Replacing double caps with two singles - am I missing anything?

[MattG]

This is somewhat of a follow-up to my earlier question, about capacitor life expectancy in regards to heat from nearby power tubes, particularly in a combo cab with sub-optimal ventilation.

I have found out that my caps are the JJ TC 529 50+50. The specs aren't particularly encouraging IMO: rated for 1000 hours at 70C. I searched the web for comparable 50+50uF 500v caps. I couldn't find anything significantly better in the dual/2x format - the "best" ones seem to top out at 5000 hours at 85C.

However, Mouser has the Nichicon UCY2H560MHD: a single 56uF, 500v, 12,000 hour rated at 105C. They are also only 18mm in diameter, where my current double cap is 35mm, so if I've got a mm or two to spare, I think I could make two of these work.

My question is: am I missing something? Is there any reason not to use the Nichicons, which appear to have significantly better longevity prospects?

Thanks!

[mhuss]

No, from a technical point of view you're losing nothing and gaining MTBF. Safe mounting is the only issue.

[Phil_S]

I think you are missing what the spec actually means. If you run it at 70C for 1000, it will probably give up the ghost. But you aren't going to be running it nearly that hard and it will last many more than 1000 hours. The 85C 5000 hour cap will probably outlive you. If you get the same form factor as the JJ, replacement will be simplified.

[MattG]

I guess the question is, under typical use, how often do those caps get to their 70C (158F) rating?

I tried to take some measurements. Ambient temp was 68F. Before I turned it on, I used my IR thermometer on various parts of the amp, everything was 68F +/- a degree (as expected).

I played for about two hours at "bedroom level", then took some more measurements:

• Exterior top of cabinet (i.e. right by the handle): 90F
• The dark-colored base of the tubes: jumped around between 164F and 180F
• 50-50uF filter cap closest to power tube: 140F
• Paper wrapping around power transformer: 180F

I suppose those temps are probably OK. But I consider this the best case scenario: low ambient temp and not pushing the amp much --- and that's with the 6V6 tubes. I'll repeat this tomorrow. Next weekend I'll try to put the KT66 tubes in and take some measurements.

[Phil_S]

If you feel temperature is a concern, add a 12V computer fan to the 6V filament circuit to exhaust the hot air. It will turn at half-speed and you'll never hear it. If you have a job at a place with an IT department, they will have a bone pile where you can get one off an old chassis for nothing. Those guys save everything.

[TUBEDUDE]

If it's for studio use, get a new one, with other than ball bearings. Even at lowered speeds they become noisy. Here's what I use. Mount with rubber/Sorbothane grommets.

20211031_091424.jpg