100uF Motor Run Caps

[AndrewT]

you must have very good reason for using an electrolytic motor run capacitor in audio equipment. Cost is not a good enough reason.

Use the metallised plastic film motor run caps, if you need high capacitance AND high voltage..

[piero7]

electrolytic motor run capacitor? Huh?

They are NOT electrolitic caps!

[AndrewT]

Originally Posted by piero7

electrolytic motor run capacitor? Huh?

They are NOT electrolitic caps!

you are right, they are not solely or even electrolitic.
They are available in plastic film and electrolytic.

[Matt.B.H.]

He, He,
There are two types of motor cap. Motor start and motor run. Back in the old days I have seen electrolytic motor "start" caps. They are only in circuit with an aux. winding until the motor gets up to speed. This is controlled by a centrifugal switch. the dissipation in these old buggers was massive and they had a very short life. I have not seen one for maybe 10 years. Most if not all surviving examples are long dead or replaced with oil filled caps (perhaps due to better plate etching allowing a higher unit volume/capacitance). You dont really see the oil filled ones often now apart from high reliability apps. The older and maybe more resilient paper/foil/oil boys only live longer because a fault causing arcing between plates will in a way self heal. This of course also causes a slight decrease in capacitance and in some cases a considerable decrease in working voltage, this is of course no real worry in a valve amp (unless you are using the cap. beyond its voltage rating)
Anyway all new motor type caps(this is a broad brush) are polly/foil and as far as I am concerned are very good caps. As for being inductive or not, show me any large value cap that is not inductive? (Evox Rifia PEH169 electrolytics being rather good, but still not a "pure cap")
Someone wrote about AC caps being 1.41x rated DC. This of course is correct. I appoligise for not quoting directly but I cant see the post.
Cheers Matt.

[kevinkr]

Originally Posted by Matt.B.H.

He, He,
<snip>
Someone wrote about AC caps being 1.41x rated DC. This of course is correct. I appoligise for not quoting directly but I cant see the post.
Cheers Matt.

In some cases the dc rating is significantly higher than 1.414 x the AC voltage rating because other factors such as dissipation may come into play in ac applications. Corona discharge in oil filled poly/foil motor runs is often cited as a limiting factor in dc voltage ratings for these capacitors. I can refer to ASC made motor runs I use which are have quite disparate ratings for the reasons I mentioned.

Motor run caps are generally much cheaper than standard film & foil or metalized film types in comparable ratings, generally are nicely packaged and have very high voltage ratings and excellent reliability IME. (It's all about the number of ac motors out there as compared to tube amplifiers - much more demand for motor run caps which just happen to have an ideal blend of characteristics for use in tube amplifier power supplies.)

Incidentally the non-polar electrolytics commonly used in the past (I think they are still comparatively common on this side of the pond) for motor start applications are quite different than the PPIO types generally used for motor run. I am not sure whether or not NP electrolytics were ever common for motor run applications - I've never seen any, but my experience is (very) limited to motors in electronics and small US made appliances.

[Bob, W7ETA]

I recently bought DiversiTech caps on eBay.
The seller answered my question FAST, combined mailing costs. I bought 4 caps for two different integrated tube amps. I seller had to look up dimetions for me since I had space limitations.

SO, I now have a 70uF cap inside the SCott 299B I'm rebuilding, and by using 3 caps, I have 70uf in a Heathkit AA-151

Best from Tucson
Bob

[TheGimp]

The two reasons I can see to use Motor Run caps in an amp are (1) they won't dry out like Aluminum Electrolytics will and (2) you can readily get them with higher voltage rating than Al EL caps.

Tests I've done so far using 25uf and 60uF MR caps in the power supply of my 10W pp amp show no decernable difference in operation compared to the 100uF AL EL. They sound no better and no worse than the AL EL. Granted, I was not comparing uF for uF, but the AL EL caps I have on hand are all 100uF or larger (I have some 1000uf 200V caps).

The life expectancy of the moror run caps is probably 10X that of the Al El so you never have to change them.

If you are below 450V B+ and don't mind changing the caps every 10 years, it is probably cheaper to go with Al El caps like the Panasonic TS Series (450V, 82uF - $3.43 from Digikey).

I got my MR caps locally and paid $17 for the 60uF one (new from electrical supply store that caries Heat Pump parts, etc). The 25uF one was a loaner from a friend, and is used.

The dissadvantages of MR caps is their size. They are large compared to modern AL EL caps of comperable capacitance.

If I could get the MR caps as cheapely as I could AL EL, I'd go with them dispite their size. If I build an amp that has B+ Greater than 400V I'll switch from 450V AL EL to MR.

[zigzagflux]

Motor runs also provide a very convenient terminal for soldering point to point connections. You can typically make this a star point for those wiring purists. The height of the terminal is just right for 3D wiring, too.

Fortunately, the needs in tube equipment tend towards a higher voltage, requiring less capacitance (it's that handy 0.5*CV^2), so motor runs fall in the right range for power supplies and output stage cathode bypass.

Compared to exotic film caps, they are inexpensive, readily available, and will be around for many years to come. A nice hammered enamel paint, and the presentation is excellent through the top plate. Parts-obsession can be satisfied with bypass caps, I suppose.

Hard to go wrong.

[sea_bird200]

Originally Posted by i45one

I am starting to source parts for my 1st tube amp project. I am looking to construct Porkchop's KT88 se circuit.

Does anyone know where I can find a suitable 100uF motor run cap? It seems as though no one really carries anything that large in the voltage required. Angela carries 50uF, but I would rather not have to parallel two caps if I can help it.

Are there any other alternatives?

Thanks!

Here is it:
http://stores.ebay.com/G-G-MACHINERY:)

[TheGimp]

Does anyone use X2 or Y2 MKP or MKT caps for cathode bypass? Too small since typically they only go to 3.3uF or so?

I have a bunch of X2 and Y2 caps from .010uF to 3.3uF 375VAC (manufacturer samples) and am wondering how useful they would be in tube amps.