He added that the 450 VDC that I had measured on the output transformers' center taps was a little high and added: "higher than normal voltages usually translate into shorter tube life".
This amp has the original and precious Dynaco/Mullard EL-34s and this made me realize that I better do something about this higher than normal B+. I thus bought a 5U4GB.
This tube, however draws too much current (1.1 A) for the 5V winding of my Dynaco power transformer. Someone in a post said the overdraw shouldn't damage the latest power transformers. But mine becomes so hot after an hour of operation that I decided to buy a separate filament transformer to power the 5U4.
With the VTA board, the fixed bias circuit is now on the board itself and this frees enough space under the left channel output transformer to put a filament transformer there. It needs to be of a size similar to the choke found under the right channel output transformer.
Fortunately I found exactly what I needed: a Hammomd model 166-L5 power transformer with a 5 VAC, 2 A secondary. And what is even more interesting, it has EXACTLY the same same dimensions as the Dynaco choke. It fits perfectly inside the chassis! Obviously, transformer laminations and cores come in standard sizes.
After installing and wiring the filament transformer and re-adjusting the bias and AC balance, I measured the voltages and now the primaries' center taps show a 410 V which should be fine and allow the longest practical life for my output tubes.
I'm including a picture showing the modification. Notice on the rectifier socket 2 1N4007 diodes to block inverse voltages and protect the 5U4 as some have suggested.
Surprisingly, the Dynaco power transformer is now distinctly less hot.