Not A Replacement For Eros 2200uF @4V

[Grainger49]

I bought some of these just because.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=150509160608&ssPageName=STRK:MEWNX:IT

I had seen Paully's schematic and know the 2200uF 4V cap acts like a cathode bypass capacitor for the EF86.  

If I have this right Ed Fallon selected the two capacitors that are attached to the EF86.  They are very specific.  They are very well chosen for the needs of the circuit.  Shortly after the Eros release I posted on the old board and was told that the 2200uF needed to have short leads, have an exceptional high frequency response, have low ESR... you get the picture.  The search back then came up with other capacitors made by the same company, with the same dielectric as the stock cap bur all smaller values.

About a week ago I bought these (hey, they were cheap!) and later, a lower post, found they probably can't be used in the Eros to replace the 4V 2200uF cap.

 

[edfallon]

Graninger,

The polymer electrolytic parts were selected for compact size, price, spec, performance and availability.  If you can find another cap that fits and performs well, go for it.  The parts we chose were an attempt to mimic the well regarded Sanyo Os-con.  I would recommned the Sanyo to swap if you can find them in stock somewhere. 

-Ed

 

[Grainger49]

Ed,

I think Sanyo was what I have seen. They are like the stock caps, about 1/4 of the top is marked blue.  But these others, the linked caps, are a comparatively huge value, 0.47F, yes I didn't leave out the micro there isn't one.

And thanks for responding.

 

[Grainger49]

I searched some posts on the old board.  According to Paul Joppa the 4V cap should not have the value increased:

...Nevertheless, the cathode bypass capacitors do carry signal current and are candidates for replacement. That would be the 2200uF/4v and the 100uF/160v caps on the PC boards. The low-voltage cap actually runs at 1-1.5v typically, so be sure any replacement will function correctly at such a low voltage. And I'd avoid anything much larger at this critical high-gain point in the circuit, as it may pick up noise. That leaves a very small field of candidates - believe me, we looked pretty hard at what was available before selecting these caps.

 

[Paul Joppa]

Good sleuthing, Grainger! But I'm afraid I was not very clear - I meant to avoid anything physically larger, which would present a larger surface for capacitive pickup of noise.

It s probably OK to increase the capacitance of the 2200uF cap, but I can't provide any certainty - these two caps set the poles of the DC feedback loop (the bias servo) and feedback loops are potentially unstable if the poles are not working together correctly.

One other point - most electrolytics, including the polymer types such as the Os-Cons,have a good deal of leakage. Larger capacitance leads to larger leakage. The leakage must be substantially less than the EF86 quiescent current or the servo will effectively be shorted out. The other feature necessary in this cap is low ESR at high frequencies.

You may find these specs in a data sheet, if not then you can't predict whether it will work, you'll just have to try it and see. Check the EF86 plate voltage first, if it's too low then the cap leakage is probably the problem. Then listen, especially for distortion or incorrect equalization - i.e. changes in the high frequency response level. If it sounds good then it is good. Watch for very slow oscillation, which would indicate an unstable servo loop - not likely, just something to watch for.

 

[Grainger49]

Paul,

Thanks for a good complete answer.  Letting me know what to look for is a big leg up.  I'll go looking for the spec sheet on the Os-Cons and the "Supercaps."  I can also put my scope on the output and watch for high frequency oscillation.  I can see low frequency oscillation on the sub.

I got the new caps today. I just had to buy them as cheap as they are.  I will post a picture of them and the ones I have that are like the Os-Cons used in the Eros.  I have some.  The new ones are a little larger than the Os-Cons and don't look like a transistor with one lead cut off.  I would guess the outer covering on the Os-Cons helps shield it?

 

[Grainger49]

I can't find the Os-Con caps I have but this is a photograph of the Supercap with a pencil for size reference.  The Os-Con is only a little larger in diameter than the pictured pencil.  This is a physically larger cap.

 

[Grainger49]

I decided I could use two of these in series and bypass the heater supply in my Seduction.