The AA8V 6AG7 Amplifier
by Greg Latta, AA8V

Main Page and Exterior Photos

6AG7 Amplifier

6AG7 Amplifier Pages:
 6AG7 Amplifier - Main Page and Exterior Photos  Tank Coil Construction Details
 Interior Photos of the Finished Amplifier  Schematic Diagrams and Circuit Descriptions
 Construction Photos  Testing And Preliminary Work
 Typical Operating Conditions  Why Use A 6AG7?

Introduction and Historical Background:
After using the 6CL6 One Tube Transmitter and the 6146B amplifier on the air for several years, I decided that it would be great to have VFO control, rather than crystal control. Crystal control worked fine, but was limiting. In particular, I found that many modern operators did not know that they should tune up or down a few kHz after calling CQ, just in case someone using crystal control was calling them back. The result was a fair number of missed QSOs.

I had built an amplified digital VFO and had successfully interfaced it to my venerable Eico 720 transmitter. The combination worked beautifully, but I wanted to use the VFO with a homebrew transmitter, rather than a commercial transmitter. Plugging the output of my amplified DDS VFO into the crystal input of the 6CL6 transmitter (via a 1:4 transmission line transformer) worked well at driving the 6146B amplifier, but it was a compromise. The 6CL6 circuit was designed to be an oscillator, not an amplifier. It provided plenty of drive for the 6146B amplifier, but by itself it didn't give that much power out, and I wanted to run it barefoot as a QRP transmitter. I figured that I could probably get more power out if I designed a new one tube amplifier optimized for use with the DDS VFO. I could then use the VFO with the new amplifier as a QRP transmitter, or I could add the 6146B amplifier for higher power.

Remembering that I had a small crystal oscillator transmitter in a corner of the shop that might serve as the basis for the new amplifier, I began testing and preliminary work to see if I could use the little transmitter as the basis for a new amplifier. After much experimentation and a total rebuild on a new chassis, the result is the 6AG7 amplifier that you see in these pages.

Exterior Photos of the Finished Amplifier:

Overall View:
In this view of the amplifier the power transformer and 5Y3 rectifier are in the back, and the 6AG7 and other components are towards the front. The 6AG7 is the black metal tube in the middle of the chassis.

The chassis is actually a 9 inch cake pan, in keeping with the tradition of many homebrew transmitters built in the 1950s and 1960s. The cake pan came painted with an attractive, durable finish, and I can't imagine a better or more attractive chassis for the amplifier.

6AG7 Amplifier
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Front Close Up View:
The 6AG7 amplifier is a band switching amplifier that uses two plug in coils. The band switch is in the center of the front panel and has four positions, so a single coil form can actually accommodate four different bands. More inductance is switched in as the switch is rotated counterclockwise from D to A.

I am currently using the amplifier on five bands, so I used one coil to cover three bands and another to cover the two remaining bands. The coil visible in the photo covers the 80m, 60m, and 40m bands, and another coil covers the 30m and 20m bands.

As in many other tube amplifiers, a pi network is used to match the output of the tube to the antenna. The plate tuning capacitor is on the left, the tank coil is in the center, and the loading capacitor is on the right.

The key jack is at the lower left, and the power switch is just to the right of the key jack. The pilot light on the right indicates when the amplifier is turned on.

6AG7 Front
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Side View:
In this side view of the amplifier the rear panel is to the left and the front is to the right. The rear panel contains a fuse holder and the input and output connectors. The 5Y3 rectifier tube is behind the power transformer on the left.

The 6AG7 tube can be clearly seen near the middle of the chassis. The loading capacitor is at the upper right and the plate tuning capacitor is at the lower right.

Connections to the plate tuning and loading capacitors are made through grommeted holes in the chassis. These can be seen just above and below the left side of the coil in the photo.

6AG7 Amplifier Side View
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Rear Close Up View:
This is a rear view of the 6AG7 amplifier. On the top of the amplifier are the 5Y3 rectifier tube on the left and the Stancor P-6010 power transformer on the right. The transformer was cleaned and then airbrushed with gloss black Rustoleum to restore its original appearance.

The rear panel contains an SO-239 coaxial connector for the output on the left and an RCA phono connector for the input at center. The 1 Ampere fuse on the right protects the power supply in case of component failure, and a strain relief is used to protect the grounded power cord.

The chassis is a 9" square cake pan with a very durable finish that I obtained from the local surplus store. Many homebrew transmitters from the 1950s and 1960s used cookware for the chassis, since the chassis was usually one of the most expensive parts of a project.

6AG7 Rear View
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6146 Amplifier Thumbnail IMageClick here for pictures and information on the matching 6146B Linear Amplifier

Wingfoot Exciter PictureClick here for pictures and information on the Wingfoot VFO 2E26 Exciter

813 Amplifier PictureClick here for pictures and information on the Wingfoot 813 Amplifier

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