The AA8V 6AG7 Amplifier
by Greg Latta, AA8V

Typical Operating Conditions

Class A Linear Amplifier:
The 6AG7 amplifier is different from most RF power amplifiers in that it is a class A linear amplifier. Most RF linear amplifiers are either class B or class AB. A class A amplifier will, for a given amount of drive, produce more output than any other class. Since I had a very limited amount of drive available from my VFO, a class A amplifier was the way to go. Class A amplifiers are also the least efficient of all amplifiers, but that is not a consideration here.

The 6AG7 was chosen because of its high transconductance (11,000 umhos) and its relatively high plate dissipation (9 watts). Compared to other tubes such as the 6CL6, 5763, 6AQ5, 6V6, and 12BY7, it has the best combination of gain (transconductance) and plate dissipation. Higher plate dissipation means that the tube can be operated with less bias, which translates into higher overall gain and more output. According to the RCA tube manual, a 6AG7 can deliver 3 watts of output when operated as a class A amplifier, which is more than enough output to drive the 6146B linear amplifier and just enough for QRP operation when used barefoot.

In a class A amplifier the plate current (and thus power input into the tube) does not depend on the drive level or power output from the tube. It remains essentially constant, unless the tube is overdriven into the nonlinear region. Plate dissipation does, however, depend on the power output from the tube. Since the sum of the power output and the plate dissipation is the total power into the tube, higher output means lower plate dissipation.

This behavior can be seen from the following measurements of cathode current and power output made on the 6AG7 amplifier:

Though cathode current is not the same as plate current (it is the sum of the plate and screen currents), we can still see that the total input to the tube is essentially the same, regardless of the frequency of operation and the amount of drive. This is typical of the class A amplifier. These measurements were made into a 50 ohm dummy load with a 1:4 transmission line transformer between the VFO output and amplifier input.

The decrease in output on 30m and 20m is not caused by the amplifier, but is due to less drive from the VFO on those bands.

Operating Bias:
The gain of a class A amplifier increases as the bias on the tube is decreased. Unfortunately, the plate dissipation increases as the bias is decreased. If the bias is set too low, the plate dissipation will be exceeded and the tube will be damaged. In the 6AG7 amplifier, the 180 ohm cathode bias resistor was carefully selected to provide the least bias possible while keeping the plate dissipation safely under the maximum of 9 watts, even if no input signal is present. Thus, the tube operates within its ratings even there is no signal input, no antenna or load connected, and even if the output circuit is not tuned to resonance. It's a robust circuit!

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu