Can an EL34 tube be replaced with a KT88 or 6550?

Generally no. There are some autobias amps that have a large enough range to handle it but they aren't common. I would not assume that your amp can do it.

The counterpart of the EL34 would be the KT77. The 6CA7 can also be used. But check with the amp manufacturer first to be sure it is autobias and that these substitutions will be fine.

Arthur

Thanks for the info, Arthur. I tried looking up info, but all i found was the site www.audiofi.com. It has a contact link, but there is no email or phone listed. I thought maybe the KT88 amps were more expensive mainly due to the higher cost in the tubes. It appeared like the input sockets all looked the same, so I was hoping I could do some tube rolling with various tube types. Alas, it is not possible.

Thanks,
Clinton

All EL34's don't sound alike, not by a long shot, so even if you can't use 6550's and KT88's, you can get some tubes which will sound significantly different. You will still need an auto bias curcuit or the ability to re-bias the amp with the new tubes. Ditto small tubes except you don't need to re-bias for them. Good luck......

Depends on the topology/design of the amp.

Thanks for the advice. I think i'll just get a quad of either fat body Electro Harmonix EL34, Mullard EL34 or JJ Telsa...heck maybe i should just get em all and see what characteristics they have. My amp does have auto bias, as I took it to a repair shop to get some knobs fixed, and the guy said it was auto bias. Plus there are no bias holes anywhere on the top or back of it. Woo hoo for auto bias!