flex 6300 and keying sb-200

Clint,
If the SB-200 is stock then you should not connect it directly to the TX key output of the 6300. The SB-200 presents around -125VDC on the key input jack which is beyond the capability of the TX output on the Flex. You either need an amplifier buffer of some type (such as the one from Ameritron) or you need a softkey modification installed in the SB-200. The softkey mod from Harbach gets good reviews.
73,
Doug K4DSP

What radio were you keying the SB200 with before ?
I think its safe to say that any modern rig should use a buffer to key a tube amp of this vintage.

Here is an MFJ but there are others.

http://goo.gl/ss0AIy

Good thing you didn't test it out with the Flex! Wow, that's a lot of current.

The radio might not survive either the voltage or the current. Buffers ie;

http://www.hamgadgets.com/UKA-2  $19.00
and
http://www.hamgadgets.com/UKA-3  %22.00

might be cheaper than the shipping costs to have the rig repaired.

My UKA-2 assembled in 15 mins, and has always just worked.
The Defective TX1 relay jack on my 5000 is the direct result of my googling for advice about the need for buffers with a Flex. Seems the 3000s are happy, with Heathkits the 5000s are not. I stopped reading the advice too soon, I guess.
I now buffer the Heathkit, It doesnt hurt it  any, and it keeps it from hurting anything else I connect to it. BTW the previous owner of the amp mentioned that the Harbach Soft Key mod had been installed by the owner he purchased it from.
Safe or Sorry, You get to chose!
73, Jay - NO5J