[Amps] New SB-200 owner

[Peter Chadwick]

Carl said:
>Peter, just what would you consider a good economical tube if building from 
scratch for 6M? Ive not seen any REAL verified IMD numbers published for any 
Russian tubes except maybe the 4CX800 and have hesitated in even trying one.<

Do you mean if buying a new tube, a used tube, or as I usually manage, getting for free?
It IS possible to get good IMD figures from tetrodes, but it does require negative feedback. See Pappenfus.  4-125 and 4-250 can be good with NFB, and they'll work FB at 50MHz..
I'd likely look at the 3-500Z, as a good workhorse.
For real low cost, a pair of 4CX250  with lots of feedback and an electronically regulated screen and bias supplies. Even better 4CX350J.
Part of the economy is what's available for a plate supply, too. And are you building one for you, or designing for production. There's a lot of difference, as I have no doubt Tom can tell us.
Finally, unless you have exactly the same layout etc, feedback can make things change so much at 50MHz that you may not necessarily reproduce a set of IMD figures. 
73
Peter G3RZP