Why high voltage in coupling caps?

[TonyTecson]

I knew people had been killed in the 60's due to valve amps.
I wasn't aware it was due to the mains plug and capacitor.
I obviously got things twisted from my little knowledge of the matter.
I apologise.

However I personally would still use a high voltage capacitor on the input.
If a valve gets bashed and the anode hits the grid then there could be a problem.

agree with this too, my reasoning is that i want my coupling caps to be able to withstand higher voltages in a tube amp even in an unlikely event of failure...

my rule of thumb, if B+ is 300 volts or lower, then i use 400 volt dc rated caps, if above 300 volts, i use 600 volt rated caps...

 

[luckythedog]

Yes, for instrument amps a far more common hazard is/was hot chassis due to a mains wiring or filter fault, not least because strings are directly connected to the chassis AFAIK.

Seperately, in that rugged environment amps get dropped, valves sometimes smash or become internally damaged, so I suppose that increases chance of g-a short. Seems sensible to rate input capacitor to safely tolerate g-a short to me.

 

[TheGimp]

I agree that for Guitar/instrument amplifiers it would be good to be sound practice to use a cap with a high voltage rating for the reason luckythedog brings up, mechanical damage.

For stereo amplifiers, I believe it is not necessary, but won't negatively impact the performance of the amplifier since the capacitors used in this position are not electrolytic capacitors.

 

[woody]

Walter Jung had a paper on audio caps. He did a lot of testing and showed that
all other things being equal the higher voltage rating the less distortion. That is say
if panasonic has a line of mylar caps the .1uf 600v one will display less distortion than the .1uf 60v one.

 

[soulmerchant]

With regard to coupling caps, specifically in front of input tubes (12AX7, etc), I often see values of .01uF, .1uF, etc, and voltage specifications of 400~630VDC or even higher.

I certainly understand the need for HV capacity in power stage smoothing caps, and also where HV will be present, such as B+. But I don't understand why the need for high voltage specifications for input coupling caps.

Can someone explain? Thanks!

I have actually seen failures where the coupling capacitor saved an output valve (tube).

Of course I have seen many more failed coupling capacitors recently... especially EXPENSIVE foil and oil ones....

Back in the day, when we all used cheapish polyester caps nobody thought twice about not using ones with high voltage ratings. They were pretty cheap in bulk... and you just never saw coupling caps fail either... it just didn't happen.

Maybe my ears are getting old, but to me ERO MKT1813 still sounds just as good as some fancy oil encapsulted stuff when used in a well designed circuit using DECENT iron. The ERO's tended to FIT well too

Just saying...