Capacitor type for tube preamp

[cl1238]

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)
Thanks

 

[rotaspec]

Polystyrene would be the best for tone caps.
Polypropylene (mkp or mpp) or paper-in-oil for coupling are good but you may have to check the size will fit your PCB.

 

[BinaryMike]

Polypropylene film is generally best for most positions. Polystyrene is an excellent choice for tone controls if available. Polyester (PET) is okay in audio applications only if the voltage rating is far in excess of requirements.

 

[ericj]

Between different types of poly film caps, I find that dogma about the dielectric is largely just dogma. There are good and bad caps for each design with each dielectric.

Wima caps are good, but not as good as the hype would suggest.

 

[JonSnell Electronic]

A coupling capacitor from the anode to the tone control should be able to stand HT, the rest is not important. Polystyrene are best suited. Don't waste your money with oil and paper, if they were that good, they would still be making them.

 

[DF96]

What type of cap is appropriate - wima, map, mks, mkt, ceramic disc, CBB etc..

Wima is a brand, not a type. Avoid ceramic disc, as they are usuall high-k and so nonlinear. For most purposes polyester/mylar are fine for coupling. Polystyrene are good for filters, as are low-k ceramics (e.g. NP0, C0G).

 

[crazyfrog]

Lot of people building tube amps only swear by PIO caps. However, for HV power supply, oil capacitors can stand high DC (motor run type) at a very decent price.

 

[HollowState]

If you want to make an informed and intelligent choice about which capacitor to use where, read this article.

 

[Nikolas Ojala]

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?

 

[ericj]

No love for mica?

.:Sent by pneumatic tubes

 

[Nikolas Ojala]

No love for mica.

No love for mica?

Do you like hysteresis?
Ref: The Sound of Capacitors

 

[cl1238]

Thanks everyone for the info.

 

[Bob Richards]

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.

 

[ericj]

Electrolytic capacitor chemistry has come a long way in even the last 20 years. It generally makes sense to replace them.

.:Sent by pneumatic tubes

 

[juanitox]

No love for mica?

.:Sent by pneumatic tubes

i prefer silver mica than Polystyrene in coupling application more natural Highs for me .. cheap to try

 

[dotneck335]

DIY Audio guru Rod Elliott says:
"Once someone starts sprouting utter BS about the "audibility of capacitors" they are no longer credible.*There is no credible evidence that any film cap is aurally different from any other."

 

[Gnobuddy]

..."There is no credible evidence that any film cap is aurally different from any other."

-Gnobuddy

 

[SY]

This is true IF the corner frequency is well below the lowest frequency of interest, and that none of the caps are pathological (e.g., "hand made" with PTFE dielectric).

For filtering, it's plausible that some caps might sound different.