You can't go wrong if you choose a working voltage higher than the HT voltage for polypropylene and similar types.
Electrolytics should be chosen to have a working voltage higher than the maximum they are likely to encounter. This will be easy to determine for cathode bypass and similar applications but for HT smoothing it is important to realise that the HT voltage can rise considerably above its working voltage during the warm-up period. This is particularly important as you are using solid state rectifiers that have no warm-up period of their own. (Valve rectifiers can have similar warm-up periods to the output valves so there can be no appreciable rise in the HT voltage at switch on.)
For the HT electrolytics you will need to know the peak voltage from the HT winding on the mains transformer because that's the voltage the DC HT will rise up to under no load conditions, the voltage of the HT winding on the mains transformer will be given as or will be measured as a RMS voltage so you will need to multiply that figure by 1.414 to get the peak voltage, whatever that figure turns out to be that will be the DC voltage the HT electrolytics will charge up to and be subjected to under HT no load conditions such as the amplifier valves not yet conducting, eg: when the amplifier is initially switched on.
Lawrence.
You will have to excuse my extreme ignorance here. If we take a transformer with the following values do I take it that I should have the caps at a rating of 300v (to allow for rectifications) OR at max main voltage of 240v?
Mains 10-0-200-220-240 50/60Hz
Full Wave rectifier 300-0-300 VRMS 125mA DC
Heater 6.3 Volts Centre tapped 3.1 Amp
Heater 6.3 Volts 1 Amp
(Suitable for Mullard 5-10 mono amplifier)
Re: Post#23:
Hi Rob, multiply the 300 volts by 1.414, that will give the voltage across the HT electrolytics before the amplifier valves start conducting, that voltage turns out to be 300*1.414 which equals approx. 424 volts DC, that means the electrolytics should be rated to at least that voltage, so a rating of 450 volts DC would do.
Lawrence.
Peak voltage of a sine wave is SQRT(2) * RMS voltage. So for your 300V transformer that's 1.414*300= 424V. Since the winding is off load until the valves warm up it may be a little higher. You definitely want 450V (or higher) rated caps for smoothing.
Many thanks for all the help so far - is it wise then to aim at 450v for ALL the caps in the circuit?
I have to say I'm feeling an absolute dunce here - obviously I cannot cram the knowledge you guys have (which is why this forum is so helpful), but can anyone recommend some reading material so that I can get a background to understanding here (I am aware that this may not be straightforward as it sounds).
many thanks
Rob
Any cap that can see full HT needs to be at least 450V rated. This may include screen decouplers that get full HT (via a resistor) until valves warm up. Cathode decouplers can never see full HT (unless the valve is horribly duff) and are often rated at 25V or less. Actual rating to be determined by the designer using ohm's law. If cathode resistor is (say) 1200R and the valve is passing (say) 10mA max 10mA*1200R = 12V. So that's the minimum rating of the cap.
The one common exception to these simple rules is the tone correction cap if one is fitted. One end is connected to the anode of the output valve so it can see full HT plus the audio. An example with 300V HT and 100Vp-p swing ont he anode that cap has to be at least 400V rated. you would normally use a 630V or sometimes even a 1000V part here.
I don't think we need to worry at the moment about TV circuits and very high power amps. Capacitor requirements can get quite tough in some positions.
Many thanks Lawrence
Regards
Rob