500p coupling cap type ?

[Colas LeGrippa]

Hola amigos, what would be  the most reliable cap type  500P /600V to be used between the preamp and the PI ? I  am modding my  2204 clone to build a dual showman head.


(One  of the  guitarists who plays in the woodstock tribute had such a nice tone last week-end with his fender showman head, better than mine with my marshall clone ....but he's not as good performer as I am    hey hey....)


There are many out there: ceramic disks, polyester, silver mica.....
Obviously, the tonal caracteristics is prefered to the longevity .
Thanks for suggestions.


Colas

[tubeswell]

Any type that works reliably

[sluckey]

The original would have been ceramic.

[separateness]

In amps and such higher voltage applications I always use silver mica for pico range caps.  I am not a drinker of mojo koolaid, though.

[Colas LeGrippa]

In the Fender Showman amplifier schematics ( AB763 ),
the normal channel shows a 7025 tube with the first cathode  grounded with a 1k5 resistor, the other cathode with a 820R , while both voltages show 2 V.  How can that be possible ?
Error ?  Fender's 7025 was  mismatched ?
Something I didn't see ?  Leo was  tired that day ? My glasses are dirty ?


I have ordered a 12AT to be used as the PI.
Meanwhile, I put a 12AU7 which sounds pretty good though.


Colas

[separateness]

Because that 820 resistor is connected to two cathodes, doubling the current through it, which in turn doubles the voltage on account of E=IR

[sluckey]

In amps and such higher voltage applications I always use silver mica for pico range caps.  I am not a drinker of mojo koolaid, though.

Neither was Leo. He used what was easily available and affordable. At the time that was ceramic caps. Look at some pics of AB763 amps. They all used ceramic caps for pico values. They were plentiful, cheap, and they rarely failed. I don't know that silver mica was even available at the time.

[tubenit]

I have had maybe 2 or 3 silver mica be problematic in creating unwanted noise. Twice when used as a bright cap on a potentiometer. Not hiss as one might expect but a very annoying buzz problem. Not had that problem with ceramic.


With respect, Tubenit

[separateness]

I'll be, I was promised greater reliability. FWIW I would imagine that ceramic caps are the least audiophool type given how little they cost.

[Colas LeGrippa]

Because that 820 resistor is connected to two cathodes, doubling the current through it, which in turn doubles the voltage on account of E=IR


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You're totally right. I hadn't seen the other
cathode from upper tube connected to it.
Thanks.


Colas

[PRR]

....I don't know that silver mica was even available at the time.

Mica caps go to the 1920s at least; there was less use for small caps before tubes. Mica caps (often silvered) were critical technology in WWII. The good mica came from a conflict zone, Burma or outer India, I forget. There's a 1940s Western Electric study showing that less-good "reject" mica could be acceptable in many uses.

Ceramic studies may have been advanced by mica supply difficulties.

When I was a boy mica was best, and always 500V or more, and not too costly. Lately I notice the price is up, and you can get 50V mica which won't last in tube work.

Yeah, the 500pFd in a Fendery tone-stack or coupling can be 500V Ceramic, because <1000pFd with leads will always be NP0, flawless ceramic. Sizes over 0.01uFd may have coloration due to over-salted ceramic blends.

[Colas LeGrippa]

Here s a pic of old Micamold cap

[Colas LeGrippa]

.....the small one is SANGAMO make