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RE: The fallacy of "reforming" capacitors

Posted by Sherwood Forest (A) on December 23, 2015 at 13:26:27

In Reply to: The fallacy of "reforming" capacitors posted by Sherwood Forest on December 21, 2015 at 14:00:31:

Hello all,

Thanks for the thoughtful replies. A couple points. First, I should have titled this differently. I am referring to reforming of USED capacitors, not NOS. On that subject, I discovered by reading some of the articles posted here that proper procedure to reform a cap is by applying full working voltage through a current limiting resistor. Bringing amps up on a Variac doesn't really mimic this at all; you are simply ramping up voltage rather than forming it to the full rated voltage. Additionally, you have no current limiting resistor in place so any leakage current is going to be unchecked as you do this.

Also, regarding the 6db or so of headroom I got from the EL84M from Jim McShane, I should note that part of the reason for such an impressive gain in, well, gain is that the old tubes were no longer drawing all that much current. They were drawing about 17.5 - 19ma a piece, which is pretty cold for this amp. After putting in the new tubes, the draw went to 23-24ma per tube and that is no doubt where the headroom is coming from. They are great tubes to be sure, I just didn't want to leave the impression that there was some magic here. No magic, just higher current pull than my tired old Dutch Amperex 7189s. EDIT -> 6db is a seat-of-the-pants measurement based on my ears alone. I particularly noticed a dramatic increase in the ability for my S-5000 to pass the bottom octaves, and was frankly astonished by how much less large bass notes robbed power from mid-frequencies than before. This thing can handle 90's hip-hop records now with little complaint and startling authority.

Here are my overall points:

1) Reforming caps is a gamble at best, and a long-odds gamble when you're talking about used, heat and ripple stressed caps.
2) The potential damage to the amp if the cap fails hard is not worth the savings.
3) The time you have to invest in reforming makes replacement even more economical
4) Even a cap with no leakage is likely to have less capacitance due to drying of the electrolyte
5) Bringing up amps on a Variac does not simulate the reforming process, it merely ramps up voltage rather than apply full voltage through a current limiting resistor.
6) A brand-new can cap costs less than a fried 5AR4

It's all of those reasons that I find reforming old used caps in amps to be a losing proposition.

Edits: 12/23/15

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