Real Radios Glow In The Dark

By Ed K2OHK

In the 1960's it was very ordinary for hams to build their own amplifiers. Not too many around now, I thought you fellows would like to see mine. 

The two amplifiers shown here are in regular service at K2OHK. WA2KKD, Walt (SK) and I built them around 1964. The top one in the photo on the left was his. It was built from an article in QST, January 1960. The photo shows his beautiful cabinetry work. The RF chassis is shown in the second photo - extremely neat work. The most interesting feature of this amplifier is the meter on the right side of the front panel. In those days hams could not afford an output wattmeter, or they were not available. The meter is an RF sniffer that samples the outgoing current. It has an arbitrary scale from 0 to 10 and a pot to keep it in the center of the scale. Using it, you tune it for maximum output. Rules of the time required hams to record their input power, with a maximum of 1KW. The plate current (left meter) was multiplied by the known plate voltage, which gave your input power. This being a Class B amplifier, you figured your output to be around 50%. In recent years I have replaced the original 866A mercury vapor rectifiers with diodes. I have also rewired the plate circuit to completely separate it from the filament supply. I have installed a manual switch controlled soft start circuit and changed the original 811A tubes for 572B's.

The amplifier with two drawers was built by me. It has four 811A’s - a very common configuration of the time and indeed also today, as with the Ameritron 811H. I copied the Collins 30L, B&W and Heathkit amplifiers. I took what I wanted from each. The RF deck is shown in the photo on the right with the cabinet. The large rectangular object on the right side of that photo is the filament transformer. The rest of the power supply is in the lower drawer, also a very common configuration. It is very heavy, with an iron core choke and oil filled capacitors. Just now (November 2011), it is sitting on my worktable, after some rewiring, waiting for a ham friend to pick it up for me and put it back in the cabinet. 

The most interesting feature about this amplifier are the separate grid and plate meters which enable me to keep an eye on both functions at the same time. I have found that if the amplifier gets out of linearity on voice peaks, the grid meter (on the left) will drive across the scale and pin faster than can be observed with a scope trapezoid pattern showing "flat topping," the traditional method of keeping an eye out for overdriving. The plate meter, by the way, has a hand-wired shunt that was made from a piece of nichrome wire used in toasters. Now THERE is a neat old ham trick! The amplifiers were built in the time when the driving transmitters had PI network outputs. They could tolerate looking into a load with an SWR. These amplifiers have no input tuning. The coax runs straight into the cathodes. Our modern transceivers will cut back on their output to protect the final transistors and they require looking into a 50 ohm load. Like a lot of other hams I just put a small antenna tuner in the line between the transceiver and the amplifier instead of modernizing the input circuit. Unlike the new solid-state amplifiers, changing bands does require tuning up. Well, I do like playing with the knobs and I do like telling other hams what I am using.

Ed K2OHK