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.

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.

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.

When I worked for an electronics store back then, I don't ever remember Dynaco saying it was okay to use them. Here is a copy of the owners manual where on page 9 it states, "A 5U4 type of tube can be used for testing purposes." I have never seen the 5 volt filament ratings that I can remember. A 5U4 draws about 50% more filament current than a 5AR4. They run warm enough with a 5AR4 in them in my opinion. I don't think it's worth the risk, especially if you have a '59 to '62 built unit, with the better transformers a lot of people like.