determining which tube is the tremolo tube

[pdubs410]

Hello,

I've got a Danelectro Twin Twelve Model 300 Series F I recently purchased (I'm guessing somewhere around a 1964.) It sounds great, except that the tremolo gets very weak after about 20 minutes of being on. I'm trying to determine which tube would control it, so I can replace it-any ideas on how I can determine which tube might control it?

Thanks!

 

[pdubs410]

Ah, thank you! Was hoping to pick one up before going to the studio this weekend, but it's not going to be that easy...