02-05-2017, 12:08 PM
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.
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.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
