Something I've always wondered about but never had to try.

Old capacitors especially those found in old valve based gear have capacitors marked in 2 voltages, working volts or "VW" and surge volts.

I gather manufacturers had to do this as in old valve gear the rectifier valve often conducts before the other valves and the main HT rail can go quite a bit higher than the normal "running voltage" of the set.

My question is, if a radio had say a 600v capacitor for the main B+ filter cap, and I was to replace it with say a 450 or 500 volt modern one, will the modern cap get cranky when it gets "overvolted" for a while?

I've never had to worry about it much because I could always use caps with a higher voltage rating and be sure all is well, but try and find caps above say 500 volts and they are rare and expensive.

I ask as I've got a couple of radios to do up that have electrodynamic speakers that use the field coil as a filter choke, the input voltage after the rectifier on these sets can go above 600 volts when first switched on.

Thanks,
Raff

My advice is never to go below the VW rating of a condenser ever. If the manufacturer fitted one rated at 600 volts they will have gone to the extra expense to ensure reliability.

Steve Savill has indicated that he is getting more condensers in. (He's been selling them for a long time). There's other suppliers that members sometimes use too that stock caps rated at 600+ volts.

If you feed 600 volts into a 500 volt condenser it'll go bang - loudly. When one blows up it sprays its electrolyte (usually some sort of oily muck) and the foil plates everywhere. Getting the higher values (when needed) is worth the effort in the long run.

The 525SV cap was the mass production cap used in nearly everything. The 600V in sets above the most common 250V volt rails.

Modern caps rarely list surge volts. Filament rectifiers without a cathode sleeve (80) Silicon Diodes & Metal Rectifiers, will always produce a surge voltage and the common generalization was twice EMF, prior to the heater tubes conducting. Filament tubes will conduct quicker.

I will not go under 500V on a 5Y3 and in some PYE / Astor sets & others where the 6V6 was run at maximum plate voltage, you would see 600V types. Do not forget that with 250V rail the voltage at the cathode may be around 300.

Shop around 600V are still out there, but don't buy to cheap. Some of the Sprague's are also physically big, but high current. Some of those 450V RB's I have had fail in no time on a heater rectifier and their ripple filtering is not that good. There is also a film type that some US restorers are using as they do not have the polarity & aging issues of electrolytics.

Marc

Another way to get there is to have a pair of caps in series. Then you can use caps with lower voltage ratings, at least more than half the B+ plus some safety margin, say at least in total 65% of B+. The individual caps' capacitance needs to be twice the desired amount you want seen by B+. Two rules to follow: use a pair of the same kind and capacitance and voltage rating, and wire in parallel of each cap a resistor say 470K 1 watt. This resistor will help keep the voltage across cap to be half the total voltage of the B+. The resistors will also help bleed off stored voltage charge after the set is turned off, helps with safety when working on the radio.

Heathkit CRO I have uses that method for EHT. Do not forget the equalisation resistors.

The best way to avoid the need to worry too much about residual voltage, I find is to get it right first time.

Marc

Thanks for the input guys / gals.

Odd thing is that the old Desoto (made by AWA) car radio I just repaired had several 600 volt capacitors in it.

The highest voltage I measured anywhere in the set was around 300 odd volts DC.

It makes me wonder if higher voltage caps were specified for other reasons like perhaps leakage current value or ripple current ability.

I have read somewhere that the capacitors used to tune the vibrator power supply that are across the transformer are quite critical in voltage rating and capacitance to minimise sparking of the hard working vibrator contacts.

In any case I used capacitors as close to, and often in excess of the voltage ratings of the originals.

So far the old girl is working once again.

I just have to locate a better speaker for it, the old one sounds bad.

Raff I still have some 600 volt electros left my stock is dwindling as I only get a 600 volt stock each two years, as they are expensive but I like the best, as with 600 volts you have plenty of room to move !

I still have a few speakers too !

What needs to be looked at comes down at times to inventory and when it was made. After the war there was a lot of surplus that went cheap & needed to be liquidated.

Some equipment seems to have been made to take advantage of this & I suspect EF50 is in that category. The majority of the old common "universal" electrolytic caps had surge volts of 525V (SV). It would be cheaper to mass produce like this.

The car radio valve 6X5 will not produce the same surge as a filament tube like 5Y3 so a 600V cap would be excessive on a "B" voltage of 250V.

With other non polarised types, we have gone to a similar situation as the old Ducon's by getting rid of voltages after 100V and making 630V which cuts the inventory & costs considerably.

Marc