Lowering B+ with zener diode

[zhyla]

So I wired up my 5B6 this weekend and it is functional but the B+ is at 510 VDC instead of 400. So I ordered a 100 V/50 W zener diode. My understanding is I put the zener between the PT secondary center tap and ground.

My question is about capacitors. I was reading this page: http://www.aikenamps.com/BackBiasing.html

If you look at the 2nd circuit there is a gigantic capacitor in parallel with the zener. There's a discussion of why that's there but I didn't quite follow and I definitely don't know how to choose the cap size.

Later on he shows a modified approach where the 1st filter cap is attached to the center tap. I don't quite see how to apply this to the circuit I'm building as the center tap is already grounded.

[TD_Madden]

Back in the day. Mission Tweedys came with the zener since his PTs weren't voltage-corrected. I don't remember putting a cap in parallel with it.

[metalangst]

That link shows how to get a bias tap for a fixed bias amp using a power transformer that has no bias tap. The circuit you posted is cathode biased, so you won't need a set up like that back bias. I don't know if it will drop the B+ in that configuration. You may want to look at dropping 50v off the B+with that zener. Did you get the 510v reading with all tubes loaded and the amp in play? Gotta have that load on the supply. I would look at using that extra power upstream. Drop the output plate voltage by increasing the preamp voltage. You may be able to "fix" it with a couple resistors. I did use a 75v zener to drop a hot B+ in a custom amp. That was years ago, I don't remember how it was connected, but I remember it got HOT. It was mounted to a hole in the chassis and the chassis got hot as well.

[zhyla]

The 100 V zener dropped the B+ almost exactly 100 V. But I had something wired wrong so I really only needed to drop 50 V - new zener is on the way. I love tubes and all but dang you can do some cool stuff with silicon.

They do indeed get hot. I bought two, a 100 V and 120 V, and one of them came with a little plastic piece that I guess goes between the diode and the chassis? Didn't make much sense unless it's a thermal conductor. Seems like thermal grease would be more worthwhile.