Tone caps - what voltage do they really see ?

[chopstuck]

I'm looking for some quality ceramic disc caps for use in the tone stack.
The COG/NPO caps are not available in 600-2KV in every size as they are in a 50v package.
Been looking for hours....

My question is: if the small value (pf) cap is following the coupling cap, does it see much voltage at any time ?
Sometimes at start-up wonky things happen so I'm asking the veterans if they would put lower voltage ceramic reliably in the TS and as treble bypass caps.

[jjman]

In a Fender BF/SF stack it will see the voltage on the plate of the 1st stage. About 200VDC. If it’s truly DC blocked by another cap there should be no concern about dc. AC volts matter too but those are typically low in typical tonestack areas. A specific circuit would need to be referenced to know for sure.

[martin manning]

In the case of a FMV tone stack the treble cap will see DC voltage equal to the tone stack input voltage, which would be either the plate voltage (200-250V) or the cathode voltage (100-200V) of the driver tube, and you can add to that half of the signal voltage (another 100V say) for the peak. The other caps in the tone stack won't see much above the input DC voltage.

If the amp is powered with the driver tube pulled, then the DC voltage will be either zero for a cathode-driven tone stack, or the power supply node voltage for a plate-driven stack (300V or more).

Coupling caps are in the same situation as the tone stack treble cap, but a bright cap on a volume pot will usually be downstream of a coupling cap, and so it will see no DC and at most something like the peak AC signal voltage.

[billc]

Something I've noticed when checking plate voltages on V1 and V2 is that you can sometimes see close to full B+ voltage there when the amp is first powered up until the tubes warm up and start conducting (then the voltage quickly drops to design level) , this prompted me to switch a 200V coupling cap out for a 600V...tone caps may or may not see that depending on the design but figured it was worth mentioning

[martin manning]

Yes, good point. This will happen at every power on if there is no standby used, or if the amp is powered up with no tubes installed.

[chopstuck]

Thanks for the input. I guess I will stay with the higher voltage ratings but may have to get caps that aren't as temp stable as cog/npo or X7R.
any suggestions ?

[LOUDthud]

I saw the guts of a Roccaforte amp recently. It had 3KV Y5P caps.

[Joe L]

Low voltage ceramics are very non-linear and sound pretty crappy. That's why you only see 1kV and up ceramics in amps.

The only amp I know of that uses a lot of low voltage ceramic is the Marshall Silver Jubilee. It uses some 100v axial ceramics that look like resistors throughout the amp even in critical spots. Even the Return effects coupling cap is a multilayer ceramic.

It's easy to make a better sounding Jubilee than the original.

..Joe L