7199 vs 6U8

[Kegger]

Yes, your post prompted me to post about it..

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And there is nothing wrong with those 6AN8, there fine tubes.

But they run into the same basic issues as the 7199, no longer
made, and there the only tubes with there particular pin-out to.

Going to the 6U8/6GH8/6BL8 family of pin-out tubes makes sense
as there are just so many of them, so set your circuit up properly
an roll away with a never ending supply, an to me the 6BL8 rocks.

 

[Tom Bavis]

6U8/6GH8 etc. have the advantage of being at least CLOSE to the 7199 footprint (three pin difference). 6AN8 might be the closest electrically, but it takes more rewiring - even worse, it has its own unique pinout - there are NO subs, just like the 7199. Whereas, some time in the 23rd century when the 6GH8s run out, we'll still have some 6LN8, 6KE8, 6MG8, 6HL8, 6LX8 to play around with without rewiring...

Keg's point is that the operating point of this stage is CRITICAL for best performance, and may not be optimal without adjustment. This is true with 7199s as well - when somebody says a particular brand sounds better, ask them if they measured voltages - some may be far from the best DC operating point. Dynaco copped out and didn't give voltages, saying that they "vary". But lowest distortion will occur at a particular operating point - ideally all would fall close to that, but practically they vary a bit. One advantage of the replacement boards - most use a long-tail pair as phase inverter with more undistorted voltage swing, so even if it IS DC coupled, some offset from the "best" operating point won't hurt performance much.

One more thing - most tubes won't take more than 100V from heater to cathode for long... biasing the heater string positive in the ST-70 might help prevent some tube failures due to h-k breakdown.