pompeiisneaks wrote: ↑Tue Oct 19, 2021 7:55 pm
EIther way, though, 105C rated caps are more than plenty for what kind of heat tube amps see on a regular basis inside the chassis.
In my particular case, the cap I'm most concerned about is the 50-50uF / 500v PSU filter cap which is really close to one of the power tubes.
Here are pics of my build. In the pic, I can't see the label for those caps, and I looked up another guy's build pics, I can't see the label on his either. I don't have easy access to the amp at the moment, so not sure if they really aren't labeled, or I'm just unlucky with the pics. Either way, it seems those high voltage "multiple caps in one can" are not particularly common. Though, I was able to find a handful of different 50-50/500v caps; some didn't have any temperature rating, and others fell between 55C and 85C. Definitely couldn't find any 105C rated caps in this style.
However, with a quick Mouser search, I could find 500V 47uF and 68uF "single" caps with 105C temp rating. I think I'll start with the fan idea; but with some mounting creativity, it seems possible to install more robust caps.
sonicmojo wrote: ↑Tue Oct 19, 2021 11:29 pm
The most common 40/20/20/20 CE can caps made today are only 55C/131F rated. I’ve seen many Princeton Reverbs fail with “not that old” new caps. Try to find caps with higher ratings.
How old is "not that old"?
R.G. wrote: ↑Wed Oct 20, 2021 3:14 pm
Mild air flow from natural heat-rises convection is what most amps use, but as you guessed, this takes some careful layout. Tubes-up and tubes-down are the easiest and what nearly always gets used, but tubes-horizontal is the best for natural convection.
I think you would call my combo "tubes-down". I feel like my combo's arrangement is such that there really is no where for the heat to go, except to "pool" in the little compartment created by the cabinet's rear panel. (You can see that rear panel in the pics I linked above; it goes right across where the tubes hang down from the chassis.)
R.G. wrote: ↑Wed Oct 20, 2021 3:14 pm
This idea is nearly always viewed as an abomination, but one of today's low speed and nearly silent computer fans will drop the air temp in an amp down to nearly ambient. Running a 12V "silent" computer fan on 6-7Vdc can make it so quiet you have to stick your ear near the openings in the amp to hear it. And your caps will live a long, long time if nothing else kills them.
I've been building my own PCs for many many years now, I have no shortage of silent 12VDC computer fans.
![Smile :)](./images/smilies/icon_smile.gif)
I think I'll modify that back piece of wood such that I actually hollow most of it out (i.e. kind of turn it into a "frame"), and the void can be filled with that metal mesh I've seen on other amp heads and combos (even my cheapie Monoprice amp has this). That will keep the tube area protected, but also allow some ventilation from the fan I plan to install.
Thanks for that, very helpful! That's actually what I was thinking when R.G. mentioned installing a fan, the low voltage secondaries for the filaments seem ideal for powering a fan.
Thank you everyone for the helpful feedback! Much appreciated!