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.

[Richard]

Part 2 - Construction


Parts required;

2 off 6D22S or EY500 valves - 6D22S are cheaper!

2 off B9D Magnoval valve bases preferably ceramic, top or underside chassis mount.

2 off ¼” or 6mm valve top cap connectors ideally ceramic types.

1 off ¼” or 6mm rubber grommet.

About 30cm/12” of preferably PTFE insulated wire.

Valves bases and top cap connectors are available from various distributors and of course the well-known auction site!


1) Obviously power down first and the usual caveats apply about knowing your mains voltage and plus-or-minuses!

2) Remove the 5U4Gs

3) Disconnect the wiring from the valve bases, noting which are heater (purple) wires, which are transformer HT (black) wires, and which is DC HT from the 5U4 “cathodes” to the smoothing choke and positive of C1

4) Remove the Octal bases and fit the B9D bases in their place in the same holes trying to keep the orientation similar with regards to wiring; turn 180 degrees in this example

5) Drill a ¼” hole through the chassis between bases, de-burr and fit the ¼” grommet

6) Rewire the heaters to Pins 4&5 on each valve, and here it is necessary to maintain the pin connections ie pin 4 to pin 4 and pin 5 to pin 5.

7) Wire the Transformer HT (black) wires to pin 8 only, one wire to each valve, do not use any other pin than pin 8.

8) Connect short leads 6”/150mm or so, preferably PTFE insulated wire, to both top caps, pass them through the grommet from the top and connect both to either pin 4 or 5 of one of the 6D22S heater pins, obviously trim length to suit, but not over-tight.

9) Reconnect the feed from HT smoothing choke and CI capacitor to either pin 4 or 5 of the other 6D22S heater

10) With all valves removed plug in power and turn on. Immediately check the heater supply to the 6D22S heaters. It should be about 6 Volts ac

11) If all OK then power down and refit all the valves including the 6D22S not forgetting the top caps

12) Power up and check the 6D22S heaters which should be approx. 5 to 6V AC. Check for 425V AC between each anode (pin 8) and star earth on the PS tagboard/white transformer wire. Check for approx. 475V DC on the cathodes (top caps) with a warm-up time of about 30 secs for the HT to come up to nominal.

Please ask if not sure and feel free to add comments that may help others