• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Help Please!!!!!!

mongiak

Member
2004-10-22 6:07 pm
Hello
I´ve built a guitar tube amplifier, it´s a 15 W class A amp, and it doesn´t work.
I think that the trouble is in the phase splitter (shown in the attached picture) tube were I have 42 V betwin the catode and the grid, this is because I have a small current throught the resistors R17 and R18, so I think that the tube is in the cut off zone, and i don´t know the reason, I thought it was because the capacitors were no good, but I changed them and it´s still the same. (perhaps the new ones are not good either, don´t know).
in the rest of the circuit the V are just what they have to be.

Please can anyone tell me what it´s wrong with it?

Thanks to all.


Urko
 

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moamps

diyAudio Member
2002-10-11 8:15 am
Croatia
www.moxtone.com
mongiak said:
.... so I think that the tube is in the cut off zone....

Hi,
The anode current of each triode is V(R19)/2/R19, which is around Ia=0,7mA; perhaps not the optimum but certainly not in the cutoff zone.
Vg=- 2*Ia*R14=-1.7V
Your voltmeter has an input impedance around 1Mohm, which causes the high impedance network measurement error that SY pointed out.

Regards,
Milan