A great performer and affordable tube amp is the st70. They were originally built from 1959 up to the early 90’s and according to Wikipedia over 300,000 ST-70’s were produced, this is an amazing number of amps, and thanks to this huge volume of these fine amps in the field, an entire industry has sprung up around this amp. You can get everything from simple mods, and repair kits, to new reproduction chassis, and even entirely new circuits using the ST-70 iron.
The ST-70 produces a clean 35 watts per channel with a 1.3 volt input. The output transformers have 4-8- and 16 ohm taps. It uses 190 watts from the wall, and was designed by Ed Laurent, and Bob Tucker. And if you had a time machine you could go back to 1959 and buy the kit for 99.95 and the assembled amp for 129.95.
Dynaco produced 2 main version s of this amp, the Dynakit, and the Dynaco, they were exactly the same except for the fact that the Kit was exactly that a KIT, where the purchaser could save some money and assemble it themselves. The kits usually sell for less since the quality of the construction varied on who was building it and there was no real quality control. The dynamos on the other hand were factory assembled and were generally built a little better.
The ST-70 uses 4 el34 (6ca7) power tubes, in a push pull configuration, 2 tubes for each channel, combined with 2 output transformers, and 2 driver tubes (7199), they were powered by 1 large power transformer and rectified by a 5ar4 (gz34)
Each channel has its own bias pot, and as you can see in the photo below I removed the octal connectors on the front and replaced them with inputs, then I put to test points to connect my meter to so I can set the bias. I also added some updated binding posts for the speaker connections, and since my speakers are 8 ohms I disconnected the 4 and 16 ohm taps and just zip tied them under the chassis.
I also rebuilt the driver board with original value components, and replaced all of the caps. It now sounds as good as it did in 1959.