My first post so be gentle. I am interested in purchasing either an ST120 or two M125 kits from Bob with my tax refund money. The decision on which to buy will probably be made by she who must be obeyed.
But the reason I am posting in this thread, after a lot of reading, is that I have been thinking about this mod, the two diodes inserted before the rectifier tube, and it doesn't make a lot of sense to me. Here is why:
The maximum PIV of the GZ34 is 1500V. The maximum PIV of the 1N4007 is only 1000V. So the GZ34 will do a good job of protecting the 1N4007. Right?
The forward voltage portion of the AC sine wave does not stress a rectifier tube unless current is excessive in which case the power transformer is vulnerable as well. It is reverse voltage that can cause flashover. And the reverse voltage across the GZ34 will be virtually the same whether the 1N4007 is there or not because the reverse resistance of the GZ34 will be much higher than that of the series silicon rectifier which will form a voltage divider and the GZ34 will drop almost all the inverse voltage whether or not the silicon rectifier is there or not.
Now does using series balanced diodes help? A little, it will increase the PIV of the diode pair to 2000V protecting the diodes from failing, but still not protecting the GZ34 from a reverse voltage flash over.
My conclusions are placing the 1N4007s in series with the plates of the GZ34 is OK if you wish and if it gives you a feeling of confidence, fine. There is no possible damage from doing this. But I don't think there is any real benefit either.
Further, I think there may be some benefit from adding a single series balanced pair after the rectifier tube to provide protection from the ON - OFF - ON rapid cycle.
In my case, I am going to be using some NOS NIB Mullard Blackburn GZ33's or one if the ST120 is the choice. I am going to combine that with Bob's delay circuit board and use a Tower Manufacturing 30339015 15 amp GFCI 5-Outlet Adapter which will prevent ON - OFF - ON as this GFCI requires a manual reset to restore power.
I am still considering placing the series balanced diodes after the GZ34 (GZ33 in my case) and welcome any advice from this forum. I also welcome any critique of my post and reasoning. Even flames.