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kalee20 · 23rd Sep 2022, 10:20 am

I played around making amplifiers out of old TV's... Frame output transformer makes a fair heater transformer; Audio output transformer for its intended purpose; HT straight off mains; etc.

Even with a 6V6 overrunning the poor little OP transformer, bass response can be pleasing. What the figures looked like, I don't know - I only measured the hours of pleasure and the number of smiles.

But, what makes a good valve amplifier? Low distortion (sub-1%); adequate frequency response (30Hz - 18kHz +/- 1db); low output impedance to give good loudspeaker damping, are generally accepted. To that I'd add smooth overload characteristics, reliability, long-life.

How to achieve that? Lots of ways... I'd suggest running output valves at no more than 75% of rated maximum anode dissipation. Using good-quality components (paying attention to voltage rating of metal-film resistors). Plenty of iron in the output transformer. Good regulation in the power supply, unless using pure Class A. Plenty of decoupling between stages - using common HT impedance to hold-up bass response by positive feedback is flaky! Aiming for good frequency response and low distortion open-loop, and using NFB as the icing on the cake (and to keep control of OP impedance, if using pentodes or BT's), are all 'musts' for me.

23rd Sep 2022, 10:20 am	#43
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: What makes a good valve amplifier? I played around making amplifiers out of old TV's... Frame output transformer makes a fair heater transformer; Audio output transformer for its intended purpose; HT straight off mains; etc. Even with a 6V6 overrunning the poor little OP transformer, bass response can be pleasing. What the figures looked like, I don't know - I only measured the hours of pleasure and the number of smiles. But, what makes a good valve amplifier? Low distortion (sub-1%); adequate frequency response (30Hz - 18kHz +/- 1db); low output impedance to give good loudspeaker damping, are generally accepted. To that I'd add smooth overload characteristics, reliability, long-life. How to achieve that? Lots of ways... I'd suggest running output valves at no more than 75% of rated maximum anode dissipation. Using good-quality components (paying attention to voltage rating of metal-film resistors). Plenty of iron in the output transformer. Good regulation in the power supply, unless using pure Class A. Plenty of decoupling between stages - using common HT impedance to hold-up bass response by positive feedback is flaky! Aiming for good frequency response and low distortion open-loop, and using NFB as the icing on the cake (and to keep control of OP impedance, if using pentodes or BT's), are all 'musts' for me.