Rubicon high ripple miniature electrolytic caps no longer manufactured.

[David G4EBT]

In recent years a number of forum members, myself included, have sung the praises of Rubycon miniature BXW and BXF series high ripple rated miniature electrolytic capacitors which are ideal for re-stuffing reservoir/smoothing can type capacitors. So small in fact that three will fit in a can if need be. They're 105 degrees C rated, with a stated life of 10,000 hours.

Unlike many capacitors on which there is little or no information as to the ripple rating, these have a comprehensive datasheet which gives the ripple rating at a range frequencies from 50Hz to 100 kHz, though it's mostly 50 Hz and 100Hz, that we're interested in for half-wave/full wave rectification in valve radios.

I referred to them in post #6 in this thread:

https://www.vintage-radio.net/forum/...hlight=Rubycon

Seeing a current forum post about ripple rating prompted me to look up the Rubicon caps on Farnell's website as it's a little while since I stocked up on them. There are two case styles - the pencil slim type are the BXW series, which still seem to be available:

https://uk.farnell.com/c/passive-com...nge=bxw-series

However, the BFX series of 'dumpy' style cans are no longer manufactured and only a few are now available until stocks are exhausted:

https://uk.farnell.com/rubycon/400bx...c%20capacitors

Before anyone jumps to conclusions I don't change reservoir/smoothing caps without first checking if they can be reformed. Back in 2009 I made the rather splendid 'Capacitor Tester and Reformer Unit' designed by Stefan Niewiadomski (Forum member 'Saddlestone Man'). The project was featured in the winter 2008 issue of the BVWS Bulletin and first appeared in Radio Bygones June/July 2008,

However, often, the capacitors are beyond redemption and quite how long capacitors which are now more than 70 years old will last when reformed is a matter of conjecture, (though I have to say that the BEC ones in Bush DAC90As never seem to be at fault). In a Murphy A124 (1948), which I'm presently restoring, a twin can and a single can had low capacitance and high leakage. (Also a 50uF 25V can type cathode bypass cap), so I've re-stuffed those, albeit not with Rubycons. When I opened up the cans and unravelled the foil/paper layers, the electrolyte in the caps was as dry as dust.

So, in conclusion, I wonder if other forum members have found a source of branded high voltage electrolytics which have their ripple rating stated, either on the can or in datasheets? Typical values that we need are 10uF, 22uF, 33uF and 47uF - the modern equivalents of 8/25/32/50 uF. (I'm not impressed with tacking physically larger ones beneath the chassis, and prefer to re-stuff the old cans).

[dave cox]

For Panasonic, the types are listed here PANASONIC. Probably EE-A or ED-A types seem to fit your requirements.

RS has at least some of these series in the 450V rating, and in small quantities!!

HTH, dc

[David G4EBT]

Thanks for reading the thread Dave and for the info.

The Panasonic ones from RS look ideal. EG, 47uF 450V radials in packs of two:

https://uk.rs-online.com/web/p/alumi...citors/7112141

[trobbins]

Wrt reading the specs on ripple - for 10uF 450V the Rubycon BXW is 130mA, and the Panasonic is 350x0.3=105mA, both for 105C and 10khr.

[David G4EBT]

Thanks for reading the thread and for your comments.

Yes, despite their small size, their ripple ratings of both Rubycon and Panasonic caps are well up to the task as replacements in domestic radios.

The ripple current rating has to be reduced to 80% for half-wave rectifiers as the ripple current is higher than in full wave rectifiers, as is the ripple voltage, which is the source of low level 50Hz hum in radios such as the commonplace Bush DAC90A and similar radios which use half-wave rectification. Tune off a station and 50 Hz hum is there in the background, but when tuned to a station, isn't so noticeable.

It was acceptable to listeners when such radios were new, and we can surely live with it today. The 1940s & 50s were austere times, and that reflected in the design of radios. Not difficult to see where corners were cut - auto-transformers & live chassis, half-wave rectifiers, AC/DC designs in which a dropper was cheaper than a mains transformer, (and many UK homes still had DC mains anyway).

It might be helpful to less experienced forum members to explain the difference between half-wave & full wave rectification, as depicted in the two sketches below which explain how ripple current and ripple voltage arise, and why they're higher in half-wave rectification.

I've drifted off the topic of 'where to get small caps with high ripple ratings?', but relevant enough I hope.

[John_BS]

I'm now feeling conflicted, so would appreciate some feedback. The original 20+20uF dual capacitor in this Radford amplifier is date coded 1960, but looks quite healthy. It has "reformed" as follows:

C = 27 + 27uF
ESR = 1.6 ohms
Leakage: <500uA at 420V


Should / can it go back into service, or re-stuff with Rubycons?

John

[David G4EBT]

I'd say they've reformed very well and don't think they would be any cause for concern - not to me at any rate. The capacitance is close to the original, and when new, smoothing cap tolerances were often stated as having wide tolerances in any event. The leakage is low, (and ESR isn't really a concern in valved rectifier circuits, but in any event it's low).

[John_BS]

Many thanks David.

BTW, there's a guy on ebay currently selling some BXA capacitors which have a decent life / ripple spec.

John

[snowman_al]

John_BS, We are a little off topic but remember the Radford is drawing 250mA from the HT so the ripple will be considerably more than the little Rubycons (or Panasonics) can handle. Especially if you enclose them in the original can.
Better, if you decide to, replace it with a good quality 32uF+32uF 450/500volt (for example from F&T or ARS) that is made for the application and has ample ripple specification on the Red terminal.
There is room if you side mount the larger cap clip.
Alan

PS, those BXA series ripple figures are quoted at 100kHz multiply that by 0.45 to get the 100Hz figure...
The data sheet is here: http://www.bdtic.com/datasheet/rubycon/bxa.pdf

[John_BS]

Alan:

I looked at the F&T caps but couldn't find a spec! Do you know where I can find one?

John

[snowman_al]

Not easy to find, but here is the 32+32uF data:https://www.tubesandmore.com/sites/d...2-32-500ft.pdf
Quote ''0.27A at 85 degrees C''.
The 50+50uF is of course higher at 0.33A / 85 degrees.

Mundorf have a data sheet too https://www.hificollective.co.uk/sit...atasheet_0.pdf

[ms660]

Originally Posted by David G4EBT

The ripple current rating has to be reduced to 80% for half-wave rectifiers as the ripple current is higher than in full wave rectifiers

As an approximation:

Ir = Vr2PiFC for half wave.

Ir = Vr4PiFC for full wave.


Ir = Ripple current (RMS)
Vr = Ripple voltage (RMS)
Pi = 3.141 (approx.)
F = Supply frequency.
C = Capacitance (in Farads)

Lawrence.

[John_BS]

Originally Posted by snowman_al

Not easy to find, but here is the 32+32uF data:https://www.tubesandmore.com/sites/d...2-32-500ft.pdf
Quote ''0.27A at 85 degrees C''.
The 50+50uF is of course higher at 0.33A / 85 degrees.

Mundorf have a data sheet too https://www.hificollective.co.uk/sit...atasheet_0.pdf

The BXA spec for 33uF is 350mA @ 120Hz @ 105 C

Has anyone ever looked inside an F&T cap?

John

[snowman_al]

You are correct.
It does make you wonder why such 'small' caps have better specs than larger 'made for the job' caps...