I was just watching some Psionic Audio videos, and in particular one where he checks the bias on a cheap Vox Ac10 (?). Ha made a comment along the lines of having bias on one of these EL84 cathode biased amps at around 110-120% of plate dissipation seemed to be the sweet spot.
This has always confused me. I have been biasing my amps to around 60% because mostly they're fixed bias and I can't really hear much difference between the internationally decreed default setting of 70% and 60%, and setting it cooler apparently helps prolong tube life. So in a fixed bias amp I love the sound of 60% bias.
But in cathode-biased amps it always sounded good but never did the amp break-up in the fabled way. I recently tried a Clark 5E3 and the way it broke up was what I wanted from my 5E3s, but mine always had crazy headroom. I have lowered the values of various caps here and there in my 5E3s to reduce the ridiculous amounts of low end a 5E3 traditionally has. Great pedal platforms, classic sounds, beautiful clarity but not the traditional Deluxe break-up. I didn't have my meter to check voltages but the Clark did have a 270ohm cathode resistor.
So after hearing what Mr Psionic had to say I tried a 270ohm cathode resistor in my latest 5E3, which meant that it was biased at a smidge over 100%. THERE WAS THAT SOUND! And even if I cleaned the amp up by reducing volume to around 3 I could still get a very similar sound if I used the Hudson Broadcast clone I just built. Fab!
So then I tried a smaller cathode resistor in my over-built 5E3 - Mercury Magnetics Fatstack PT, 22µF filter caps, choke... so now it's biased at between 110-120% and it sounds incredible. It's the 5E3 of my dreams!
Thing is there is none of the weirdness that you get with too high a bias voltage in a fixed-bias amp. If anything the amps seems happier with the smaller bias resistor. There's no excess noise or hum, no nasty brittleness, nothing.
So I will monitor the tubes over the next few months and see how they go. You know how it feels when something clicks and you finally achieve something great? I am hoping that's the feeling I have right now!
This has always confused me. I have been biasing my amps to around 60% because mostly they're fixed bias and I can't really hear much difference between the internationally decreed default setting of 70% and 60%, and setting it cooler apparently helps prolong tube life. So in a fixed bias amp I love the sound of 60% bias.
But in cathode-biased amps it always sounded good but never did the amp break-up in the fabled way. I recently tried a Clark 5E3 and the way it broke up was what I wanted from my 5E3s, but mine always had crazy headroom. I have lowered the values of various caps here and there in my 5E3s to reduce the ridiculous amounts of low end a 5E3 traditionally has. Great pedal platforms, classic sounds, beautiful clarity but not the traditional Deluxe break-up. I didn't have my meter to check voltages but the Clark did have a 270ohm cathode resistor.
So after hearing what Mr Psionic had to say I tried a 270ohm cathode resistor in my latest 5E3, which meant that it was biased at a smidge over 100%. THERE WAS THAT SOUND! And even if I cleaned the amp up by reducing volume to around 3 I could still get a very similar sound if I used the Hudson Broadcast clone I just built. Fab!
So then I tried a smaller cathode resistor in my over-built 5E3 - Mercury Magnetics Fatstack PT, 22µF filter caps, choke... so now it's biased at between 110-120% and it sounds incredible. It's the 5E3 of my dreams!
Thing is there is none of the weirdness that you get with too high a bias voltage in a fixed-bias amp. If anything the amps seems happier with the smaller bias resistor. There's no excess noise or hum, no nasty brittleness, nothing.
So I will monitor the tubes over the next few months and see how they go. You know how it feels when something clicks and you finally achieve something great? I am hoping that's the feeling I have right now!
