Hi Mehfuzhoss. I have found that there's a lot to consider when trying to reduce B+ voltage. My experience may differ from others, but I have just about always found that it's best to get a PT with a voltage you're happy with. Here's why.
It took me a while to learn the lesson that B+ from a tube rec (assuming you have a tube rec) is usually around 35-40% higher than half the stated Vct. Hammond says the 290BX is 660Vct, so depending on the type of rectification, I'd expect a B+ of at least 450 or higher. Which is what you have. So now you're going to be fighting that high B+ all along the way in order to get the amp to sit where you want it.
Of course, if you use a 5Y3 instead of a 5AR4, you'll get less B+, which sometimes works in a pinch, although it usually won't reduce the B+ by much more than 20 or so volts (in my experience), and you want to drop about 50 from what I can see.
What used to confuse me most was that some designers and builders will recommend PTs that kick out much more plate voltage on the secondary side than is sometimes needed, at least for me. I finally learned to work with what made me more comfortable regarding the final product.
One of the reasons I found zeners and dropping resistors aren't always the best option is because they will drop the voltage, but that voltage converts into a considerable amount of heat. Depending on where your transformer and tube rec are located, and depending on the size of the interior of the chassis - and depending on how well ventilated it is (any air holes in the chassis?) - things can get very very hot, almost guaranteeing a shortening of the life of the amp, and leading to a lack of confidence generally in your own skills as a builder.
But if you're set on that PT and install a zener strip, a good fix if you have too much heat is to install a 120mm computer fan in the cabinet. Some run almost silent. They are usually designed to run at 12vdc, so I also install a small 5v switching transformer (quite common and quite cheap because they're used with bluetooth etc, although they may have to come from China - I get mine from Bangood). The 5volts will only run the fan at half speed, but use a high speed fan (2200rpm) and you'll get a very nice 1100rpm that keeps everything back there happy.
I've tried installing bucking transformers and other "fixes," but if I were in your shoes, I'd simply bite the bullet and buy a transformer with a 550Vct-600Vct. It'll still produce plenty of B+. Again this is just in my experience, but I have never found a problem with voltages being "too low" in my builds. A lower voltage PT means I can increase the B+ anytime simply by going to a 5AR4 or 5U4 from a 5Y3 (as long as the PT can handle the extra mA current draw of the 5AR4).
But it's not so easy to go the other way. Ultimately, anytime I have a PT with a too-high output, I feel like the boy with his finger in the dike. Depending on your perspective, that's either fine or annoying.
Hope some of that helps you in your build.
Topic: 6v6 Plexi B+ voltage suggestion (Read 2576 times)