diytube.com

I bought some K40Y-9 caps for the "original" ST-35 and wondered if they are too big in diameter. The caps that come stock with the Dynakit are Illinois 0.1uF 630V which are 10.5mm dia. The K40Y-9's are 14mm dia. So, that's about 2mm extra from centre of cap which in turn, gets me 2mm closer to the output tube. There is still a gap (as you can see from the pictures), but it's narrow - maybe a couple of mils. I don't think the heat from the tube will be an issue, but should I be concerned? The K40Y's seem pretty robust (military spec).

I could mount them on the outer edge of the chassis, getting them further away from the tubes, but I would like to snug them up on the board if I could.

Please advise, Thanks

That's pretty darned close to some hot stuff. It might take a year for the innerds to cook-up, or it might be a week. I wouldn't be comfortable with that placement. I've seen output tubes burn the paper covering of non-belled transformers that were too close. The plates go to about 350 degrees and up. Get one of those $39 IR remote thermometers at the Homer Store and shoot the temps around your amp. You can use it to check for an overheating plate that's not red yet but may be higher than the rest. Clues to current mismatches and changes for the worse over time.

I think your new caps are probably "paper in oil" caps. If so, they will be much more sensitive to heating effects both in length of life and electrical characteristics then say, polypropylene or many other modern dielectrics.
I wouldn't mount them in the position that you show.
Can you hear a difference with the new caps? If so, try reinstalling the originals and check again to see if you notice a difference. Ask others that have no knowledge of the caps if they can hear a difference.

If possible I would recommend mounting them under the board and wrapping the metal bodies of the caps with some teflon plumber's tape to prevent any shorts.

The two caps closer to the output transformers will have to go under the board; they won't fit on top. It may be difficult to fit them snug to the board but easy to suspend them somewhat. I'm thinking of adding more depth to the chassis anyway. Should the caps be damped somehow if they are suspended?