• 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.

Oscillation in hybrid amp, how to solve?

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Hi to all!

I'm an electronic eng. student and for the "tesi" (the final exam) I made an hybrid audio amplifier, here is the schematic:



the 10k resistor in VT1 and VT2 now is 2.2k to drop less voltage.

the problem is that this amp work properly without the capacitor in parallel with the feedback resistor, if I add this capacitor the amp start to oscillate at about 500kHz...no matter on the value of the capacitor, 10pF give the same frequency of 100pF.

Measuring the frequency responce it give GBW at about 100kHz!!! and above 30kHz the responce is not flat and goes up and down!!

What can I try to tune this amp?

Thanks to all and sorry for my bad English!
If it is OK without the capacitor, then omit the capacitor! This capacitor has two roles:
1. it sets the HF roll-off on closd-loop gain,
2. it gives some phase advance on the feedback, which offsets phase lags elsewhere and so aids stability.

However, it sounds marginal without the capacitor as you have HF wiggles. Your amplifier forward path may have too high a bandwidth, with cascode input and cathode followers. The natural roll-off provided by Miller effect has been designed away, but you actually need some of this to cope with the phase shift created by the output transformer HF resonance.

As you are an EE student you will know about servo systems and stability (see Bode, Nyquist). The same rules apply to negative feedback.
Hi DF96, thank in advance for your reply!

Yess I know theory of servo sistems but I need to measure the phase at GBW and with Lissajous figure is impossible cause the wave at such high frequency (ca. 100kHz) is not sinusoidal and have an awful shape... tomorrow I make some pictures... here you can find a document with measures I do misurazioni.rar

Another thing, with electro harmonix EL34 the amp reach only 34Wrms @0.8% THD before wild clipping, with JJ EL34 the amp reach about 43Wrms @0.9% before the same wild clipping.... is this a sign that internal impedance of this valve higher than declared?
Instead of Lissajous try using a dual-trace scope, looking at input and output. You don't need an accurate measurement, estimates are good enough. If you can see 90 deg, 180 deg points and measure gain then you can do a rough plot and see where the trouble lies. A smaller amplitude might give a cleaner waveform.

Differences between valves could just be due to biasing - only 1dB difference between 34W and 43W anyway.
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