Replacing capacitors in Dynaco A-25s

[Horswispr]

Hello all, I'm hoping to generate some dicussion of capacitor replacement in Dynaco A-25s.

I recently received a pair of A-25s for refinish, and noticed that the treble was unusually smooth. Poking around inside, I found that the original capacitors had been replaced with Solen 6mfd poly caps (the fat black ones).

Inspired, I ordered a few 6.2s from Parts Express (they were very helpful and returned emails promptly) for about $4.50 each. I took a deep breath and did my first ever cap replacement, in a Dynaco A-25 I had that looks gorgeous but had reduced output from the tweeter.

I was able to remove the old cap, so I hot glued the new one in in its place and soldered it in.

Yep, the sound of this Dynaco is now slightly sweeter and smoother than its brother. Cymbals in particular sound clearer, with less hash. And the overall sound is more relaxed.

I'll do the other one when time allows.

A couple of questions for possible discussion: Is 6.2 the best value to use in Dynacos? I notice that the original was a 5.0. I then opened up a Scanspeak I am refinishing and found that the original in there is a 5.0 as well.

In theory, a 5.0 would raise the crossover point very slightly, relative to a 6.2. If I like a bit of a "BBC dip," should I opt for the lower value? There are charts on-line that show how capacitor value affects roll-off frequency in a simple tweeter circuit; my guess is that the difference between a 5.0 and 6.2 would be very small subjectively, but it could be audible.

Another question: in the experience of those who have replaced Dynaco caps, what subjective changes did you notice? If dreaded "old capacitor drift" occurs to the "South," then the tweeter might be forced to work too hard at the bottom of its range, causing Dynacos to sound too hard or strained. If the drift occurs to the "North," then there might be a "hole in the middle" effect. As the folks at Solen and Parts Express pointed out to me, older capacitors can also become resistant, reducing overall tweeter output (as in the case of the one I replaced). Finally, the newer poly caps should smiply sound "cleaner" than the cheap electrolytic ones used back in the day, but I have no clue why. If anyone has insight into the "why," please join in.

The bottom line: I think it's worth it to replace the caps if you have a pair of A-25s that sound good but not quite as articluate as you reference speakers.

I invite any comments.

--Horswispr

ps For those who've never tried to do a cosmetic restoration on a pair of A-25s, I'll try to upload a before and after on a pair of ugly Scanspeak bottoms I just worked on. Almost anything is possible if you take your time.