for the EL84 a 2k2 resistor for the screen which i think it's the recomended value for the EL84... and a bigger value for the cathode probably 150ohm/4W
Changing the cathode resistor is essentially changing the negative bias voltage on the grid - for really high power amps you normally supply a negative bias, which will usually be adjustable, in order to get maximum power from it. For a small amp cathode biasing is beter, and it's self adjusting.
if i'm not wrong... EL84 must decipate 12W so i'm going to measure the voltage between anode-cathode and the current the tube draws so i can regulate it... is that ok or i'm still obsessed?
i just don't want to destroy something which it isn't as cheap and also it's not so easy to find as a simple transistor
The PSU voltage probably won't change a great deal, particularly with using a semiconductor bridge rectifier, and a much larger electrolytic - original valve amps would be a half wave valve rectifier, and a 16uF or so. Guitarists still like to use that configuration, as the supply sagging excessively adds to the distortion.
btw my output tube has also a blue glow... and it's very nice! and as far
as i know that can happen with high rush of electrons hitting things inside the tube and releasing photons.
Don't measure the voltage from cathode to anode.
Measure the voltage from cathode to ground.
This tells you two things...in simple circuits.
Since the control grid is at ground potential (because there is no leakage from the coupling cap connected to the triode plate, and the grid resistor to ground...done very poorly in this circuit) the cathode voltage tells you the grid bias.
Cathode voltage wrt ground also tells you the current going through the tube by Ohm's law. I=E/R (cathode resistor).
That is why I asked you much earlier to tell us the cathode voltages.
I've just been back and looked at the original circuit, and the ones I posted.
Your original used a 135 ohm cathode resistor, I suggested increasing it - moving to 150 ohms is hardly much of an increase at all. Notice the circuits I posted, the first used 560 ohm, and the second 300 ohm.
I would suggest moving to at least 300 ohm, and see what happens then (and as suggested, measure the cathode voltage to check the current through the valve).
Well you haven't reduced the anode current worth mentioning - an obvious method based on the circuit you posted is to simply remove one of the parallel 270 ohms and half the current. Notice that's still about double the current used in the Mullard 3-3 amplifier (from the people who designed the EL84).
Phase shift in the output transformer will cause the amplifier to oscillate at a high frequency and destroy your tweeter when too much negative feedback is added. The 220pF capacitor and parallel resistor have values selected to match the transformer to reduce negative feedback above a certain frequency.
EDIT: There is no negative feedback with the feedback parts feeding a huge capacitor to ground.
I don't know what data sheet you are refering to.
On the EL84 pin9 you could try an electrolytic cap to ground.
See if the hum is even less. I don't know what value. Use what you
have for now or try an 8uf. Just make sure it's voltage rating is high enough.
Much higher than the max power supply voltage before the tubes heat up. (450 vdc?)
No, it's not a fixed degree of phase shift, it will vary with frequency - basically due to the transformer. It's NOT an opamp, you can't just add feedback as you like - check the earlier designs I posted.