There have been several threads at various times on the suitability of subbing a 5U4G rectifier tube for either a 5V4G or a 5AR4 in a ST-70. I asked Will Vincent, a respected re-builder of ST-70s (among others) for his opinion and received the following email:
Harry: Never use a 5U4 in a Dynaco 70.....The power transformer is rated for 2 amps on the 5 volt winding....The 5U4 is rated at 3 amps......Not good......Will
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.
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.
I have been wanting to follow up on my post above about the 6L6G in my Stereo 70. Initially I thought it had a very impressive midrange, but as time went on and I tried different 5U4 rectifiers, it became apparent that I wasn't so crazy about it, after all. It seemed kind of fuzzy or something and the top end didn't sound quite right to me. Whatever. I know a lot of folks like 6L6 in the Stereo 70, so to each his own. My favorite so far is the Gold Lion KT77. I understand that the EH 6CA7 is probably also a very good choice. I have reason to believe some Chinese tubes have really good potential in the 70 if you can find them properly screened and with straight pins.
The Phillips version of the 5ar4 gz4 is a big built type and fantastic sounding several reviews on them they are an inch + taller and look great . up there with the best ever made around $200 the cryoedMatched kt66 with this tube and RCA 50s 7199 Black plates are a killer combo ,and you want a great cap set up. Audyn reference caps in my opinion better then any Mundorf cap and I have tried them all. In fact I like this cap better then their top Copper cap and priced decent one thing I am doing putting in all Teflon Copper gold sockets noticably bettersonics plus Stabilant-22 on all cleanse tube pins and sockets ,as well as everyconnection .a Nasa favorite and last up to 14 years if not disturbed and temps to 450. Tip of the day. And get a Furutech Copper gold IEC socket with filtering built in $65.