substituting capacitors

[stusnyder]

because when you put electrolytics in parallel, in theory it should work, in reality the caps can slightly differ from each other, and then you have a balance problem. Best to replace with the single proper cap. You can go a bit higher on uf and voltage, but never lower. Also I would suspect your tester if the short light showed up on every tube tried, but on just one, it's probley the tube.
That link I gave you has a good page all about capacitors and what and not what to do.
I usually check my tubes using 2 different testers.