Oscillation

[alhe]

Hi there. I am having trouble with a Ceriatone build. This amp is oscillating past 5 on the bright channel, and 7 or so on the normal channel. This amp is a factory build, but had a blown output section. I don't know if the two issues are related, but it seems to work fine with new EL34's.

I have traced the oscillation to V2 and the surrounding circuitry. It will still oscillate when V1 and V3 are pulled, and no instrument is connected.

I would much appreciate if anyone cared to have a look at the schematic, and maybe shed some light on why it may be oscillating. My scope shows the oscillation is present on both V1 and V3 as well as in the EQ section. The EQ, and especially the mid-band, alters the intensity and onset of oscillation.

Thank you

[danman]

You may want to try replacing or paralleling another preamp filter cap to see if it cures the problem.

[Haze13]

I've build JTM45, Plexi, JCM800, JCM800(78) preamps and had same oscillations with Plexi preamp. It has some thing to do with 500pf/470k connected to the grid. Remove the cap (500pf) and see if this continue to happen.

[rgorke]

What does it sound like when it is oscillating? Do you have a short clip? Usually doing a YouTube video is easiest to post.

[alhe]

Sure enough, it was the filter cap. I should have known that.

Sorry I took so long. Seems Metro went down just after I posted here.