Capacitor type for tube preamp

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I recently bought a 12ax7 Baxandall tone control PCB from eBay. I'm a confused when trying to obtain capacitors as there are so many type and not sure of the required voltage. As the HT is 250V I'm going to get 400V+ (which ever is cheaper) to be on the safe side.
What type of cap is appropriate - wima, map, mks, mkt, ceramic disc, CBB etc.. For
100pF, 4700pF (tone control)
O.1uF, 0.22uF, 0.33uF(coupling)
How to prevent microphonic effects?

I would have probably chosen some polypropylene pulse capacitor and used it without thinking more about it, but after microphonics were mentioned, it is true that those are microphonic, so something should be done to prevent unwanted effects.

The capacitor could be first soldered to a dedicated small PCB and attach that to the main PCB with some elastic attachment components (intentionally not specifying what kind) and flexible wires.

Other good ideas?


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Electrolytics should be replaced if they're old and weren't used much (the applied voltage apparently maintains the integrity of the dielectric over time, and lack of it allows deterioration). With regular use, I've got many electrolytics that still work great after 30 years. Some ceramic caps are significantly non-linear and temperature sensitive. I only use those in the power supply circuits (a .01u 3kV across the AC line and across the primary and secondary power transformer winding). Most plastic dielectric caps are fine. I try to use polyprops or polystyrenes when I can, and go with other plastic dielectrics (mylar etc.) when I can't get what I need in PP or PS. The bigger issue might be the manufacturers. Most are fine, but there may be some "boutique" brands that don't really know what they are doing. In a tube circuit, make sure the voltage rating is more than enough at turn-on, when B+ is significantly higher than when the tubes start conducting, and load down the power supply.
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