fizzy sound

Hello,

I have completed the A15Mk1 kit and after some slight changes the amp is working now, the clean channel works fine but the gain channel doesn't sound very well. It appears then when I turn on the amp after one day, the first couple of minutes the amp sound fine after that it sounds very thin and fizzy. It is hard to describe the sound but it sounds like it is coming from a very small speaker (e.g. 8").
I use a Fame 2x12" cabinet loaded with Celestion vintage 30. I have measured the bias and I think it is okay -24 Volt. The voltages at the resistors on the powertubes are about 34mV. I also measured other voltages and everything seems to be correct?
Does anybody have any suggestions or ideas where to look?

Thanks

Hugo

Re: fizzy sound

Thanks David,

I tried that and for the first 15 minutes the amp sounded great, but after that the fizzy sound comes back.

Thx. Hugo

Re: fizzy sound

Checked the wiring, but found no mistake.

The tubes require some time to warm up. right? Could this be the problem, or are there other parts that could cause this effect. Is there a way to check a tube with e.g. a multimeter?

I rechecked the voltage on the preamps tubes and the voltage on pin 6 is a bit too low, could this be (a part of) the problem?

V2 pin 6 146 Volt (measured) instead of 158 Volt (according to schematic)
V3 pin 6 102 Volt (measured) instead of 114 Volt (according to schematic)
V4 pin 6 168 Volt (measured) instead of 181 Volt (according to schematic)

Thx Hugo