Exactly What Is A PETP Capacitor?

[Grainger49]

I have some KKs that are K73-16.  They are PETP, but what is PETP?  And how good is that as a dielectric?

Thanks in advance.

 

[xcortes]

PTFE

 

[Grainger49]

These don't look like the Russian Teflons, PTFE, they are green on the ends and I believe were sold as PETP.  Sounds like a lot of alphabet soup.

 

[Paul Joppa]

From Wikipedia:  "Polyethylene terephthalate (sometimes written poly(ethylene terephthalate)), commonly abbreviated PET, PETE, or the obsolete PETP or PET-P..."  Think Dacron sails and polyester Nehru jackets. Also called Mylar.

 

[Grainger49]

Thanks, I know PET as the plastic in soda bottles.  Mylar I also know but didn't know they were the same stuff.

Were Nehru jackets Polyester?  (remember them well, didn't have one)

Are they supposed to be good sounding caps?

 

[jrebman]

Mylar is used in many caps, and even sometimes is also found in the paper of some otherwise labeled paper caps.  I believe the Jupiter HT series is actually a paper/mylar blend of some sort, but it is still described as paper.  I believe only the orginal "wax candle" type Jupiters are the only ones that had a pure paper dielectric.  Also, they now say so, but I suspect that the original Audio Note PIOs -- copper, silver and tin -- also had a paper with a certain percentage of mylar in it.  And of course there are dozens of metalized polyester types.  Auricaps, for example.

Have you given them a listen yet?

-- Jim

 

[Doc B]

I think that Auricaps are metallized polypropylene rather than polyester.

 

[Grainger49]

Jim,

I have used them as power supply bypass only.  I will give them a listen in something soon.

Thanks for the information.  It is encouraging. 

 

[ToolGuyFred]

I find that the relative permitivity or dielectric constant is often a fair guide to a material's "goodness" for use in an audio capacitor:

Vacuum	1 (by definition)
Air	1.0006
PTFE	2.1
Polypropylene	2.2-2.36
Polystyrene	2.4-2.7
PETP	3
Polyimide	3.4
Paper (unoiled)	3.5
Transformer Oil	4
Mica	3-6
Glass	5-10

(Information from Wikipedia, ElectricianEducation.com and Goodfellow.)

Hope this is useful.

 

[jrebman]

Dan is right, I mistated on he auricaps.

John, thanks for the table.  Don't see cotton, silk, or polycarbonate on there -- there are some new caps coming out with polycarbonate dielectric now, but I have no idea how they sound.

-- Jim

 

[Grainger49]

Fred,

Thanks, I have some air dielectric parallel plate capacitors.  I probably won't try them though. (Picture turned upside down to make more sense)

Good list!  Funny, it follows much of what I have experienced and heard elsewhere.  My true confusion was not knowing that my new KKs were made of old Nehru Jackets (Dacron/Polyester).

 

[ToolGuyFred]

John, thanks for the table.  Don't see cotton, silk, or polycarbonate on there -- there are some new caps coming out with polycarbonate dielectric now, but I have no idea how they sound.

Can't find any values for silk.

Polycarbonate	2.9
Cotton	1.3-1.4

The polycarbonate & foil caps I have heard were OK but only available in low values.
Cotton will only maintain this value if kept bone dry - some sort of oil or resin would keep the moisture out - but I suspect that would seriously impact the dielectric constant.

 

[jrebman]

John,

Thanks, and I am also looking at this from a wire insulation perspective as well.

Lie Grainger, most of this agrees with what I've heard (though have not heard any polycarbonate caps myself yet)  That said, I'm not a huge fan of teflon on wire, but in caps, I do mostly like it.

-- Jim

 

[Dyna Saur]

Then there are the caps which are not really that  which they appear to be:

Saw this one  over at the www.diytube.com website

/ed B in NC

 

[Paul Joppa]

Yow! That's scary.

Speaking of scary, thanks for the link - I didn't know of that site. Under SC35 there's a thread on standby switches in the center tap of the power transformer - exactly what I've been telling people for years based on theory, and nobody every buys it. But it's true nonetheless - switching high voltage DC will destroy switches. Nice to see that experience nears out the theory!

 

[ToolGuyFred]

... But it's true nonetheless - switching high voltage DC will destroy switches...

My day job involves industrial control equipment. You will very often find a switch or relay contact rated at a given current for 240V AC or 24V DC. Experience also supports this recommendation. If you exceed the DC rating then the insulation bears up OK but the contacts don't last long at all. If you have to switch high voltage DC then use an appropriate MOSFET, IGBT or SCR and only ever for control: don't rely on this solid state stuff where your safety is concerned. If they fail they usually fail short-circuit.

(Edit)
On reflection, probably best to avoid SCRs for amateur use: you can latch them on OK but switching them off while they're passing DC current can be a bit of a chore...

 

[Grainger49]

Ed is that a 35V 2200uF inside the can of a 60V 6800?  I can't quite read the voltage on the outer can.

I have heard of counterfeit capacitors.  This is the first I have seen.

 

[Grainger49]

... But it's true nonetheless - switching high voltage DC will destroy switches...

My day job involves industrial control equipment. You will very often find a switch or relay contact rated at a given current for 240V AC or 24V DC. Experience also supports this recommendation. If you exceed the DC rating then the insulation bears up OK but the contacts don't last long at all.  .  .  . 

I used to work for Alcoa in a rolling mill.  The big motor was 14,000 Hp (an Westinghouse motor built in 1942).  I wish I could get a picture of the DC contractor for that motor.  It was HUGE!  It wasn't allowed to close with voltage across it.  But an emergency stop would open it under load.  It was like a cannon going off over your head.

 

[ToolGuyFred]

The big motor was 14,000 Hp (an Westinghouse motor built in 1942).

I used to work as an application engineer on Mitsubishi variable speed drives. Never played with anything bigger than 500 Hp (still big enough to command a certain level of respect). Even a 20Hp drive makes a fair bang when the power stage fails - not unlike a .22 rifle going off. (I was once standing next to a 20Hp servo drive when it failed - despite ear defenders my manager looked like heart failure was imminent but commented that I didn't even flinch - maybe too much time on the rifle range?)