Stock Dynaco ST-70 using 7199, 6GH8A and 6U8 tubes - video

Thanks for posting this, Bob

It would be nice if he could also test the 6AN8 too, given its use in the other Dyna amps. Dunno if there are any ready-made 6AN8-7199 socket adapters, though.

Nice find, Bob!

Keep in mind that these tests are not of the tube itself, but how the tube performs in that given circuit. It makes sense that the 6U8 should perform worse in the Dyna circuit since it has the lowest mu triode section and the lowest Gm pentode (compared to the other tubes mentioned). All that translates to lower open loop gain which means less loop NFB - and as a result there is hiigher THD with the 6U8.

The 6AN8 is getting quite scarce, I suggest it be reserved for the Mk III and such where it was originally fitted. In fact triode-pentode tubes in general are getting hard to come by. There don't seem to be many out there except dribs and drabs.

I've got a box full of 6GH8s that I collected about a dozen years ago, various brands, all used but good condition, anybody want or need any, CHEAP !