So, i have checked the circuit für several times.
at the first time i found a problem with the voltage reegulator V1. it becomes very hot after a few seconds and so the heating voltages of the tubes decreases to a value of about 2V.
Then i cooled down V1 with a cooler spray, the amp works about 1 minute and then the proplem came back.
I changed V1 but the problem is still there.
Could it be that the 18V ouptput of the transformer is broken or wihich parts could cause my problem?
I hope that the Support can help me or can give me a hint to solve my Problem!
Thank you very much