[HAM] Best 12AU7 now 6550

[David Anderson]

On Mar 9, 2008, at 2:16 PM, Ian Hooper wrote:

> At the risk of marching into left field : some folks don't consider  
> beam
> power tubes like the 6550 to be 'true' pentodes.

Beam tetrodes aren't true pentodes, but that's just a simple  
difference: beam forming plates vs. a suppressor grid. It all goes  
back to a patent issue.

It gets confusing, though, when, for example, some 6CA7s are beam  
tetrodes and some are pentodes like the EL34. They're  
interchangeable, but they don't always behave precisely the same.

But for Leslie purposes, all 6550s, KT88s, KT90s, etc... are beam  
tetrodes or beam power tubes. KT = "Kinkless Tetrode."

As a complete aside, in a given circuit, all other things being  
equal, a beam type output tube will often have punchier, better- 
controlled bass than a pentode.

David