Coupling capacitor voltage rating

It is a narrow margin of safety you have there, in fact there's none!!

Thing to remember is that the capacitor is there to block dc voltage in this case 250 volts, for high frequency ac the capacitor should couple it through to the next valve section - hence the reason its called a coupling capacitor.

So the voltage rating of the capacitor should be above the dc blocking voltage which is 250 volts...

I would suggest you increase the voltage rating of the capacitor to lengthen its life.

John.

Electrman is right. you should increase the rate by 300v or 400v. although dc rate generally greater than its withstand voltage.

In old valve tube set, this capacitor is used to use 300v, sometime [people use 250v as reduce cost.

the signal voltage on the capacitor is not equal to 250+100 ! if it would be resistor load, but it would be not, inductor for instance, or as a snubber, you should choice more voltage value. sometimes iss doulbe.

Thanks for the replies, i see what you guys are saying. What I really want to know is do you have to allow for the ac signal in the voltage rating (VDc) of the cap? Maybe I should email a cap maker like wima?

I guess your meaning.

Can a 250v rate capacitor was applied by a 250v ac signal?

My answer is no. unless you use it as temporary measure at short time.

you should select at least equal or greater then the peak voltage and energy.

E=1/2 CU^2

During the 40 years I spent repairing amps, TV's etc. All the companies I worked for stocked mostly 600V Caps. The only exceptions were Flyback circuit caps (1000 -4000 V) and Buffer Capacitors in 6 Volt Car Radios (1600 Volts). Because 600 Volt was the standard, they were actually less expensive than other values due to quantity pricing. I would not even consider anything under 600 Volts. Be aware that there are a lot of businesses that have sizable stocks of just about any value. Some types of capacitors dry out or change value, some may develop leakage over many years of non-use. Either buy newer product or insist on proper testing of older stock to assure values are still within acceptable tolerance.

If it bursts, its going to cost you a lot to clean the amp up, or even completely rebuild again. Change it out asap.

None of you guys is answering our guest question....

He simply asks if the AC signal voltage should be added to the dc voltage for the purpose of deciding the capacitor working voltage ratings.

The answer is NO, as long as the AC picks are not higher that the max dc positive value.

However all those who recommended that you increase the DC rating of your capacitor, are absolutely right. as you did not allow for any safety margins. AC ratings of a none-polarized capacitor, as is in your case, are usually somewhat higher than their DC ratings, but this is for reasons other than insulation properties of the dielectric material.

your DCVx2 will give you good margin.

Wangito.

I thought my answer #1 gave a clear picture that the ac doesn't need to be added to the dc voltage?

I don't think I could have made it more clear without insinuating that the questioner was an idiot

John.

Agree.

My answer was far too long. I should have said simply, NO.

(and that what we all should have said)

Wangito.

12AX7 is a double triode for preamps, similar to ECC83. Probably it's used as an class "A" amplifier. In this case if the supply voltage is 250 V the voltage on the anode is about 125 V and the AC component will be less then 250Vpp. I would use at least 400 V rated capacitors though.