Tube replacement for Dynaco ST 70

A few years back, I modded a ST-70 and used 6L6GCs for the output. However, I had to replace the bias potentiometer in order to have enough range to accommodate these tubes.

Triode Electronics in Chicago has some some driver upgrade boards available that include excellent instructions for a variety of modification options.

I have the JJ E34L's in mine and have been very happy with them.
I feel that the EH tubes are really good in guitar amps, but prefer the JJ in the Dynaco. notice that these are E 34 L not EL 34- so they draw a touch more current. Mine started sounding pretty good after about 10 hrs. They didn't sound BAD at first , just started to open up after ten hours.
If you want to go the 6l6 route, I would definitely go with KT-66's. The KT designation specifically refers to "Kinkless tetrode" which means that they were designed to remove a "kink" that was present in the frequency range of the 6l6.

First of all, could you tell us about the modifications? I've personally used several different mods of the ST-70 with a number of different tubes. I've recently used Siemens, pre-Sensor Svetlanas, Winged C's, Shuguang Treasures, RCA's, Mullards, and the reissue Tung-Sols. Mullards are usually my favorites, but good matched pairs/quad are just skyrocketing in price. I'm also a big fan of the Siemens; they seem to be a little more neutral in the presentation than the Mullards. I might say that the Shuguang Treasures are the best of the bunch right now. If $200+ for a quad, then I would suggest getting the Winged C's. They're essentially the same as the pre-Sensor Svetlanas which have a very similar presentation to the Mullards.

JJ E34L as suggested above are a well extended and well well balanaced EL34 type that works well in the Dynaco. They can be had for a very good price from Eurotubes.com I like the blue glass versions . I have been using them for years.

I own one of Will Vincent "Audiospec" Stereo 70's.
Recently upgraded the output tubes to the Gold Lion KT66
and have not looked at other EL34 tubes. The soundstage just opened up. With this amp the improvement is huge.
All the best....