Non Polarized Filter Caps

[dpm309]

I am working on putting together a Fender Twin Reverb silver face amp that has been heavily modified.  The owner recently relocated from California to Phoenix and brought me this work in progress to put back together.  He purchased a new set of transformers and choke from Mercury Magnetic that I have already installed.  The fiber boards have been replaced with turret boards as well as the filter cap board.  One of the filter caps is a non polarized, 20uf/450 volt Sprauge Atom cap.  I have attached a picture showing the new filter cap board installed along with the old board.  It appears the old board was recently re-capped with the same 20uf non-polarized cap.  Can a non-polarized filter cap be used in this situation?  I have never seen this before in the power supply.

Thanks,

Dan

[FYL]

Non-polarized caps are basically two factory-formed caps connected in series like this + - - +. They are OK for AC applications but behave very poorly in DC power supplies because their equivalent series resistance is much higher.

I'd change all the caps for some modern elcos - the date codes on the Sprague caps are 1979 and 1983...

[dpm309]

FYL,

The board on top of the picture is the new board with all new caps. The board on the bottom right is the old one.   From what you are saying, I should change out the 20uf non-polarized with a polarized cap.  Is that right?

Thanks,

Dan

[HotBluePlates]

You don't have to change it out, but don't go out of your way to use a non-polarized e-lytic in a power supply in the future.

[TIMBO]

Hi guys,i have wondered if non-polarised caps would be good for cathode biasing as i can get 1,2.2,3.3,4.7,6.8,10,15,22,33,47,68,100,150,220,300,400uf @ 100vDC caps from a local store.These are values that are used a lot. Thanks

[HotBluePlates]

Why use them?

They are generally somewhat more expensive than ordinary electrolytic caps. And if you really want non-polar and are willing to pay more, why not go all the way for a film cap (especially in the lower voltage ratings)?

The only time a non-polar/bi-polar e-lytic makes sense is in speaker crossovers (but you'd usually want a better-performing film cap) and in certain coupling cap locations in transistor circuits. In most filtering or bypassing uses, a normal e-lytic will outperform and cost less.

Unless you have a source that's ultra-cheap (you might then want to ask why they are such a good deal).

[Willabe]

Hi guys,i have wondered if non-polarised caps would be good for cathode biasing as i can get 1,2.2,3.3,4.7,6.8,10,15,22,33,47,68,100,150,220,300,400uf @ 100vDC caps from a local store.These are values that are used a lot. Thanks

Good question Timbo! I don't ever remember this being asked before, as far as -NP- lectro's being used for cathode biasing?

AC & DC at the cathode of the PT/s (when bypass cap is used) but maybe AC goes thru bypass cap only???


     Thanks,      Brad       

[TIMBO]

HBP,they are much more costly but i was hoping that they might be a big plus for amp building and set a new trend in this world of great ideas (just trying to keep up with the Jones when it comes to great ideas  ) Thanks

[FYL]

From what you are saying, I should change out the 20uf non-polarized with a polarized cap.  Is that right?

I would, but this isn't mandatory.

[FYL]

i have wondered if non-polarised caps would be good for cathode biasing

Film caps are really nice there.

[FYL]

AC & DC at the cathode of the PT/s (when bypass cap is used) but maybe AC goes thru bypass cap only???

The cap sees the DC bias voltage - from app. 1V and more in a preamp stage to 10V and more in power stages.

[bluesbear]

I considered using film caps. I don't mind the higher cost; it's not that much. The problem is the size of these suckers! I mean, they're huge! You'd have to put them on the outside (like Fender) but you'd have to have covers custom made. Suddenly, the higher cost is much higher, not the mention the pain in the butt factor.
Dave

[FYL]

You may use ABS boxes as doghouses. Cheap and plentiful.

[HotBluePlates]

AC & DC at the cathode of the PT/s (when bypass cap is used) but maybe AC goes thru bypass cap only???

I mentioned speaker crossovers as a place where you might see a non-polarized e-lytic. In this application, there is generally only ac, and the size of a film cap might be prohibitive.

Then again, a crossover designer of a nicer and more expensive product would juggle the values of the caps and inductors in the circuit to allow the use of a film cap, which generally outperforms any electrolytic.

Stop and consider that the reason different dielectrics exist in capacitors is the size for a given capacitance. The dielectric that results in a cap closest to the ideal also allows only a small capacitance for the area/volume of the cap. For example, tuning capacitors usually use air, but are limited to 10's to maybe couple-hundred pF. Film caps are slightly poorer performers, but you can easily get values up to 1uF at a fairly small size (especially if the voltage rating is low). Electrolytic caps exist because you can get a very large capacitance in a physically small size, but they are the furthest from an ideal cap.

FYL pointed out that there really is no such thing as a non-polar electrolytic cap; they are really a pair of polarized e-lytics back-to-back. Therefore, for a given rating, they have all the sins of a regular e-lytic x2.

[FYL]

Therefore, for a given rating, they have all the sins of a regular e-lytic x2.

And twice the ESR...

[TIMBO]

AAAAAAAAAAAAh, ESR and RIPPLE.  I have a good local supplier of electronic parts and when it comes to caps the range is endless and there is usually a few of the same value that have different ESR and RIPPLE values.What should i be looking for when choosing caps. Thanks

[FYL]

All quality modern caps are OK. Illinois, F&T, Nichicon, etc.