In the vast world of high fidelity and tube amplifiers, certain designs hold a special place thanks to the combination of vintage engineering and refined sound. In particular, amplifiers based on the 300B tube are appreciated for their sweet tonal response, their ability to recreate an articulated soundstage, and a “warmth” that many enthusiasts associate with true hi-fi listening. When we are faced with a device that uses 300B output stages or similar, and perhaps a circuit that includes negative feedback or construction choices that are not trivial, servicing it may require an expert hand and a keen eye. In this case, the story begins like this: They brought me this amplifier because, according to the owner, it had gone completely silent:

Here are some photos of the inside:

I don’t want to say it, but a socket doesn’t cost much…

The photos below, on the other hand, give me the kind of satisfaction that only a few details can…

For those who are not familiar: the red pin is the positive speaker terminal, while those two resistors with the capacitor feeding back into the circuit carry the negative feedback signal. And excuse me, but I LOVE IT. In those magazines where they publish projects and schematics, everyone waves around zerofeedback as the only path to good sound, to win over the narrow-minded masses devoted to the creed spread by certain gurus, risking being snubbed if one dares say otherwise. But then… well… I LOVE IT. As my teacher G.M. used to say: “Tell people that amplifiers are built the way they think they should be built, but when you make them… make them properly.” I have never been able to be false or avoid speaking the truth. But a 300B with feedback… and it’s not even my design… I LOVE IT. The remaining issue is all those who don’t understand it, who claim to build them “as they should be built” and then actually make them exactly as they say, along with all those who follow them. And perhaps, Diego Nardi, you will also have to forgive all the people to whom you said the amplifier was “zerofeedback”… but I understand you: faced with prejudice, sometimes all you can do is nod with a “yes, yes” and then do what you know is right, since they wouldn’t have the ability to figure out whether that negative feedback is there or not anyway. Diego Nardi, you’re my idol: I even forgive you for the tubes soldered without sockets.

Anyway, after spending a bit of time desoldering the tube pins to measure them and confirm they were in good condition, and after replacing two electrolytics that had swollen, the amplifier returned to proper operation.

The amplifier delivers about 10 watts with a damping factor of 2.66. Below are the usual reference graphs that I keep and publish for future comparisons with devices of the same type:

Spectrum at 1 watt on resistive load.

Bandwidth at 1 watt on resistive load

And on reactive load

Square waves at 100 Hz – 1 kHz – 10 kHz