ToddD wrote:
If I use the GZ34, should I get the B+power down to 360Vdc? Spec sheet for the 3663A list 400Vdc Max on the plate voltage. I think I know the answer, but I would like confirmation. I have been using the 5U4GB for now. I think I am okay with the measured B+driver voltages because of the plate load resisters, but let me know if that should be a concern.
Ok, lets break this down logically.
First, don't worry about the driver stage. All these voltages are just fine.
Now, the design point for the power stage is Vp=261v and Ip=90mA. This puts plate dissipation at about 23.5W (i.e. acceptable). Since this is a self biased amp, the total B+ circuit (transformer, tube, and bias resistor) need to be taken into account for B+ determination. The transformer has a 94Ω primary resistance and therefore a voltage drop of 94Ω*0.090A≈8.5v. The bias voltage is 0.090A*910Ω≈82v. So the required B+ voltage is 261v+8.5V+82v=351.5v. This is about what is on the load line design sheet (1v different). Whatever you do to the component values, this approach remains the same.
Now let's look at your GZ34 measurements. B+=374v and Ip=110mA. You used an 820Ω bias resistor so the bias voltage should be 0.110A*820Ω=90v. The transformer drop is about 94Ω*0.110A=10.3v. This makes the plate voltage 374v-10.3v-90v=237v and the plate dissipation 237v*0.110A=30.1W. This is too hot to run the 6336. In my build, I ended up with about 200Ω of series dropping resistance in the power supply to get the B+ down to where I wanted it with the GZ34.
So you have two choices, stick with the 5U4GB and be happy, or add some series resistance to get the B+ down and use the GZ-34. In the end, it's really about the plate dissipation of the power tube. The design point runs the power stage at about 80% of the rated plate dissipation. Under these conditions the 6336A tubes should last a good long time. At 30.1W you'll burn (literally) through power tubes much more often. Since these are generally pretty expensive tubes, I recommend lowering the plate dissipation (by lowering the B+) if you want to use the GZ-34 rectifier.