M-125 Tube Rectifier 5U4 or 5AR4?

When I ordered my kits three years ago it was suggested that the tube rectifier be a 5AR4. I also ordered a pair of Weber WS1-T solid state rectifiers to try out, which I've been using ever since comparing my two choices at the time.

I use a Variac to lower the voltage down to 117V which is a good compromise for all the internal voltages when using the WS1-T.

So, one question is, why was the 5AR4 recommended instead of the 5U4 which is still listed as the recommended rectifier on the M-125 page?

The tube compliment when I built the kits was: KT88, 12AX7, 5AR4 then decided to use the WS1-T.
It is currently: KT120, 12BH7, WS1-T.
The only mod is R28=8.2Ω

I've only got one 5AR4 that works, the other one went kerflooey (sorry for using a technical term). So I'm wanting to order tube rectifiers, but would like some guidance as to which type to get and why. Should I be trying the 5U4?

ttocs,

You already have built a high quality system with some premium components. I recommend to not stop and go with a quality GZ33/GZ34. This is not only one of the most beautiful glowing tubes, it is tough as nails. You pay a premium for it but it has the looks, it has good sound, and you probably can get by without the Variac if you had to or so desired. I cannot recall reading a single article in this forum where someone had a blown CZ33 or CZ34, but anything can happen. Deepee99 is unfortunately no longer with us, but he had the best quote, something like " women cannot keep from disrobing themselves in front of the beauty of the CZ34."

This is a rough time to purchase quality NOS tubes like Mullard and Gold Lion with Russian stock being non grata. You can check if Jim McShane has any in stock. Upscale Audio used to sell guaranteed quality Mullard tubes but looks to be sold out of these. I do have one spare that I can sell to you at cost, if you can find another from another source, or you can take your chances with eBay or Etsy. I got mine (GZ33/CV5745) and a spare from Upscale. They run a quality organization. Best of luck...
Mike

Technically a 5ar4 should work going by tech sheet specs. However new production versions rarely measure up. As I know all too well. The GZ33 suggested by AmpedUp is the Cadillac solution. Another stout answer is GZ37. Ive been running GZ37's in my M125 for 5 years now, zero issues.

LeGrace, what is the difference in the GZ33 and the 37 ? I've been looking at the 37 from TubeDepot.
Also remember DeePee tried but couldn't kill his 37 !! Thx, Dale

Either works fine, price and availability is a larger consideration.

Thank you all for the replies! Very helpful!

I now recall reading the info Bob linked to when I built my amps. Totally forgot all about it. I've got electrostatic speakers, so as such, I will keep running the SS Rectifier.

Things are good, but my main goal was to reduce the internal voltages by swapping over to tube rectifiers.

As far as 5AR4 vs 5U4 here's a few things to consider

5AR4 is indirectly heated and 5U4 is direct so B+ will come up faster with a 5U4. 5AR4 will give a more delayed turn on if you are worried about cathode stripping.

5u4 heater current is 3A vs 1.9A for the 5AR4. As the 5U4 was originally listed for the M-125 it shouldn't be an issue for that amp but the power Xformer will run cooler with the 5AR4.
Definitely something to keep in mind in other apps. For instance, in an original ST70 with the stock transformer, it ran quite warm to begin with. A 5U4 might push an old transformer over the edge in that amp.

The 5AR4 has a higher PIV @ 1700V vs 1550 for the 5U4
However, the 5U4 has better current ratings rated at 1A continuous and 4.6A pk vs 825mA and 3.7A pk for the 5AR4

And finally, the 5AR4 has less voltage drop with 17V at 225mA vs 44V drop for a 5U4 at that current. The 5U4 will have more voltage droop on heavy demand.
In theory you should have tighter, deeper bass with a 5AR4.
On the other hand, if you have high line voltage the 5U4 might help bring the operating voltages closer to spec in the amp.

It's possible to use the 5V rectifier heater winding as a bucker for the 117v transformer.

see How to wire a 'Buck' transformer - diyAudio