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Re: [Amps] Tetrode calcs

Subject: Re: [Amps] Tetrode calcs
From: Manfred Mornhinweg <>
Date: Mon, 27 Apr 2015 19:45:28 +0000
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How do I calculate G2 voltage from the Anode voltage?

Basically you decide at what input power level you will run the tube. Along with Anode voltage that gives you the anode current. In a single ended amplifier, the peak current is 2.8 times that average current.

Then you decide whether you will run this in class AB1 (positive peak grid voltage is zero volts) or AB2. If AB2, you decide how much positive you will be driving the grid.

Then you look up the tube's datasheet, and try to find out from it what screen voltage you need, to give the desired anode peak current at your chosen positive peak grid voltage. Some datasheets might be more helpful in this than others. In some cases you might have to try.

And then you check whether the resulting screen voltage is within the tube's maximum ratings!

Tube is 4-400a.  Class AB2.

The design calls for Va of 3200V, but I only have 2500V so I will just
go with that.

Design EG2 is 600V, but that will be different for a lower Va ?

To get the same power at lower plate voltage, you need more current. That means, either more drive (along with more grid current), or more screen voltage (and current), or both.

The maximum allowable screen voltage is 800V, but a high screen voltage along with a relatively low plate voltage typically leads to excessive screen dissipation and low efficiency. And the maximum allowable plate current (DC) is 350mA, so you are limited to 875W input power if you only have 2.5kV available.

To be able to drive the tube to 350mA DC, meaning 1A peak, the datasheet tells you need about +20V positive peak on the grid, when the screen is at 500V. That might be workable. The grid dissipation should not be a problem at that level. With a higher screen voltage, you would need less positive peak grid voltage. But even at 500V on the screen, that tube starts going into saturation (distortion) at about 1100V! So the IMD will be poor, if you drive the tube to a reasonable output. With higher screen voltage, this will turn even worse.

I would think that a 4-400A doesn't combine very well with a 2.5kV supply, unless all you want is relatively low power operation. If instead you used a 3.5kV supply, you could just run in class AB1 for the same output power, with saturation starting at 800V, giving you a better combination of efficiency and distortion.

There are other tubes that work fine at 2.5kV.

All I wrote here is judging from the tube's datasheet. I have never used that particular tube myself, so I can't talk from practical experience. Take what I wrote with a grain of salt.


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