Using a silicon diode rectifier instead of an AZ31 valve

[wireman]

Do fit the rectifier snubbing caps, they stop any mains buzz caused by the slow turn off (1 to 10 uSec) of standard recovery rectifiers.

Given the price of a couple of resistors I would not ommit them from a capacitor chain, the caps might not blow up straight away but they might go bang after some years of service, especialy if one cap is nearer a heat source than the other. Keeping electrolytics cool dramaticaly extends their life.

With any older live chassis gear an isolating transformer is a very good idea.

[Old Sparky]

Having fitted a 1N4007, with a 100 ohm surge limiting resistor in series, the no load HT voltage is 325v, measured with my DMM, and with 240vac from the mains supply. I therefore need to reduce the HT to somewhere around 200-220v, which, if I've calculated correctly, will need a 5watt 1800 or 2200 ohm resistor, the stated HT current being around 45mA according to the service sheet. No doubt a capacitor dropper would be better, or, of course a new rectifier valve, if one is available anywhere!

[AlanBeckett]

The AZ31 Data Sheet implies that a Series Resistance of around 600R should be about right for that Voltage. The forward Resistance of the 1N4007 will be lower but I wouldn't have thought that much. A bit of experimentation is called for methinks.

Alan

[Alan Douglas]

A capacitor dropper won't work with a rectifier: the cap simply charges up until the rectifier passes no current.

[pwdrive]

Having fitted a 1N4007, with a 100 ohm surge limiting resistor in series, the no load HT voltage is 325v, measured with my DMM, and with 240vac from the mains supply. I therefore need to reduce the HT to somewhere around 200-220v, which, if I've calculated correctly, will need a 5watt 1800 or 2200 ohm resistor, the stated HT current being around 45mA according to the service sheet. No doubt a capacitor dropper would be better, or, of course a new rectifier valve, if one is available anywhere!

That R value looks a bit high to me, are you confusing peak (no load) with RMS ?

Subtract the working HT voltage from the nominal AC mains supply voltage, divide that by the working current to get R, the internal resistance of the silicon diode can be ignored, the resultant R should be around what Alan Beckett has suggested.

Having said that you need to make sure the res. capacitor is up to it, you can always incorporate a NTC R into the series limiting if you want a "soft start"

Lawrence.

[pwdrive]

Can't find the Ia/Va curve for the AZ31 but the AZ1 has the same electrical properties, the Ia/Va curve for the AZ1 is here:

http://www.r-type.org/pdfs/az1.pdf

The curve is per anode, you may be able to work out the forward resistance of the valve from that curve, bearing in mind that in the P5A the forward resistances would have been in parallel.

Lawrence.

[Old Sparky]

I've now completed the repair of this set, which involved, besides replacing the usual waxies, 3 resistors and the blown reservoir/smoothing capacitor, fitting a substitute o/p transformer and LF smoothing choke, the latter two items being obtained from a scrap EKCO/Ferranti, and BUSH VHF 41 respectively!. With a 1N4007 diode as the rectifier, and an 820 ohm 5 watt resistor in series, the AC at the 4007 anode is 208 v., and HT at reservoir cap. (22uf 450v) 200v, with 180v at the Smoothing cap (also 22uf 450v) Whilst these, and other voltages are a tad lower than those specified in the service sheet, I'm inclined to leave things as they are to hopefully prolong the life of the by now 67 year old valves, especially since the radio works beautifully, putting out a little over 2watts into a 4 ohm load from several MW stations ( I haven't yet measured the o/p using my sig. gen!) Everything runs cool after maybe 1.5 to 2 hours operation, and All DC conditions remain stable, with the set out of it's cabinet. Incidentally I've fitted a 2n2 class 'Y' disc capacitor across the 1N4007 to bypass any 'spikes' on the supply. This cap. is rated at 275vac (ca 780vDC if I've done my sums correctly!) which I feel should be adequate for the job. Anyone care to comment on any or all of the above? NB As the AZ 31's heater was O/C - hence the use of a 1N4007 - I've removed the wires from the heater winding on the Mains TX to it's heater pins. Presumably no harm will result from there being no load on the (4v) heater winding?

[Old Sparky]

Further to the above, I plan to replace the exsisting on/off sitch & Volume control assembly with a pot. having a double pole mains switch, so that both L & N mains leads can be switched. As some members will know the original has only a SP switch connected in the Neutral line, which is switched to chasis, this radio using what is basically an autotransformer to provide the HT supply.

[AlanBeckett]

That sounds like a Plan

Alan

[Alan Douglas]

Presumably no harm will result from there being no load on the (4v) heater winding?

None at all.