Replacing old caps in the SP8 pre-amp

[enilsen]

My Audio Research SP8 is 25 years old now and I was thinking of replacing most of the Capacitors in this pre-amp.

I know there are numerous threads on caps and who's got what and where to get them, but I'm more interested in hearing about those that have actually replaced these parts with success.

In spite of the fact that this pre-amp does sound very good for its age I'm of the impression that heat from the tubes will have to a degree taken its toll on the larger caps as the labelling has started to peal off.

One thing of interest is that the upgraded versions of the SP8 doesn't use as many of those big yellow caps (Electrocube) which mine has a lot of. I can only assume this was due to cost savings on the newer models or maybe the newer replacement parts were of better quality.

[refference]

Hi ,

Do you want my sincere and humble opinion ???

Do not replace anything !! Let all stuffs stay the
way they are now !!!

If you really want to do anything , replace ONLY
the electrolytic caps in the power supply or eventually
one (s) in the signal path , with brand new ones , it will
be enough !!! Nothing more than that !!

Congratulations , you have a nice equipment .

Carlos

[enilsen]

I have already replaced the electrolytic caps in the power supply once. They had leaked and there was a noticeable hum when in use.

That was 15 years ago. The rest of the caps could be facing the same faith. I do believe though that "If it aint broke, don't fix it" so this isn't something I'm committed to, but I also recognize the harsh conditions these caps have been thru hence I would like to be prepared.

I have a stock of brand new replacement tubes and I never throw out the old ones just in case I need them some day for comparison (which might never happen).

Murphy's Law works for me and things break all the time so why wait until after the fact. Your advice is well noted as I would have given the same advice to others but for now I don't what to miss a day without this pre-amp.

[Eli Duttman]

IMO, Carlos was on target in suggesting replacement of any 'lytics in the signal path.

As for the "Electrocube" caps., ARC designs use loop NFB, which has a definite homogenizing effect. You should be fine using SCR (France) made Tin foil and polypropylene film parts as replacements. The SCR made caps. are available under Axon, Angela, and L_RD knows what other labels.

I don't know if the "Electrocube" parts really need replacing. The ink used to label the parts could easily be more fragile than the "guts". The photo supplied does not show signs of melted plastic.

[Brian Beck]

I agree with Eli. I’ve seen the outer tape sleeves on these plastic film caps come loose. In my experience the epoxy sealed inner workings were still OK. Unless metallized layers become exposed, which I’ve not seen, they’re probably good. You can always check them with a meter. These parts should last a long time. The electrolytics are another story of course. Were it my SP8, I’d leave well enough alone.

[enilsen]

Well the picture provided was borrowed from ARCDB to give anyone who's looking an idea of what I'm talking about. My caps still have the writing on them, but the plastic tape used to wrap the caps has pealed and is showing its age.

I suspect Electrocube fell short when producing a quality product by wrapping their labels on the caps instead printing on the cap itself.

In addition to this I also have discolouration around the tube sockets on the print circuit from extended heat exposure and wires that obviously are taking on that crispy look when residing too close to the vacuum tubes. Even the top perforated steel plate is warped and does an interesting buckle when removed.

I'll look into SCR made caps and see if they have some acceptable operating temperatures.

[Jaimo]

I replaced the Electrolytics on my Rev 7 SP8 with nos Sprague Atoms and this cleaned up some of the 60Hz hum I had been experiencing. There was also an overall improvement in the bottom end performance so I will recommend going this route if you wish to keep your SP8 in stock condition.