300B amplifier loud AC noise ?

[santitrucco]

Why does my 300B amplifier make a relatively loud AC noise for the first 3 seconds after turning it on?

 

[grovergardner]

If it's using AC filaments, they will often hum until the high voltage kicks in. What is the amp? Pictures?

 

[santitrucco]

Schematic some similar. Interestage Transformer.

 

[grovergardner]

What is the power supply?

 

[as_audio]

Switch on noises are to be expected.

 

[grovergardner]

Not necessarily. I just worked on a pair of 300B monoblocks. There was no noise at switch-on. It might be normal or there might be something wrong.

 

[6A3sUMMER]

Suppose that the single ended E/I air gapped output transformer is near to either the power transformer, or to the B+ choke. The AC magnetic fields from the power transformer or B+ choke magnetically induces a voltage on the output transformer.This is maximum when the 300B tube is cold at power on. The plate resistance is near infinite at first power up (Cold 300B). But when the 300B is warm, you effectively have 700 Ohms 300B plate resistance across the output transformer. 700 Ohms shunts out the magnetic fields from the power transformer and B+ choke.
Or, the output transformer is far from. the power trasnformer and B+ choke, but you made the mistake of using a magnetic steel chassis.
Did I discover the cause of your trouble????

 

[Shef]

Try to switch the 300s' grids from IT directly to Ubias, what happens?

 

[sumotan]

If your PS is tube rectified try changing that first. Experienced this once with a dying rect tube

 

[santitrucco]

The amp have one rectifier tube per channel , and both do the AC noise 3 second.

 

[euro21]

PSU schematic?
Mainly secondary current capacity, its DCR, rectifier type, first capacitor value, load current.