5U4G suitable for Dyanco ST-70?

You can absolutely use a 5U4G in a Dynaco ST70.

In fact, the Dynaco MKII monos did use that rectifier. Along with the power supply choke and the KT88 output tubes, one of the improvements the successor, the MKIII offered over the MKII was the 5AR4 rectifier. Their next amplifier, the ST70 stereoblock, carried that forward.

And the 5AR4 represents a true upgrade over the 5U4G, offering much tighter, more immediate sound, with deeper, more impactful bass. In fact, the improvement is far more than one would expect. I can't name one area where the 5AR4 takes a back seat to the other rectifier. When you factor in the increased time of the soft start (though most will be surprised how much of a soft start the 5U4G actually provides), there's not much of an argument outside of the 5U4G looking so much cooler than a 5AR4.

That said, I understand the audiophile curiosity. Obviously, that's what drove me to try this rectifier. But after trying it (several variants, in fact), I found there was absolutely no comparison.

Dynaco DID list the 5U4G as a suitable substitute in the ST70. Likewise, they named 6L6 acceptable in the output section. Of course, they didn't believe either tube provided the sonic performance of the recommended varieties.

While the Russian 6L6 has become a bit of a cult favorite in the ST70 over the past several years, as with the 5U4G rectifier, I found the performance in comparison to the EL34 quite lacking, though to a far lesser degree than the rectifier. I could see if one had an already bright system they were trying to tame, the 6L6 might prove appealing. However, I could not accept the loss in low frequency tautness, clarity, and overall refinement.

On page 10 (or 12 of 16 counting the .pdf way)of Hifitime's manual, "The 5U4 or 5U4G can be used in place of the place of the GZ34." Not that everyone here doesn't already know, but the GZ34 is the 5AR4.

Another Dynakit ST70 manual states, "using this tube [5U4G vs 5AR4 - Trelja insertion] type will cut down on the maximum power rating of the amplifier."

As we know, during that era, the goal was to deliver as many watts as possible, and in addition to far better sound, that's what the 5AR4 provides.