Loud shrill feedback sound when probing plate voltage

[Erik Aa]

The guitarist in my band has an ashdown amp with two el34s. It worked ok for a while (a couple of years) but now all of a sudden it started producing this awful high pitched noise when he turns it on. When master volume is above 2 the noise is unbearable. It is reminiscent of the sound you get when trying to Connect negative feedback and the wires are switched. Anyway, I took it home and tried to switch tubes. No change. The voltages on the plates and screens and grids on the el34s look ok. But when I measure plate voltage on one of the el34s I also get this loud shrill noise. What is going on? Any ideas? Thanks!

[dude]

Try another set of known good EL34's

[Latole]

Try another set of known good EL34's

Wich amp ?

Tubes may need a bias adjustment with new tubes than other the same brand and model

[PRR]

> when I measure plate voltage on one of the el34s I also get this loud shrill noise.

That may be "normal". Your meter lead radiates big plate signal into small signal stages.

There's no real reason to poke plates. The DC voltage there can't be much different from B+, even if the amp is real sick.

[Erik Aa]

Ok. Having plugged in an instrument, the noise starts when turning up the master volume. When I touch the strings. The noise can also start when wiggling the first preamp tube. Could it be bad socket soldering on the first socket?

[Williamblake]

Once you post a picture of the PCB, Sluckey will point out a bad solder joint in no time at all. Maybe several.