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.
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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.
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.
Then again, one can also use 6L6 as a sub for the EL34 and four of those would cut the current draw enough to allow use of the 5U4 with much less risk.
Dynaco used at least three power transformers in the ST-70, large stack cloth lead, small stack cloth lead and small stack vinyl lead. The small stack transfomers are really compromised as it is. If you want to screw around with additional current draw and have one, it might be a good idea to switch to one of the higher current new transfomers available from Triode Electronics, Dynakitparts or have Heyboer make one up for you.
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.
I recently finished building a new Stereo 70. It has the new Dynakitparts PT wound for 122VAC line. I put a thermistor in the PT primary and did some other enhancements, but the circuit is still essentially the original.
The amp sounds great. I just got a set of the new Tung Sol 6L6G and have tried them with 5U4GB and 5U4G rectifiers. There was no need to change-out the two 10k resistors in the bias supply. The greater voltage drop of the 5U4 and probably also the 122VAC PT make this possible.
Ive tried this amp with the 5AR4 with JJ E34L, EH EL34 and JJ KT77. The 5U4/6L6G combination is better than all of them, to my ears.