Dynaco MKIII high B+ 5U4 low bias

I have a pair of MKIII's I recapped 20 years ago with the help of a retired tech, who is no longer available.  I have not used these in a long time, but I am having a pair of AR3a's restored and I want these guys for the power.  I am now listening to them with a pair of EV-4s and realize that I should have been using them all along.  Some of the B+ voltages are about 40V higher than called for, however with the 5AR4s they both Bias at 1.56. I have a couple of 6550's that develop light red plate glow, when I use 5U4s that goes away, but the best bias I can get is around 1.26 on one and the other one about 1.50  I am wondering what to look for as the cause of the one only doing1.26, it also has some hum.  Thanks for the help, I am happy I finally found this forum.

I do not see a way to post pics.

Welcome to the forum. You will find all the info you need for those amps here. Although you will get many varied opinions.

The cause of the higher B+ may just be a higher line voltage than original. Assuming you have a DMM you can check your house outlets for AC. Most places are running much higher than the 115/117 VAC that was common long ago and is shown on the Dynaco schematics. Higher house AC will translate to higher amp B+. If that is the case do not worry about it now. See below

If both amps bias to 1.56 VDC at the test point with a 5AR4 then the amps should be fine. If one 5U4 results in one good reading and one low reading then maybe one tube has one bad section. The correct tube is the 5AR4. Stick with them.

As for red plating at full bias voltage. Turn the bias so the bias voltage at the test point is 1.2 -1.25 VDC. This will drop the output tube idle current to the 50 mA/tube region and extend the tube life. Very important if those are vintage 6550s.

All three responses above have been discussed many times in the forums posts. Do a search and read many of the threads. There are many solutions to the high B+, the value of the 5AR4 over the 5U4, and the lowering of the output idle current. Bob has posted a sticky thread about how to post photos. Read it and you will get it. It took me a try or two as well.

Okay thanks for the response, I will put the 5AR4's in and cut the bias back. I am using Jan 6550's right now but I have some Tungsols to try later. Thanks again.