5U4G suitable for Dyanco ST-70?

I would not use a 5U4 or a 5V4 in the ST-70. The 5AR4 is a better sounding tube and is indirectly heated so it has a soft start that puts less stress on the output tubes and circuitry, and if you subscribe to the phenomenon of cathode stripping in outputs, it should mitigate that as well.

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.

FWIW, an older tube manual I have confirms that the filament draw of the 5U4 (all suffixes) is 3 amps. The 5V4 (all suffixes) is 2 amps, and the 5AR4 is 1.9 amps. All at 5 volts.

I have no knowledge of the ratings of the ST-70's transformer.

Regards,
-- Al

I wouldn't use a 5U4 in a ST-70 either. Also, Dynaco didn't recommend it back it the day. Nowadays, the voltage from the utilities is higher in a lot of areas than it was back then too. That means the 5U4 could be harder on the amp's power transformer than it was in the '50s to '60, when the AC line voltage was typically lower.