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Re: Dynaco Question and 5U4

Posted by Steve O (A) on December 9, 2006 at 19:47:29

In Reply to: Dynaco Question and 5U4 posted by Paully on December 9, 2006 at 18:54:40:

You didn't mention which Dyna you're dealing with here so I'll assume it's either an ST70 or a MK3. Regardless, a 5U4 will work just fine in either. It can handle both voltage and current. The problem is that the voltage drop under load is greater than the 5AR4 so you'll get less max power. Another issue you need to watch out for is that the 5U4 heats up much faster than the output tubes so initial voltage surge will occur and you're PS caps must be able to handle this. IMO it's a good idea to have PS caps capable of withstanding full surge voltage because one possible fault condition is that of no output tubes present. You'll get full surge voltage regardless of rectifier in this case.

With the advent of low cost and reliable Chinese an Russian 5AR4s you might have an alternative to the silly cost Mullards. Also consider the US made 5AR4s by GE and Sylvania or the Japanese Mullard copies by Matsushita or whomever. All are quite good in my experience and 1/2-1/3 the cost of NOS Mullard.

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