As far as I understand it, despite being marketed as identical / drop in replacements, there are minor differences between EL34's and 6CA7's.....this is copied from another forum, chap explains it much better than I can, I'm no technician lol!!
"Technically speaking, the old production, US made "fat bottle" 6CA7s are beam tetrodes in that they have only two grid windings. Beam forming plates take the place of g3. They look somewhat like healthy 6L6GCs and the beam forming plates are plainly visible if you you look close. AFAIK, only GE and Sylvania/Philips ECG made these although they were rebranded by others. They're very rugged and apparently have a following in the guitar world. There are current versions being mfgd by Sovtek and others and these too are beam tetrodes.
EL34s are true pentodes in that they have three grid windings and no beam forming plates. They're almost always in tall, smaller diameter bottles compared to 6CA7s. True pentode EL34s have been mfgd just about everywhere but the US."
I have only had EL34's in my Snappers so it's never been an issue, but as I spotted it on Manley's Snapper page, thought I'd pass it on
