Replacing electrolytic capacitors with plastic film capacitors....

[Jack_Hester]

Our music leader at Church scanned and emailed an article to me about this topic, authored by a fellow named R.G. Keen.  After reading it, I located the same article here:

https://www.premierguitar.com/Magazine/Issue/2008/Oct/The_Immortal_Amp_Mods_Pt_4.aspx

It was very interesting, and I did not find a similar discussion here.  If it has been discussed, please post a link to the thread(s).  Is this a common practice that I should do up front on a build?  Please elaborate.  Thanks.

Jack

[kagliostro]

Related problems are:

1) availability of adequate values

2) price

3) required space

for little value I've read more than one time that this mod is a good mod

Kagliostro

[Geezer]

Related problems are:

1) availability of adequate values

2) price

3) required space

for little value I've read more than one time that this mod is a good mod

Kagliostro

(1)Solen makes the adequate values, but they

(2)cost very much more than similar value electrolytics that are readily available, and

(3)are HUGE in comparison to the same value electrolytics.

If you have the space & extra cash, go for it.

I think I have read that a film cap of smaller value placed parallel to the electrolytic can give some of the same "benefits"......anyone else have any info or opinion on this? Or is there any benefit at all?

G

[kagliostro]

About the use in PS also I have read about the benefits of using a small capacitor in parallel with the electrolitic

I've also seen it in some Made in China Vox AC30 PS schematics
(may be they know the quality of their electrolitics ..............)

about the use in parallel with the cathode bypass cap
don't know if this can be a good pratice may be it is

Kagliostro

[Dave]

Something tells me that they would not work well as reservoir caps. E-caps charge up to operating voltage and help to keep operating voltages relatively constant with changing current demands. Film caps would not "store" extra umph for those needed moments.

I'm thinking more sag and an overall squishy power supply.

Dave

[sluckey]

Film caps will charge (and discharge) exactly like the electrolytic caps. There is no magic extra umph storage characteristic associated with an electrolytic cap. They are physically very different but electrically they behave the same. It's all about applied voltage, resistance, capacitance, and time.

Here's a good reference...     http://www.electronics-tutorials.ws/rc/rc_1.html

[kagliostro]

They are physically very different but electrically they behave the same. It's all about applied voltage, resistance, capacitance, and time.

If I'm not wrong and don't remember badly

an important difference between the two different capacitors

is that electrolytic have (with the age) less reliability and accuracy than film caps

Kagliostro

[rzenc]

If it is possible to use film caps instead of e-caps, I use them. I built some amps using Solen Fast Caps and indeed they are huge. I think film caps has an advantage over e-caps since they will not leak goo inside the amp when they need to be replaced.
I did not perceive any sound difference between e-cap X film caps when they are used on the P.S., however I did notice a difference when using film caps as cathode bypass.

With Respect

Rzenc

[Dave]

OK, I may be wrong, but I didn't say "magic". :occasion18:

Dave

[sluckey]

I know. I said that rather than use "quotes". Hey, it's Mardi Gras, a magical season! 

[Geezer]

I know. I said that rather than use "quotes". Hey, it's Mardi Gras, a magical season! 

Nice recovery there...........

[plexi50]

I know. I said that rather than use "quotes". Hey, it's Mardi Gras, a magical season! 

Nice recovery there...........

I never drink except when it's Mardi Gras. Tonight is Mardi Gras isnt it? Thats what happens when Robin Trower is blowing my windows out. GOD what a great soul machine he is:

Personally i am so used to electrolytics where they go that i would be affraid to change my thinking and use them in high voltage spots

Then again it's all pretty much high voltage to me. Madi Gras is such a thing. Ha!

[tubeswell]

@ Jack_Hester

R.G. Keen is a regular over at the ampage forum (Music Electronics Forum) where his handle is R.G. http://music-electronics-forum.com/users/46/. There you go - now you can PM him and ask him about it for yourself.

[38Super]

About the use in PS also I have read about the benefits of using a small capacitor in parallel with the electrolitic

It is good practice in electronic design to bypass your PS electrolytics with a good quality 0.1u or 0.01u.  Electrolytics can have inductive qualities and do other unpleasant things at high frequencies.  Many don't consider that the ESR in a filter cap creates a zero in the PS frequency response, so from a filter perspective, there is a breakpoint (corner or pole which rolls OFF with increasing freq) based on the filter capacitance and resistance or inductance that is "sees".   But at some higher frequency, the gain flattens back out (a zero rolls UP response with increasing freq)) due to effect of effective series resistance (ESR) of the filter cap.  So this zero created by filter cap ESR counters the pole created and your filter cap and inductance/resistance and frequency response flattens back out at some higher frequency.  This analysis refers to a single filter stage.  Obviously HF rejection is greatly improved with cascade filter stages.

