Question about voltage ratings on bypass cap

[catnine]

When I built the two amp I have I used 500 volt sprauges or F&T and on my AA764 champ I used a 25 uf 150 volt cap on the cathode bypass resister because I had it and it fit . Both amps are no more than 3 to 4 years old and at that time I did not see 25uf 50 volt caps or I may have got them so I used the best caps I could get and what the amps were using when built way back when.

On both amps I used sprague 25 uf 25 vol caps and one 2uf 25 volt on the champ . Somewhere I read people use 50 volt preamp and second gain stage cathode bypass caps . If there is 1.5 to 2 VDC throught the bypass to ground what would be the harm in 25 volt caps or the advantage of 50 volt caps ?

I mean if over 25 volts appears through that cap what is a 50 volt going to save or are 50 volt caps more accurate or stable . Fender used 25 volt bypas caps on their twins and everything else. I was just wondering why , some go for 50 volt caps.

[Enzo]

SOme people have a more involved sense of safety than others. I tend to think more modern converts to tubes suffer this.

In the old days - look on some old can caps yourself - you would see something like 40uf 450WV. WV meant "working volts," and it was a voltage the cap was designed to handle all day long. There would usually be a surge rating, but often it was not printed onthe can. Surge rating might be 500v. That means that if you have a 450v supply, you can use 450WV caps as filters.

Modern parts don;t usually come with WV ratings, though spec sheets may have short term overvoltage performance specs. SO a lot of guys now think that if they have 450v they ned to have 900v worth of cap there. They don;t, but it certainly won;t hurt anything to have more voltage rating than you need. Any more that it would hurt something to use a heavier guage extension cord than you need. It wouldn;t hurt mom's table lamp or alarm clock to plug it into a 12ga extension cord.


Now in your tube stage, the DC steady cathode voltage may be only 2 volts, but what is the cap exposed to? Remove the cap and determine how high the signal voltage could rise on that cathode. Even if the cap doesn;t have time to charge to it, it is still exposed to it - at least potentially. Even so it is not likely to go over 25v.

Now consider the worst case. A dead short plate to cathode in the tube. And let us assume B+ is 300v for sake of argument. You would be tempted to think 300v can hit that cap. I doubt it. First, a cathode-plate short in a 12AX7 is something I don;t recall ever seeing in my 50+ years of tube work. Extremely rare. But let us say it happened. A typical circuit has a 100k plate resistor and a 1.5k cathode resistor. A shorted tube simply connects them together. The cap is across the 1.5k resistor. The two resistors form a voltage divider., so in that worst case scenario, the cap sees what is across the 1.5k. That would be 300v x 1.5k/(1.5k + 100k) = 300 x 1.5/101.5 = 4.4

So even with a dead shorted tube, that cathode sees only about 4.4 volts.

Now that discussion was about your preamp stages. Obviously your power tube bias is larger that a couple volts. But the same thinking goes into specifying a cap. But for me, 25v caps on your preamp cathodes will be just fine - as you noted, a zillio0n Fenders can;t be wrong.