The resistors in the screen grids are not large, ranging from 100 ohms to 1 k ohm depending on tube type. The wave forms show that by connecting the screen grid of one tube to the plate of the opposite one provides the screen with a signal which is in phase with the cathode voltage but above it by the value of the B batteries. Notice that B 2 is connected between the cathode and screen of V 1 and B 1 is connected between the cathode and screen of V 2 . This allows the tubes to have the higher transconductance of the pentode connection rather than the lower Gm of the triode connection. A higher transconductance means a lower output impedance, gain closer to unity, and less overall distortion.

Proper Load Impedance.

Let's work out some numbers for power levels of 25 and 50 watts. First of all, in the following tests I'll be using two 500 volt power supplies for B 1 and B 2 . If the transfer function of the tubes went in a straight line right down to zero, the peak voltage across the load (output transformer primary) would be 500 volts. Unfortunately, tubes don't have that kind of transfer function.

A very conservative minimum voltage for most power pentodes is 100 volts. Therefore, the maximum, peak, voltage across the load is 400 volts.

V RMS = 400 divided by the square root of 2.

V RMS = 400 / 1.414 = 283 Volts.

Now if we want 50 watts from this amplifier we need an impedance of,

R = V squared divided by P

R = 2832 / 50 = 1.6 k ohms.

For 25 watts we need a load impedance of 3.2 k ohms.

Load Impedance.

Wired normally it will present a theoretical load impedance of 3.4 k ohms. The measured impedance is 3.6 k ohms. That's a little above the 3.2 k ohms needed for a 25 watt amplifier so we should expect less than 25 watts in this configuration.

The other way to operate it is at half impedance. That is wire the secondary for 16 ohms and terminate it with an 8 ohm load. According to theory this should give an impedance of 1750 ohms but the measured value is 1840 ohms. This is not recommended by the manufacturer but I can hope they don't send the impedance police after me. Once again this is high for a 50 watt amplifier so we won't get 50 watts when wired this way.

Another way to wire the transformer and defy the impedance cops is to use the screen taps instead of the plate taps and wire the secondary for 4 ohms. When the secondary is connected to an 8 ohm load the measured impedance between the screen taps is 1,160 ohms. Based solely on voltage this would give us 69 watts but the maximum current the tubes can deliver will begin to limit the power long before that level is reached.

In the tables below the values of impedance presented by the transformer will be stated in the column headings.

At least that's the theory.

A real amplifier.

We will skip all that and go directly to a bootstrapped driver using a 12AX7 that will deliver plenty of voltage for the purpose.