Switching 5U4 rectifier for 5AR4 in Mk IIIs

In an earlier post dealing with use of EL 34 and 6L6 power tubes in VTA octal Mk. IIIs, Bob L. indicated that changing the 5AR4 rectifier out to a 5U4 would drop the B+ voltage into a safer range and allow use of EL 34 and 6L6 power tubes.  

Question: so would the rectifier swap be the only change required (i.e. no resistor or other changes needed to the circuit)?

Thanks,

Kevin

the 5U4 has direct heaters, and a smaller cathode area, this gives a larger voltage drop.
Look at it as an "built in series resistor", larger then the one in a 5AR4

that the "built in series resistor" of the 5U4 rectifier tube would be the only change needed to switch to running EL34 and 6L6 power tubes in my VTA octal Mk IIIs??

(Sorry to be so literal, but with my limited knowledge, I find it the only tenable option)

Thanks,

Kevin

Directly heated, the 5U4 also lacks soft-start but that didn't bother Dynaco, which used it in the Mark II power amplfier.  

As a very common rectifier in tube-type color TVs, the 5U4 is relatively common and thus often cheap!

All good info. I have a couple of us-made labeled "Hammond" which I think are GE...

Kevin