# [Amps] Tetrode calcs

Jim W7RY w7ry at centurytel.net
Mon Apr 27 20:58:50 EDT 2015

```Check out the schematic and operating voltages for the Johnson Viking
Thunderbolt amplifier.  It ran a pair of 4-400s.

73
Jim W7RY

-----Original Message-----
From: Manfred Mornhinweg
Sent: Monday, April 27, 2015 12:45 PM
To: amps at contesting.com
Subject: Re: [Amps] Tetrode calcs

Steve,

> 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.

Manfred

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