be rectified from the ac at the anode to dc at the cathode top cap.

Even if you’re familiar with anode top connectors on the likes of PL504 this is different and needs digesting.

WD cannot control what a constructor might do and cannot take responsibility for any accidents.

Part 1 - Introduction


John and Matthew have been hard at work developing a new valve rectifier for Kit6550 which may also be useful for Kit34. A short introduction follows!

Kit6550 uses 2 x 5U4 rectifier valves as 1 alone cannot supply enough HT current. 5U4 is a twin diode valve or “full wave rectifier” so each one in Kit6550 has its 2 diodes connected in parallel, and then the valves are used as 2 single diodes, to make a full wave rectifier with more current possible than from a single valve alone.

This usually works fine with sometimes just the need for balancing resistors to be added if the sections in the 5U4 are not closely matched. This is covered in another FAQ, http://www.world-designs.co.uk/forum...ead.php?t=3859

20 years on from Kit6550’s introduction 5U4 are getting more expensive and a large single diode valve 6D22S is now suggested as an alternative. In several ways this is more efficient and more elegant; 6D22S is a large single diode or “half wave rectifier”. It can pass more HT current than 5U4 and using 2 to make a full wave rectifier can supply Kit6550 easily. As each 6D22S is a single diode there are no balance problems between sections. It warms up slowly making for a more gentle start for the other valves. Its heaters draw less than 2A each compared with 3A each for 5U4. This is useful as the mains TX is unloaded a little so it will use less electricity and run cooler. It is also fortunate that this will cause the 5V heater voltage to rise when unloaded to more nearly match the 6.3V nominal used by 6D22S.

In practice this works very well and is a straightforward change and re-wiring of the 2 valve bases. 5U4 uses the 8 pin Octal base, 6D22S uses the 9 pin B9D Magnoval base which fits in the Octal’s place without metalwork changes. These both require an approx 30mm (1.125") hole with 2 mounting bolts 40mm (1.5") apart. Itmay vary slightly between brands socheck your chassis dimensions before ordering the Magnoval bases.

The attached circuit diagram shows the changes when compared with the Kit6550 circuit. The attached 2 photos ofbefore and after (Part 2 - Construction)show the work in practice. Notice that the new bases are turned 180 degrees to keep the wiring easily orientated to reach the bases. The pins in both cases are numbered clockwise from the underside.