Hello guys, recently finished my first Riaa preamp with ef804+El84 tubes. The first impresion when started was wow, really good sound, no hum at all and i am very happy with it. Later i have started playing with anode voltage and cathode resistor for ef804, and everytime the sound changes, sometimes it is worse, sometimes it looks better, so the question would be: does anybody in here have expierence with ef804 biasing? It looks like it low current tube, but there is si many variations to try…. Any tips would be a huge help guys, thanks.
I will also upload full schematic, so any sugestions are welcome.
I will also upload full schematic, so any sugestions are welcome.
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Not familiar with EF804, but the preamp's sound quality will change with tube operating point because sourcing/termination of the RIAA network will change as well, and that will change the frequency response, likely placing it out of spec as far as the official RIAA EQ curve for vinyl goes. Find the best bias point for your tubes, and then adjust RIAA R's & C's to assure compliance with the published RIAA curve.
I have designed it myself, basicly it is a mix o 3 different schematis, the original uses EF86 and says +130v for the input tube, tried this, but it does not work for EF804. Sound is thin and bright, the lower voltage i apply, the this tube plays �� The most important thing, that datasheet of EF804 states 100v as the lowest voltage, but i am using +62 at the moment, and it plays wonderfull, thats why i am suspiciuos about this thing, am i doing something wrong, or shall i trust my ears �� I have invited friend of mine, demonstrated him +62 and +120 volts sound, we both decided that +62 is much better.
I do not have enough knowledge to be able to calculate the needed values of the two resistors and the two capacitors in the anode lead of the EF804. But I do understand that because of the RIAA curve, the amplification of the lower frequencies has to be large enough to be able to follow the RIAA correction curve. So it surprises me that lowering B+ gives a less thinner sound. I would expect the opposite because higher B+ gives more amplification.
I do not understand what you write about the datasheet stating that 100 V is the lowest voltage in relation to the anode voltage of 62 V that you measured. I get the impression that you are mixing up "Betriebsspannung" (Ub), which means supply voltage (B+), and "anode voltage". At a supply voltage of 100 V the anode voltage will ofcourse be lower than 100 V as a result of the voltage drop in the anode resistors.
I do not understand what you write about the datasheet stating that 100 V is the lowest voltage in relation to the anode voltage of 62 V that you measured. I get the impression that you are mixing up "Betriebsspannung" (Ub), which means supply voltage (B+), and "anode voltage". At a supply voltage of 100 V the anode voltage will ofcourse be lower than 100 V as a result of the voltage drop in the anode resistors.