SpeakerTalk

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I have rebuilt couple amplifiers this year. Recently I did rebuild my Audio Research SP14 and Classic 60 amplifiers. I bought SP14 quite recently and it was clear that volume pot has passed best before date quite long time ago. I replaced pot with ordinary Alps 27 series dual 100K pot and and amp performed quite nicely after this.

I have had Classic 60 power amp for couple of years. Classic 60 has always sounded good to me, but nothing special. Last summer I did rebuild ARC D76A from 1977. Circuit topology of these 2 amplifiers is quite close to each other, even D76A is 12 years older design. After rebuild D76A was clearly superior to CL60. This made me think if main reservoir caps of CL60 should be replaced. I ordered 4 pc Kemet 1000UF/450V ALS30A102KF450 caps to replace the original Mepcos. I replaced the original SP14 Sprague 53D series 100UF/450 B+ cap with similar Sprague unit. I replaced also electrolytic´s of muting, control and filament supplies.

So... there were no fancy magic parts. Sprague cap was used as it was chosen when SP14 was originally built. Kemet caps were chosen as they had similar dimensions as original Mepcos. Also Kemet part number suggest´s that they are made by Aerovox tooling.

When the caps were replaced, it was clear that performance of the ARC set did improve considerably. Especially harshness of CL60 HF was reduced quite much.

After replacement I charged all old B+ caps to 20VDC. To this date there has been no leakage measured with VDM. Capacitance values were within spec and close to ea other. It is hard to think that series inductance can increase due age. ESR will most likely rise due age... But 4 caps with 0,1ohm ESR in paralell feeding output stage with 2000ohm A-A load will most likely tolerate slightly higher ESR quite well.

This all made me think if similar improvement would have been possible by cleaning capacitor mounting terminals. Screw terminals are made of aluminium, spades are made most likely of nickle plated brass and brass screws were used for mounting spades to aluminium terminals. There is quite much room for deteoriated contacts. Snap-ins are not too much better, as aluminium lead out is crimped to pin that can be soldered.

There is not too much you can do to snap-in caps. As far as I can remember, there was discussion about iridium (or something like that) plated mounting screws for caps in Audio Amateur in 80´s. They were supposed to be better than other screws for this application. If this is true, also similar material would be handy for spades.

I tried to google about this issue without too much luck. Also information about the screws in question was in short supply.

Any ideas about this topic would be most welcome.

Best Regards

Kimmo