I just couldn't resist the idea of having a go at refurbishing (if not modifying) my Stax amp. I wanted to do "just" the electroytics and the valves, those being the only components which I could reasonably assume had aged. (The rest of the components look fine, and I struggle to believe that Stax would have picked parts which are somehow inferior in such a nicely made piece of kit.)
I got a pair of NOS RTT/Matsu****a 6CG7 tubes on e-bay, so that's probably just fine.
I wanted to replace the power supply caps as well, and here availability of replacements look, ehrm, interesting. For a supplier which looks like it will be able to ship and which sells in small numbers, I found a French sub-site of Mouser Electronics which at least do a mean website.
Capcitor specs: I just took the lid off the amp and read the numbers (didn't look at the solder side of the PCB). The main high voltage supply caps are 100 uF / 500V polarised (I assume electrolytics) with some sort of radial mount (can't see if it's leads or snap-in), these are marked C11 - C14. There's also a pair of low voltage 10 uF 50V marked C9 - C10 which I might as well replace when I'm in there.
(Finally there are a few electrolytics in the delay circuit, but I'm leaving those alone - it would be a strange delay circuit indeed which made a difference to anything other than the switch-on delay, and I can't imagine that improving from where it is ).
The high voltage caps are 30 mm D / 60 mm L dimensions, marked 85 deg C.
Now, my main doubt: capacitor terminals. The ones I can find on mouser are all snap-in not leads. Is that what the PCB takes? If the PCB doesn't take those, can I simply and cruelly drill a hole or two holes in the correct diameter, and go ahead and snap in the new caps?
The rest of the specs I'm merely slightly in doubt: higher temp rating versus higher ripple current. Temps available are 85 and 105 deg C. For the caps that are in stock, the data sheet all say 3000 hours life time / stress test time.
- 85 deg C, 3000 hours : Cornell Dubilier 380 LX, ESR : 2 Ohm, max ripple: 1.1 A
- 105 deg C, 3000 hours: United Chemi-con KMS, max ripple: 0.82 A
- 105 deg C, 3000 hours: Nichicon GN, rated ripple: 0.65 A
No ESR spec on the two latter. (For comparison, Kendeil K06 are 85 deg C, 1 Ohm, 0.7 A but I can't find a place to buy those.)
Physical dimensions are all a small variety, 22x45 up through 30x30 (mm D x mm L). They're going to lok a little short, but at least if I get 30 mm D they're going to look like they belong...
So, any comments? I honestly don't know how hot it gets in there - the ventilation looks carefully thought out - so the extra max temp night not improve matters for me. The increased max current of the Cornell caps ought to go with lower ESR which ought to be A Good Thing [tm], right? Oh, and they all cost about the same - that's the downside to not shopping in Audiophile stores where there is a siginificant charge for flowery verbage about the sound qualities
PS: background : electronics engineer but working on integrated analogue circuits, not discrete or audio. Last DIY audio was back at Uni in the late 1980s but I was doing ambitious stuff back then. Funnily enough, one of my first DIY projects (back in evening classes while at secondary school) was the Wireless World electrostatic headphones for which I did put together the power supply but no more - I was first told to keep one hand in my lap while poking a live circuit and then found out why that was a good idea...