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Old 1st November 2016, 08:59 PM   #1
50AE is offline 50AE  Bulgaria
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Default Help troubleshooting oscillation

I have this oscillation problem with a 3 stage PP amplifier. It goes into oscillation (motorboating) when the volume (resistive stepped attenuator) is nearly at full volume. There is buzzing noise increasing with the volume level too and it might be higher frequency oscillation.

The problem is, I cannot measure it, because when I'm connecting the scope ground clip to any of the amplifier's ground, oscillations and noise stop. This leads me to the assumption, the whole system isn't earthed anywhere, but I'll check that tomorrow.

The amplifier has separate PT secondaries and separate grounds for the left and right channel. If I join these together and connect them to mains earth, the oscillations disappear, but there is hum in both channels that I think results from a ground loop. The interconnects are pretty long and pick up induced current. I would like to avoid this kind of fix.

Some details of the amplifier:
-No NFB
-All tubes triode strapped
-Input sensitivity - 420mV RMS to output 38W in 8R. 68k input Impedance
-3 stage using tubes El802, 6P36S and 6P45S.
-RC coupling between first stage and second, ITT between second and third
-LC(tap)RC(tap)RC(tap) filtered power supply. 3H choke -> 4400uF capacitor -> 390R -> 470uF -> 470R -> 390uF
-Q factor of LC ~0,6. Dampened mainly by the Rdc of the power transformer's windings, the huge 4400uF capacitor and 300mA of quiescent current.
-Star ground on the 4400uF capacitor. All power supply capacitors grounds go to the star ground.
-Fixed bias power supply for the 6P36S and 6P45S , RCRC filtered.
-All heaters referenced to ground via 1:1 resistor divider.
-300R, 470R grid stoppers used, G2 resistors used too.
-The power supply transformer has electrostatic shield connected to mains earth. Then the heater secondaries are wound. On top in the following order are: B1+; Bias1; Bias2 ; B2+


I'll continue my battle tomorrow, but I'll appreciate gladly if you can share some advice. I'm trying to understand the deep mechanics between such oscillations. Many thanks!
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Old 2nd November 2016, 09:34 AM   #2
50AE is offline 50AE  Bulgaria
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Location: Sofia, Bulgaria
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Ok, I managed to fix it. The system was truly floating. After earthing the source, oscillations went away.
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