Ekco U122 dropper.

[nostalgia1]

Paul Stenning in his dropper calculations article cites a worked example for the mains dropper using the same valve line up as the Ecko U122- UCH42, UF41,UBC41 and UY41.He calculates the dropper to have a resistance of 1230 ohms.THe set I have has part of its dropper bridged by 2 resistors in parallel and maximum resitance from end to end is 885 ohms.The thermistor is in series with this.I would like to add a diode in series to cut power loss. It looks as though this radio has been run with 885 +thermister in the past(perhaps on a lowervoltage).Should i add a parallel resistor to the dropper to bring the resistnce tro Pauls calculation with a diode of 527.3 ohms ?.Also can the diode go directly in the mains feed ,or in the heater chain therfore avoiding the connection to the rectifier valve anode?What is a typical power rating for a HT smoothing resaistor.The Ekco has a 3.3K in the circuit diagram?

[ms660]

For say 240VAC mains and using a diode dropper the series resistance needed taking into account the working resistance of the original thermistor would be approx. 274 Ohms rated at 5 Watts minimum, that's with the anode of the dropper diode connected to the receivers switched mains live.

For the valve rectifier surge limiter resistance I would suggest a value of 330 Ohms to start off with, also rated at 5 Watts minimum, that resistance being connected between the anode of the valve rectifier and the receivers switched mains live.

I would also suggest fitting a crowbar diode to protect the valve heaters should the dropper diode ever go short circuit, the crowbar diode (D2) should be connected as shown in the attachment, the mains in that circuit being the switched mains from the receivers on off switch, for the fuse shown I would suggest a 1 Amp quick blow.

Note that in that circuit it only shows 3 valve heaters but that's of no consequence.

For the 3.3k HT filter resistor in that receiver I would recommend a rating of 2 Watts minimum.

As a general rule it's good practice for a resistors power rating in such situations to be at least twice the calculated power dissipation.

Lawrence.

[paulsherwin]

Given that diodes cost just pennies, it's also a good idea to use at least two in series in addition to the crowbar to provide belt and braces protection against short circuit failure.

[nostalgia1]

Thank you both for your replies extremely helpful

[ms660]

No problem.

Also note that in the receiver schematic with the mains input set to 230-250 VAC the 128 Ohm section of the mains dropper will have the rectifier current flowing through it as well as the heater current, the former typically being around twice the HT load current.

Lawrence.