All caps at least 600V? Really?

[boobtube21]

I'm replacing the 250 pf treble cap on my UL Twin with a silver mica...supposed to make it sound better, we'll see about that...

Anyway the schem calls for all caps to be rated at least 600v, is that really necessary in the tone stack? I'm seeing caps rated at 250V in the power supply section... I haven't done any volt measurements at the treble cap itself yet, cause I don't want to bother taking the amp apart until I've got my parts...just lazy I guess.

Anyone know offhand if it's safe to use 500v rated silver mica caps here? That's all I can find on the Mouser site. Thanks!

[cdw2000]

I'm not sure which exact model you have, but I just looked up the Vibroverb 135W UL schematic and it says the same thing, however the DC plate voltage of the preceding preamp stage is 250V and the B+ voltage to that stage is 365V so a 500V capacitor should be adequate in this location. I wouldn't go less than that though, you always want some margin.

[boobtube21]

Thanks cdw. I actually found some 600v micas at Antique Electronic, and forgot I started this thread. I have a '78 UL 135w Twin.

I did see the 365V headed that way, then saw the 100k resistor...doesn't the tone stack also see the same voltage as the plate? It looks like they're being fed from the same voltage source:



I'm no amp tech though, so I don't usually know exactly what I'm looking at.

[cdw2000]

Yes. DC-wise the capacitor sees the 250V on the plate of the previous stage. But the amplified instrument signal rides on top of that DC-level with a voltage swing that is equivalent to the input peak-to-peak voltage times the gain of the preamp stage, so without doing a detailed analysis, the voltage could theoretically swing close to the 365V B+ voltage (given a large enough input signal). So to be safe that is the voltage you would want to have some margin over.

You will notice that there is no AC-coupling capacitor before the tone stack, that's because the three capacitors in the tone stack perform that function, so they should all be rated to have margin over the +365V volts. Another way that thrifty Leo saved on parts

[boobtube21]

Ahh, gotcha. Thanks for clearing that up cdw.

[steve25]

Also in as far as power supply filter caps go, a cap ( or caps as in serise) with twice the voltage rating over what is needed will last twice as long, unlike the myth that I have heard that running a electrolytic type cap up near it rated voltage helps them last longer.

[cdw2000]

That's very true Steve and should be pointed out. Electrolytic caps are a different beast and should always be derated in voltage since the life of an electrolytic capacitor is inversely proportional to the voltage applied.

You can even find life calculators online from some of the larger manufacturers, such as this one from Illinois Capacitor:

http://www.illinoiscapacitor.com/tec...alculators.asp

Interesting though, that this website indicates the voltage derating is more important for film capacitors. So I guess those 630V rated film caps maybe worth the extra cost.

Silver Mica caps should last your lifetime and beyond as long as the voltage rating is never exceeded. I would never exceed 80% of rated though, just to have a safety margin.

[steve25]

Also the a/c voltage rating of the cap for coupling use from the PI to output tube grid is something else to ponder as those 400 volt dc rated caps are being pushed kinda close in some amps that do not use 6V6s OR el84s!

[yankeerob]

Also in as far as power supply filter caps go, a cap ( or caps as in serise) with twice the voltage rating over what is needed will last twice as long, unlike the myth that I have heard that running a electrolytic type cap up near it rated voltage helps them last longer.

You hear all sorts of crap like that - one guy at a show was trying to tell me they last longer if you remove the drain resistors too... what a meatball... I routinely fit them to the 1st stage to make sure the supply is always drained after use... don't know if it'll extend the life but I'm sure it doesn't hurt...

Another thing worth spending a bit extra on is higher temp ratings as well - Zifa 125° rated EL's are worth their weight in gold in this respect but unfortunately cost nearly the same - we're now using F&T 105°'s on all production models and have found they're an outstanding compromise - they really stabilise the supply and the result is an even quieter amp... I wouldn't believe it if I didn't know it was true...