This amplifier is based on schematic of famous Zetagi BV131, but instead EL519 6KG6 which is very expensive today I’ve used Russian military tube GU 50, which is actually copy of famous Telefunken Wehrmacht LS50, and the main reason I used it is because I found few complete Russian military HF amplifier UM-3, at the junk yard :), with EL84 / 6P14P / 6P15P and GU50. It’s rated like 50W amplifier, and it’s classic cathode grounded circut. When I finished this little beast in GG connection I was really surprised with differencies of sensitivity of tubes. I have big stock of more than 500 GU50 tubes, and I’ve tested more than twenty in this linear amplifier, and usually they delivered a bit more than 200 Watts, but few of them were very close to half of KW!!! That’s problem with USSR tubes, they are same type, even same batch and date of production, but so, so different in gain!!!
Complete linear and amplifier is in this small cube!
Of course, that I planed completely new front panel, but I never did it.
Back of amplifier, tube socket and integrated tube cooler, 220V input and RF output.
A look under the hub, original instrument and relay were used, also switches, RFC. This torus transformer is just for heating of GU50 and relays.
Under the chassis, the secret of small size is in high voltage power supply, multiplier directly from 220V!
This is few hours modification I’ve done almost 25 years ago, when Zettagi was unobtainable and expensive in ex Yugoslavia.