Polystyrene capacitor as coupling in output tube stages

[aldovan]

Friends

Anyone have experience with polystyrene capacitors as coupling capacitor in output tube stages ?
How it sound ?

Thanks,

Aldovan

[Geek]

Hi,

Haven't tried myself and here's why... polystyrene's are inductive by nature. This could cause stability issues in some designs/layouts.

Ones that big in value are also pretty pricey.



Cheers!

[aldovan]

Inductive ? Why ?
Do not is a film capacitor as polyester, per exemple ?
I never see a post about this problem.

Thanks,

Aldovan

[Eli Duttman]

MultiCap RTX series parts exhibit very low ESR/ESL. So, inductive destabilization is not a problem. However, along with the substantial cost, polystyrene film is sensitive to heat. The environment near power O/P tube types is usually quite hot.

IMO, polystyrene dielectric caps. are best saved for use in comparatively cool environments. That's not a happy decision, when the fact that only PTFE is more transparent than polystyrene is taken into account.

[kevinkr]

I used MIT and then REL RTX (polystyrene and foil) caps in my power and pre-amplifiers for years, and thought they were great until I heard some similar Teflon types which were clearly better. I thought the RTX were a bit bright sounding, but in terms of resolution were better than the CDE and Mallory film caps I had used previously, and also sounded much cleaner as if a layer of grunge had been removed.

I never had any heat induced issues with these capacitors, I just found something I liked better.

These days I use mainly Vcaps, REL TFT (Teflon), one of the various surplus Russian made Teflon coupling caps people have raved about or transformers. All good.

[Geek]

Hi,

Originally posted by aldovan
Inductive ? Why ?
Do not is a film capacitor as polyester, per exemple ?

Polystyrene are leaded at opposite ends and rolled while polyester and polyethylene are rolled then have the leads welded, eliminating the inductance.

Take a look at the (axials show this best) polystyrene caps in a tape recorder or RIAA stage. Notice the leads are offset?

I've neasured as much as 20uH inductance on a new manufacture 3000pF polystyrene cap. A coupling cap like you're talking about, say 0.1uF, could be has high as 100uH.

Originally posted by aldovan

I never see a post about this problem.

Because they are used in stages like RIAA or precision timing or bias oscillators where a few tens uH in line is moot for the impedance levels involved.

MultiCap RTX series parts exhibit very low ESR/ESL.

I'll have to check those out Eli, thanks for the heads up!

However, along with the substantial cost, polystyrene film is sensitive to heat. The environment near power O/P tube types is usually quite hot.

x2. I forgot about that fact.

Cheers!

[aldovan]

But the polyester and polystyrene do not have the same construction ? Both do not have a film, a isolator and other film ?
I do not see differences . Polyester have a inductance, too.

Aldovan

[Poindexter]

I have recently started using polystyrene/tin foil couplers in my power amps.  They have a combination of transparency to fine detail and tonal richness I have heard in nothing else.  Mind you, I have not tried the megabuck teflon caps.  They are not terribly expensive, but they are big and heavy enough to be a mounting problem, and do not come in high enough values for my preamp output.

Poinz likes 'em.

Aloha,

Poinz
AudioTropic

[kevinkr]

Originally posted by aldovan
But the polyester and polystyrene do not have the same construction ? Both do not have a film, a isolator and other film ?
I do not see differences . Polyester have a inductance, too.

Aldovan

Regardless of construction similarities or differences polystyrene is a far better dielectric than polyester. (DA/DF) It's quite audible which is why of all of the plastic dielectrics I now prefer teflon even to polystyrene which was my preference for about 15yrs or so. Exotic teflon types are just so expensive which is why I waited so long to make the switch. Other than the eq caps in my phono stage every coupling cap in my system is now teflon. (Exception: The X-O caps in my system are polypropylene foils and are the best I can "currently" afford.)

[MRupp]

Polystyrene are leaded at opposite ends and rolled while polyester and polyethylene are rolled then have the leads welded, eliminating the inductance.

There are obviously 2 types of polystyrenes, the "laid-in" type with the connectors wrapped in with he foil, and the "extended foil" type where the leads are welded to the foils at both ends of the cap, much in the same way as other types of capacitors.