Capacitors voltage tolerance

[Strat76]

Hi everybody.
I am working on two amps projects (5e3 and Vibrolux) and I am ready to order some caps. Especially the small value ones : 10pf -> 680 pf. These are essentially the tone caps, bright caps, ...
On the schematics it is recommended to use 500 V caps but I wonder if it's really necessary to go to this voltage tolerances.
I am ready to order a batch of good caps but their voltage tolerance is only 200V - 350V ...

Do you think I can use these caps in a tube amp ?

Thanks in advance.

[nosaj]

That would be like buying a car and the instructions say use unleaded gasoline and putting in what you thing is better.

If the schematic says 500v use 500v or higher. Your only talking pocket change in the price from 350v to 500v, so why not use what's recommended?

jason

[Strat76]

That would be like buying a car and the instructions say use unleaded gasoline and putting in what you thing is better.

If the schematic says 500v use 500v or higher. Your only talking pocket change in the price from 350v to 500v, so why not use what's recommended?

jason

Because the caps I can order are really good quality caps and unfortunately their voltage tolerance is under 500V.
Trust me, if I could get these caps at the right voltage, I take them.

[nosaj]

Whatever..I wouldn't stray from the schematic's voltage ratings just because they're good caps, they could be dead caps should that voltage go over 200v-350v. Fender was just like any other manufacturer, they were not boutique in the day. They had to keep costs low to make a decent profit, so if they're schematic shows 500v that is what they felt it needed.

jason

[Bruce / Mission Amps]

Be specific... exactly which tone caps in which amp!?? ... with respect to Fender cloning, except for brown-blond-black and silver face amps... there aren't too many places in any tone circuit where there are high DC voltages so I bet you could get away with lower voltage ratings for most all the tone caps.

[Strat76]

Be specific... exactly which tone caps in which amp!?? ... with respect to Fender cloning, except for brown-blond-black and silver face amps... there aren't too many places in any tone circuit where there are high DC voltages so I bet you could get away with lower voltage ratings for most all the tone caps.

Hi,
In the 5e3 these are the two tone caps.
And in the Vibrolux, all the caps what I speak about are in the preamp and the Reverb circuits. Here is a schematic with all these caps yellowed.
I believe that at those places, the voltages are not very strong.
But maybe I'm wrong
Thanks in advance.

[loudthud]

Many times the parts a manufacturer uses are based on availability and cost, not the minimum rating needed for a particular circuit. Another factor is failure rate. When capacitors are used near their maximum ratings, a certain number if them can be expected to fail. At 90%, that failure rate can be half. So when a schematic specifies a 500V part, it is hard to know what factors influenced the designers choice. The ceramic caps used in Fender amps of the 50s and 60s were widely available at low cost. So if a part only required a 50V rating, a 500V part might have been used if the manufacturer stocked a 500V part of the correct capacitance that was used on another product.

A bright cap might only ever see 50V to 100V. The caps in a tweed Tone control, about the same unless you are feeding the first stage from an overdrive pedal. In that case the parts could see over 200V peaks. The caps in a Fender tone stack block DC so they are a little different. If you turn the amp on without using the standby switch, they could see over 500V until the tubes warm up. Most caps rated at 400V can survive 500V for brief periods, but 20 years from now they may start leaking. In fact, the test for voltage of a ceramic or film cap might be 2X rated voltage for 5 seconds. It varies widely between different companies.

So those caps with the 200V to 350V will probably be OK. Use them with caution if they must block DC. Don't use them if you see the symbol of an exclamation point in a triangle. Those are safety critical components that only should be replaced with like rated parts. Consider what other parts might be damaged if the caps fails.

[Strat76]

Thanks a lot for all these explanations. Very clear.

[J M Fahey]

Just curious: what is "a good cap"?
What difference does it have with normally available caps, which *do* stand 500V without even blinking?
Thanks.

[Strat76]

Just curious: what is "a good cap"?
What difference does it have with normally available caps, which *do* stand 500V without even blinking?
Thanks.

IMHO it just about the tone. I prefer certain caps rather than others for their musical quality. It's just my taste.

[Enzo]

Not just the standby switch, but also, if you ever fire up the amp with the tubes removed, those plates in the preamp will rise to the maximum B+, which unloaded could approach 500v as Thud mentioned.

Those 250pf caps can face the B+, the others are themselves protected by other caps and are in grid circuits, so short of a major component failure elsewhere, they will never see much voltage. A plate to grid short could possibly put some voltage on them, but those are extremely rare.

[diagrammatiks]

most of the bright caps and tone caps in an amp will have the possibility of full b+ across them.

i'd use 400vdc minimum.

As everyone else has stated, what types of caps have you found that can be found in the 200-300vdc range.

I'm not aware of any dielectric that can't make it to at least 400.

edit: more specifically on your vibrolux schem...only the 2 250pf capacitors need to be b+ rated.

the others won't have any dc on them unless something fails.

[J M Fahey]

edit: more specifically on your vibrolux schem...only the 2 250pf capacitors need to be b+ rated.

the others won't have any dc on them unless something fails.

Sorry to disagree but *all* caps in the tone controls, such as the .1 and .047uF ones also can have as much as 500VDC as stated above.
In any case, they stand the same DC voltage as the 250pF ones.
The same considerations can be applied to the .02 , .1 and .047uF caps at the plates of the following triodes.

[diagrammatiks]

Sorry to disagree but *all* caps in the tone controls, such as the .1 and .047uF ones also can have as much as 500VDC as stated above.
In any case, they stand the same DC voltage as the 250pF ones.
The same considerations can be applied to the .02 , .1 and .047uF caps at the plates of the following triodes.

op didn't circle any of those other caps.

[tedmich]

check Just Radios polystyrene caps, they are great!
http://www.justradios.com/orderform.xls