Improving the DC Heater Supply - Part 1; KIT88, KEL34, KEL84

[Richard]

KIT88, KEL34, KEL84

These amps are built on PCBs so the following way of making up "daughter" boards was found easiest. There`s quite a bit to it but if you fancy a go it should be worthwhile. In any event it`s not expensive and can be constructed off the amp and only fitted if you're happy with your handiwork. Forgive me if I take it slowly; you may only need the part numbers and diagram.

We can replace the heater bridge with schottkies which have much smoother switching and low EMI output. This interference then won`t go forward into the circuit via the heaters, nor backwards into the transformer to be capacitively coupled to the HT winding.

However the schottkies are more efficient. Just replacing the bridge will cause the heater voltage to rise by nearly 1v. This is no problem and can be reduced with a single resistor between the new bridge and board. In fact we can use this to our advantage and use a second smoothing cap with the R to make a pi filter. This will reduce heater supply ripple and therebye hum and noise at the output.

In KIT88 this reduces heater ripple to a seventh and was found to reduce hum at the amp output to a third of the previous level. Using a 1R resistor for KEL84 will reduce ripple to a quarter.

You need, in Maplin numbers;
4 x JA49 1N5821 or AL08 1N5822 diodes
2 x VH58 4700uF 35v (25v) caps
1 x JP53 board
1 x W2R2 (W1R for KEL84) resistor
1 x BL13 tinned copper wire or similar

The board can be broken to size easily after scoring along a line of holes with a Stanley knife. Bend diode legs and position as a square on board. (Legs are thick, you may have to drill board holes larger. Bend by holding leg near body with pointed pliers and bend leg don`t put pressure on body. Try to work static free by touching ground regularly or wearing an earth strap. Don`t wear your woolly socks on a nylon carpet!) Fit cap and resistor to board and 4 tinned copper legs. Play around with the positioning on the board to make the arrangement fit the board and the legs come out right. Sleeve any wire which may short on any other and solder up. Change C22 on board for the other higher voltage one. Replace existing bridge with yours.

The only variable to check will be the heater voltage. I found a 2R2 resistor was right in KIT88, which should be the same for KEL34, and others have reported 1R is correct for KEL84 but you may need more or less to set heaters to 6V - 6.6V. It is possible that the 1.6A mains fuse will blow at switch on due to the extra surge; I use 5A. If that blows you`ve made a mistake so recheck.

Do read the diagram the right way round regarding the "+" end for the line of 4 holes. Because of space it`s harder in KIT88 and the R ends up behind the board in the photo but it`ll give you the idea. You should have room in KEL84 to fit all the components on the board and make a neat job of it.

*** BR2 and BR3 numbers were transposed between the circuit and the pcb in KEL34 and KIT88 amps. The heater bridge is the larger one with all the pins inline as shown in the photo in the FAQ. It is BR2 on the circuit but labelled BR3 on the pcb. ***