Check My Math (B+ String)

[Weathered]

Alright, this is my first attempt at designing a power supply, so be gentle. No schematic yet, but here's my math (following Merlin's guide on the Valve Wizard page).

The amp is going to be 4 EL84, 3 12AX7, using a TW PT at 260-0-260, solid state rectifier.

Taking 260 * 1.4 to get a B+ off of the rectifier of 364 volts.

Using the calculations here: http://www.freewebs.com/valvewizard1/smoothing.html , I am calculating the total power draw to be 177 ma, 132 of that being the plates of the EL84s.

Here are my estimated current draws for the B+ string, using Merlin's estimation of 5ma per triode of the preamp:

B+1 - 177 ma
B+2 - 48 ma
B+3 - 30 ma
B+4 - 20 ma
B+5 - 10 ma

Desired B+ voltage at each stage:

B+1 - 364
B+2 - 300
B+3 - 200
B+4 - 170
B+5 - 150

Using Merlin's math, here are the resistors between each stage:

B+1 to B+2 - 1K3, 3W (dropping 64 volts with 48 ma of draw)
B+2 to B+3 - 3K3, 3W (dropping 100 volts with 30 ma of draw)
B+3 to B+4 - 1K5, 2W (dropping 30 volts with 20 ma of draw)
B+4 to B+5 - 2K, 2W (dropping 20 volts with 10 ma of draw)

These values look too low compared to what I've seen in most amps (the 9K1 values in a TW, 10K values in a Plexi), so I want to be sure that my math is right before I continue on with my design.....

[diagrammatiks]

you might be a decimal off.

[XgamerGt03]

You usually only get about 1.2 times the RMS voltage out of a solid state rectifier when under load.

That may slightly change your plans.

[martin manning]

Is the 260VAC secondary voltage loaded or unloaded? If it's unloaded you need a factor of 0.85 to 0.9 for losses after the 1.414 conversion from RMS to peak.

Each 12AX7 triode will typically only draw 1 to 1.5 mA, or 2-3 mA each bottle, so say 9 mA for three. Idle current for each EL84 at 70% max dissipation will be about 29 mA at say 320 volts, so 116 mA there. I'd lower the total idle current draw to ~125 mA.

[Weathered]

Is the 260VAC secondary voltage loaded or unloaded? If it's unloaded you need a factor of 0.85 to 0.9 for losses after the 1.414 conversion from RMS to peak.

Each 12AX7 triode will typically only draw 1 to 1.5 mA, or 2-3 mA each bottle, so say 9 mA for three. Idle current for each EL84 at 70% max dissipation will be about 29 mA at say 320 volts, so 116 mA there. I'd lower the total idle current draw to ~125 mA.

Right now, I'm working with unloaded figures. I will rework the math with loaded figures and see if that changes things.

Thanks for the updates to my assumptions - I was working based on the assumptions below:

EL84 plates drawing 33 ma each (132 total ma)
EL84 screens drawing ~ 3.5 ma each (15 total ma)

Merlin quotes 5 ma per triode for a 12AX7, but that did seem a touch high for what I was expecting.

Was the figure you quoted for the EL84's both plate and screen current, or is that plate current, and I can add a screen current in at 1/9 or so of the plate current?

[martin manning]

Was the figure you quoted for the EL84's both plate and screen current, or is that plate current, and I can add a screen current in at 1/9 or so of the plate current?

Plates and screens. If you have 320V on the plate, you can back-out the plate current at ~70% of max Pa (12W), so 12/320*0.7=0.0263, then increase 11% for the screen current 0.0263*1.11=0.029.