Amp Monoblock UL

Posted on Feb 6, 2014

You may be wondering why I am using four tubes. The reason is to get 50 watts without pushing them right to their limit. My tube manual gives two setups for EL34s in ultra linear. Both require a 6 k ohm transformer. Both setups use exactly the same voltages but one says you get 20 watts at 0. 34 percent distortion and the other says you get 34 watts

Amp Monoblock UL
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at 2. 5 percent distortion. The applied voltages are the same except for the AF grid drive voltage. They are just driving them harder to get more power and more distortion. Connecting four tubes in push-pull parallel lowers the load resistance to 3 k, which almost matches a transformer I have already bought, and gives more power with not a lot of distortion. I could almost live with it with no global feedback. I don`t know how Eico got 50 watts with two tubes but it is likely I am remembering it wrong and they were actually 35 watt amplifiers. See farther down this page. This is the amplifier I`m going to build. The tube manual data is for cathode bias so I`m going to see if I can`t simplify the circuit. The screen voltage regulator is already gone and if I can eliminate the negative voltage supply I`ll have a really good circuit. I`ll post it under its own heading on the front page when it is finished. Because of the extremely low powers at the low frequency end, from the Hammond transformer, I measured the distortion at various frequencies at 12. 5 watts (6 dB below maximum rated power. There is no reason to do the same for the Edcor. Distortion in the bass is readily apparent because all of the harmonics are in the range of a speaker to reproduce. Also most of the power in pop music is in the bass end. Just think of those cars with subwoofers that rattle the windows in your house as they drive by. Even in jazz or...

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