The short answer is a quality (low ESR) 0.1u ceramic, poly, or paper and snake oil cap will give you improved HF rejection in your power supply.  But you ask, why do I care about anything but the FW rectifier 120 Hz frequency?  You care because you want the power supply to reject 120Hz noise AND high frequency noise - this will help you control parasitic oscillation.

cheers,

rob

[Jack_Hester]

@ Jack_Hester

R.G. Keen is a regular over at the ampage forum (Music Electronics Forum) where his handle is R.G. http://music-electronics-forum.com/users/46/. There you go - now you can PM him and ask him about it for yourself.

Thanks for the link.  I signed up over there in September of last year ('10), but it must have been to have access to some info for members.  I didn't bookmark it, so your link probably sent me back for the first time since then.  It's bookmarked, now.  But, there are only a couple sites that I frequent (this being one of them), as I'm a bit overwhelmed by it all.  Takes a while to digest, when you've been away from tubes for way too many years.  I'm enjoying the ride.  Thanks, again.

Jack

[Jack_Hester]

Film caps will charge (and discharge) exactly like the electrolytic caps.

Are these types marked with polarity? I don't mind experimenting and using typical electrolytics, but in a final build (that I think will remain as-is for a while) it's a coin toss as to whether economics is an issue.  And, I can allow for the additional size, that several posts have alluded to as being of concern.  My builds will be small and few, while my learning/relearning curve increases.  Thanks.

Jack

[Willabe]

Film caps will charge (and discharge) exactly like the electrolytic caps.

Are these types marked with polarity?

No, film caps are not.


            Brad           

[G._Hoffman]

Looking at the Mouser site (sorry Doug, but you don't carry the parts for comparison), a 10µF 50V film cap is going to cost about 10X as much as a 10µF 50V electrolytic cap.  Now, given that all the amps we make, basically, are based on amps that were made as cheaply as - well, I don't want to say as cheaply as possible, but as cheaply as Leo Fender could get away with - we can easily see where the use of electrolytics came from (though, to be fair, I'm not even sure they could make film caps with values that large back in the `50's).  Leo Fender made some great stuff, but he was always happy to save some cash on parts or labor so he could make his price point or improve his profit margin. 

If I remember right, the explanation I've heard is that the value of electrolytic caps starts to drop at higher frequencies, where film caps do not.  So, by using small value film cap in parallel with the electrolytic cap, you give the higher frequencies a way around the electrolytic cap.  But, if you think about it, that is going to be a lot more useful on a cathode bypass cap - where you have audio frequencies - than on a PS cap where the major frequencies you need to work on are 60Hz and 120Hz (or 50Hz and 100Hz), and are well below the frequencies where electrolytics start to drop off.  However, since the caps where it makes the most sense are also the lowest voltage caps in the amp, which will also be the least expensive to replace (filter caps), you may as well give it a try.  Assuming you don't mind spending $5 on a cap, that is.  But as has already been said, a 15¢ film cap bypassing the electrolytic is going to do much the same thing, so you might as well try that first.

Keep in mind, though, that I am mostly just parroting something I've read, and which I may not be remembering correctly.


Gabriel

[sluckey]

I don't recall seeing any 10µF film caps back in the '50/60s. Doesn't mean there were none. Leo may not have had a choice. I believe he would have still chosen electrolytics based on cost/performance.

[Quatro]

however I did notice a difference when using film caps as cathode bypass.

Can you describe what the difference sounded like?

[G._Hoffman]

I don't recall seeing any 10µF film caps back in the '50/60s. Doesn't mean there were none. Leo may not have had a choice. I believe he would have still chosen electrolytics based on cost/performance.

Unquestionably.


Gabriel

[LooseChange]

Arn't these white cathode caps (25uf) plastic?  I see these in many Fenders I think as early as 1969?
These must be cheap or CBS would have not used them.

[kagliostro]

Arn't these white cathode caps (25uf) plastic?

To me they looks like normal electrolitic caps

Kagliostro

[HotBluePlates]

I've used Solen caps in a tweed Deluxe build. The primary reason I did was that the filter caps would never have to be replaced.

I can't say I've noticed any qualitative difference in the sound of that amp (in which I used a lot of expensive audio-fool parts) and another tweed build that used all the typical cheap stuff we ordinarily buy.