Are Motor Run Caps Of Any Use On Guitar Amps?

[rp]

Always been curious about this, sometimes you see those big oval oil filled cans show up surplus. You see new ones too, not sure what's in them these days, it can't be old dangerous PCB stuff.

So, for what we do are these, good, bad, useless? I always wanted to use them for the aesthetic vibe.

[diagrammatiks]

ya they are great for power supply filtering.

I use them all the time.

the round ones are easier to mount.

[rp]

ya they are great for power supply filtering. I use them all the time.

No sh*t! Wished I'd asked sooner, really curious to read others views now.

the round ones are easier to mount.

They have to be oval for cool factor, but Duh! never occurred to me I'd have to make some oval holes. That would be some nasty butchery, dang. Maybe I could just drill holes where the tabs come through? I used to see the clamps in Newark's catalog but that was ages ago. Any ideas where to source oval clamps?[/quote]

[John_P_WI]

Just a guess, maybe Johnstone supply or Grainger for the clamps??

[rp]

The ones I'm looking at on ebay say 440VOLT OVAL AC DUAL RATED - do I need to know anything regarding a DC rating?

[diagrammatiks]

like a million dc

more then you'll need ever need.

370vac gives you 515vdc minimum which is the same number as 370vac rectified.

however it doesn't directly correlate since I've seen plenty of caps rated at 400vac 630vdc.

I've never seen oval cans top mounted.

I know newark, mouser, and just about everyone sells side wall and flat mount clamps for oval caps.

http://www.alliedelec.com/search/produc ... KU=8523707

looks like digikey has the oval clamps.

http://parts.digikey.com/1/parts/170791 ... 107-1.html

oh ya in case you were wondering motor run caps that are pcb free are polypropylene suspended in vegetable oil.

basically a giant pio cap.

[Cantplay]

Those should be very low ESR. You can always bypass them with a small poly too.

John

[tubeswell]

FWIW R.G. Keen's post here

http://music-electronics-forum.com/t26247/

[diagrammatiks]

oh ya don't use motor start.

only motor run.

[David Root]

Slightly OT but I noticed in RG Keen's article he used five 10uF 250V film caps in parallel for the bias supply cap.

Good idea but I need 100uF at 75 or 100V for a 5E8A build to avoid using a Sprague 100uF 100V electrolytic. No room for ten in parallel. The Solens and Axons seem to start at 250V which makes 'em too big in 100uF.

Anyone seen reasonably sized 100uF non polar caps in 63V-100V? I could handle probably 15mm diameter by up to 70mm long. Or at least sometning bigger than 10uF?

[diagrammatiks]

you don't need a 100uf on the bias supply if you are going to use a film cap.

25-50 will be more then sufficient and will have the performance of a 100uf electro.

that being said, I don't think you'll be able to get an axial leaded film cap thats under 250v in that size, not in polypropylene at least.

you can get polyester with the vishay/ero 1813

http://www.ebay.com/itm/ERO-MKT-1813-10 ... 1117wt_684

new production here

http://www.mouser.com/ProductDetail/Vis ... 5HtofkQ%3d

You have more options in a radial lead format and they are big too so you can fit them between turrets if necessary.

mouser has the wima 33uf polyester

http://www.mouser.com/ProductDetail/WIM ... zZHVdbM%3d

rifa has a 22 in 63vdc and 250vdc

http://www.mouser.com/ProductDetail/Kem ... eH6%2fY%3d

http://www.mouser.com/ProductDetail/Kem ... XslCnYA%3d

kemet

http://www.mouser.com/ProductDetail/Kem ... QMPtwXo%3d

epcos has a 20

http://www.mouser.com/ProductDetail/EPC ... EHUczeY%3d

There are few more but you'd have to hunt to find them. They are on product sheets but aren't stocked usually by mouser.

[VacuumVoodoo]

25-50 will be more then sufficient and will have the performance of a 100uf electro.

Could you please elaborate on the above statement? 2nd part especially.

BTW, have a look at Vishay DC-link capacitors designed specifically for DC filtering application

[diagrammatiks]

25-50 will be more then sufficient and will have the performance of a 100uf electro.

Could you please elaborate on the above statement? 2nd part especially.

BTW, have a look at Vishay DC-link capacitors designed specifically for DC filtering application

for what it's worth i've got one schematic showing a 100uf/25vdc cap in that position...what is under voltaged and an 8uf 150vdc cap which is under filtered and over voltaged.

but given a 100uf/100vdc electro, the bias supply doesn't draw much current so capacitor charging not withstanding the filtering is primary function of that capacitor.

3300ohms like i have on my schematics and 100uf gives you a first order cut-off of .5 hz and 47 decibles of attenuation in an ideal model.

But, an electro isn't going to get anywhere close to those numbers, especially not an old design like a tva.

50uf gives you 41 decibles of ripple attenuation and a cut off of 1hz. A decent film cap will actually hit those numbers which is closer to what an electrolytic would get in that position.

it's a shame that polycarbonate is oos. They would be perfect for the bias supply filter position.

[VacuumVoodoo]

So you have 1Hz and 0.5Hz ripple in your bias circuit?
You don't need filter 3db points here, you need to know AC impedance relations at 60, 120,180,240Hz
50uF is roughly 53 Ohm at 60Hz, 26 at 120Hz etc. How much ripple attenuation you will get depends on both current draw (or resistive load parallel to the cap) and impedance of the voltage source charging the cap.
You need to figure out what, if any, influence ESR has on ripple filter performance.

[rp]

I read that you should keep the values modest on the bias supply caps as large caps will delay the power tubes having bias for several seconds as the caps charge up. So I just stick to the traditional values that Marshall and Fender used, 10-25mF just in case the above really matters. But even with large values you'd think the bias caps would still be ahead of the power string, especially with a tube rectifier. Any comments from the knowledgeable?

Be nice to bulletproof the bias with a poly cap, might try it if I have a roomy chassis.