This is one of the great hidden treasures at hamfests... the Brute 75. It's a 1-tube grid driven monoband HF amp built by Majestic Communications (Maco) in the mid-70's. It uses a M-2057 tube, but can be fitted with a number of different tubes commonly found, today. Most of these have a built in switchable recieve preamp. It's designed to use low level drive, 5 watts or less.
These were originally targeting CB. It has a tuned input and a built-in low pass filter, in addition to a Pi-net output stage. These are very clean, spectrally. By scrapping the tuned input and swamping the grid circuit with a 50 ohm carbon resistor, a simple band switch on the tank will allow multiband operation. These are perfect to add power to a QRP station. With the proliferation of low power rigs, SDR, and others, it's a solid and cheap amp to grab.
As for tubes... the 2057 can be substituted with a 8950 at slightly reduced output. Same with a 6LB6, with a simple change to reduce the fil voltage from 12v to 6v, most have a transformer tap. Simply change the socket, and an 8908 will work perfectly, same goes for a Russian 6P45C swap. It runs 900v on the plate, 450v on the screen, and is biased into near cutoff at -170v on the control grid.
This one had bad diodes in the hv circuit, a shorted tube, and a bad rx preamp transistor. Here's a couple of vids in the shop and on Studio B, and a few pics...
