oscillation noise in guitar tube amp

[binito]

I have a Zetang guitar tube amp . The schematic is similar to twin Reverb or Marshall JMP .
The amp have a oscillation noise in power amp section.
when I put the test probe of voltimeter in screen of phase inverter tube , the noise stop .
I dont know how its the problem .
Attached link of video of problem .
https://www.dropbox.com/s/p5c0cyqm43...52112.mp4?dl=0

[The Dude]

Motor boating is most commonly caused by by a bad filter/decoupling cap on the supply node of the effected gain stage. I'd first check or change your filter caps. You could also try temporarily clipping a known good cap across each filter and see if that makes it stop oscillating.

[binito]

filters changed , decoupling caps changed , tubes changed . if put the voltimeter in screen2 of PI the problem stop. if disconect the wire of V5 to Screen1 of PI the problem stop .

[Helmholtz]

filters changed , decoupling caps changed , tubes changed . if put the voltimeter in screen2 of PI the problem stop. if disconect the wire of V5 to Screen1 of PI the problem stop .

Without a schematic we can't know what V5 means. As you speak of the screen of the PI: Is your PI tube a pentode?
Did the amp work fine before? Any modifications or replaced OT?

[binito]

The PI is a Phase Inverter tube.
And V5 is the tube before of phase inverter tube.
I don't a schematic, the amp is a zetang. Made in Spain of 1977 more or less. And the layout seems a Twin reverb AB763 with 2 channels (clean and drive) with gains and volumen per channel. A presence pot(similar to Marshall 1959) and reverb similar to twin reverb.
I don't know if a amp worked fine before.
Arrived to me and the power tubes are bad. I changed power tubes, but the amp motorboating. Only with the power tubes and 12at7 phase inverter are installed. If I removed the PI or power tubes the amp no motorboating.

[Helmholtz]

12at7 phase inverter

THe 12AT7 is a double triode and has no screen (G2). Please clarify.

If I removed the PI or power tubes the amp no motorboating.

Your symptoms indicate a pulsating very high frequency oscillation of the power amp. Does the setting of the presence pot influence the effect?

[binito]

ouhh😅😅, sorry ,
obviously the triode of 12at7 has no screen. I wanted to say grid.
I think I have found the problem :
between the anode of V5 and gris of the 12at7 PI. there is only a 0.1uF 630V capacitor and in the schematic of an AB763 there is a 0.1uF capacitor, a 220k resistor, and a 500pF capacitor.
I installed this 220k resistor and 500pF cap and the motorboating stop .

[binito]

[Helmholtz]

I installed this 220k resistor and 500pF cap and the motorboating stop .

But then the amp never worked properly?

between the anode of V5 and gris of the 12at7 PI. there is only a 0.1uF 630V capacitor and in the schematic of an AB763 there is a 0.1uF capacitor, a 220k resistor, and a 500pF capacitor.

I don't know what 500pF cap you mean.