More Eros Capacitor Questions From The Old Board

[Grainger49]

I brought this exchange between me and Paul Joppa  over from the old board.  The Bottlehead selections for the two capacitors in question are hard to beat, but I wanted to get the information over here.

Grainger:   After being in on the assembly of an Eros last weekend I spent some time looking at the schematic. I see two large value capacitors that seem to be in the Pentode's and Triode's cathode circuits. It looks like a transistor is also in the Triode's cathode circuit so it may be a C⁴S.

Would there be any value to bypassing either of those capacitors?

Thanks,

PJ:   Those capacitors do carry signal current and may well affect the sound. The one in the EF86 cathode operates at very small signal voltages, so it is subject to noise pickup - I'd keep any bypass as physically small as possible, be sure the outer foil is the ground side, etc. The transistor is part of the servo bias arrangement, which was not present in the Repro Amp but was added so that a wider variety of EF86s would bias up correctly.

For the 6922 cathode we used a Black Gate in the first high-end Repro Amp version of this circuit. Feel free to experiment with better caps or bypasses.

Grainger:   Short leads rules out any of my Teflon KKs. They are all huge for the values.

I priced some BG caps and WOW! I'll save up for these and get all my ducks in a row before entering the fray. IIRC that is the 4V cap so it isn't as bad as the 160V cap.

Do you have any suggestions for improved electrolytics? I suppose low ESR is a good thing here?

And finally, concerning the outer foil of the capacitor, I have seen on a number of caps the outer foil is the short lead. Is that always true, usually true?

Thanks,

PJ:   For the 4v capacitor, we chose an organic polymer type because they have a pretty good reputation for sound, are very compact, and are available in low voltages. I think Os-Con is the most well-known variety, but the large value we wanted is only available by special order with a 12 week delivery quote. Ed made the final selection; maybe he'll say something? Generally electrolytic capacitors work best at 30% to 80% of their rated maximum voltage.

The main performance feature that is measurable is the ability to maintain a low ESR at high frequencies, up to and beyond 100kHz. The modern Panasonics that we've used for several years are designed for switching power supplies, so they need that HF performance. I look for a good temperature rating too, at least 85C and I generally prefer 105C but can't always make it work out - cost, availability, and size are among the many other constraints.

[mwaller]

For the 6922 cathode we used a Black Gate in the first high-end Repro Amp version of this circuit. Feel free to experiment with better caps or bypasses.

What series of Black Gate capacitors were used? Were these the "standard" series, NH non-polar series, or....?   Has anyone tried this in the Eros, and if so, did make any difference?
Thanks,
Mika

[Paul Birkeland]

IIRC, they were the standard snap-in Black Gate caps.  They're only $75 each for the few that remain available. 

[mwaller]

Did they look like this?

[Doc B.]

Yes those look about right. You will have to leave the preamp on for long stretches to reap any benefit. BGs aren't that big a deal sonically until they have run for a while, at which point they sound really nice. But you have to wait again for that "break-in" every time you turn them off. We use them in a demo Repro at shows and we just leave the preamp on for the entire show.

[mwaller]

Yes those look about right. You will have to leave the preamp on for long stretches to reap any benefit. BGs aren't that big a deal sonically until they have run for a while, at which point they sound really nice. But you have to wait again for that "break-in" every time you turn them off. We use them in a demo Repro at shows and we just leave the preamp on for the entire show.

That is great info, Doc.  Sounds like it would NOT be a good choice for me, since I only get to listen in bits and pieces. 
Are there any other caps you can recommend for this application that offer better sonics than the stock Panasonics?

On a similar topic, are there any "better" alternatives to the stock Nichicon LG-series 4v 2700uf cap?  I recall reading that this cap was chosen to mimic the properties of the Sanyo OS-CON caps, but when I compared the specs, it would appear the stock Nichicon is BETTER than the Sanyo in every respect except leakage... Thoughts?
Best,
Mika

[Tubejack]

Hi Doc -

Without opening up my Repro, what caps would you guess are in my early-on purchased version?

tks/jack

[Doc B.]

Very good ones. Probably the Black Gates or possibly Nichicons. We always use very good stuff in that preamp.

I want to caution everyone here. "Better" is unfortunately often determined for many audiophiles by the expense and level of hype from a reviewer who may or may not understand the finer points of a design. There are many factors involved in design that are never talked about in the typical capacitor shootout. The best replacement for the OSCON, to my ear, is to remove that servo circuit and put in an LED like we do in the more premium Repro. There we have some control over the user's tube choice and that is critical to using the LED. The servo is in there in Eros because, as is often the case these days, there are a few brands of tubes that just don't fit the standard curve for the tube type and need extra circuitry to work properly. Since so many people tube roll we have to include it. I think PJ was particularly clever with that servo design and it's a excellent solution, but to me the LED sounds mo' betta' when you have tubes that let you use it. 

That said, I am not particularly interested in offering step by step instructions for this modification. There may be info posted on the forum already.

[Paul Birkeland]

I would also add that the Eros cathode bypass caps are specially selected for very long lifetime and good operation at high temperatures.  In the grand scheme of capacitors, most other capacitors available are a poorer choice than what we provide.