John E. Coleman (ARS WA5BXO) wa5bxo2006 at pctechref.com
Sat Apr 7 15:56:17 EDT 2007

```Jim:

We might want to clear up something here about plate efficiency.  Many new
people get confused about all this.

Starting with a triode because it is easier to explain:

Suppose we have a triode tube operated in class C service for plate
modulation. Let's use 3000 volts and at 86 ma of plate current with an
efficiency of 75 percent. This would yield 260 watts input and 195 watts
output.

If the plate supply is then reduce to 1500 volts (1/2) then the plate
current should automatically reduced to (1/2) 43ma.  The power input and
output should be reduced by 25% which would then yield 65 watts input and
48.75 output power.  With this configuration, the plate efficiency should
still be 75 percent.  If the output power doesn't drop by the same
percentage as the input power then the tube is not being operated in the
proper class C service for plate modulation.  Thus, the RF voltage output
will not follow the audio modulation voltage.

Now, suppose one wants to maintain the same output power while reducing the
plate voltage. This can be done by retuning and reloading, i.e. increasing
the plate current in an attempt to get the RF output back up.  Under these
conditions, the efficiency may change for better or worse.  But just
reducing the voltage without other adjustments should not change the
efficiency.

Now, if we are going to use tetrodes to cachieve the same results as before,
we will require screen adjustment at the same time as the plate adjustment
but generally not at the same percentage.

Generally the plate voltage is reduced to reduce output, not to change the
circuit operating parameters (Ep, Ip, etc.)  If, on the other hand, someone
wants to achieve a certain power input and output from a particular tube,
then there one should stay within a particular window of operating
parameters.

If a tube is to be used to produce its maximum output then the window of
operation is narrow.  But if the expected power output is to be reduced,
then the high efficiency operation parameter window becomes wider.

A 4-65 has a maximum plate dissipation of 65 watts.  While the best plate
efficiency window is beyond the scope of this article, we might assume that
it is approximately 75 percent for a given voltage and plate current. Given
this parameter, the tube can produce 260 watts input and 195 watts output.

A tube used in this manner--adjusting a plate current to voltage, while
maintaining the same output power-- may prove to over heat, due to the
change in efficiency.  But if the power input is to be 100 watts and the
output to be 75 watts, then the voltage-to-current ratios of Ep-Ip could be
changed significantly and yet maintain the same efficiency.

John Coleman, WA5BXO

----- Original Message -----
From: "Jim Candela" <jcandela at prodigy.net>

The 4-65 is a Higher voltage, and lower current tube. In class C service,
the efficiency is really great at 3kv plate voltage, and is diminished at
lower voltages.

```