Zener Diodes to lower plate voltage with fixed bias

[nickfl]

I'm working on a schematic for a new project, push pull EL84 power section with fixed bias. I expect the transformer I'm using to be in the range of 360-370v B+ and I want plate voltage to be more like 320-330v. This is a non center tapped PT that I will be using with a solid state bridge rectifier.

I've used zener diodes on a few builds to adjust voltages and I've known that while you can just put them in series with the center tap on cathode biased amps it can cause problems with fixed bias, but that situation has never come up so I haven't bothered to learn why until now.

So, after some reading I understand that the issue is because the zener trick works by raising the common ground/chassis reference with respect to the HT winding center tap, therefore it reduces the voltage from the common ground to B+ but would equally increase the voltage difference from the negative bias supply to the common ground assuming the bias supply comes from the same transformer winding.

So, my question is this: my transformer happens to have an entirely separate bias supply winding (not a tap off of HT, but its own coil, 27-0 @ 0.1a) and my thought is that this means I should be able to use zener diodes on the bridge rectifier return to adjust B+ without effecting the bias supply since the bias will be derived from a transformer winding that is referenced to the common ground independently from those zener diodes.

I think I have this worked out correctly, but it is pushing the boundaries of my electronics comprehension so I would appreciate it if anyone with a better understanding could confirm or poke holes in my assumptions.

Here is a preliminary schematic showing the power supply arrangement I am thinking about:

Jetwreck Schematic.png

[sluckey]

Those zeners will not affect the bias voltage since you have a totally separate bias winding.

[martin manning]

You might want to consider a MOSFET B+ reducer as discussed here: https://ampgarage.com/forum/viewtopic.p ... 65#p332265

[dorrisant]

You might want to consider a MOSFET B+ reducer as discussed here: https://ampgarage.com/forum/viewtopic.p ... 65#p332265

+1