Big 50hz (Hum) on new 5E3P (Weber PROLUXE), no sound

[bluescaster]

Hi, i just build a 5E3P for a friend and i got nothing on output except a big 50hz noise.

I pull off the 2 6L6 and made some tests on PI. On (A) i get a guitar signal, on (C) no signal and no DC 0v, and on (B) a big 50hz saturate sinus. I change V2 with no result.

Bias on D is -49v, with no AC .

Do you have any idea please ?

Kind regards.

[Phil_S]

This looks like a typical cathodyne phase inverter (PI). The current on the inverter's plate and cathode should be nearly equal. If you measure the voltage drop across R10 (56K) and across R16 (1.5K), you can use Ohm's Law to calculate current flow. I=V/R. Also, take and provide DC voltage readings for B+ at the C node and at the cathode. Post what you find.

Where did you ground C17? Can you post a photo? We will need one of the whole build for reference and a closeup of the PI tube socket.

[JazzGuitarGimp]

Have you made the connection from point D to the junction of R21 and R22? The reason I ask is because you said you have no DC voltage at your point "C". You should have -48VDC at point C if it's wired correctly. Did you happen to notice if the 6L6's are red-plating? Best to take the 6L6's out and then measure for -48VDC at pin 5 of each of the 6L6 sockets to make sure your bias voltage is getting to the 6L6 grids.

[bluescaster]

I notice too that 50hz is present on (B) even with standby off and V2 pull off ! Bias on (D) has good looking , no AC, just -49VDC.

[bluescaster]

No idea ? 100v AC with stand-by off ! Is ground loop can do That ?

[martin manning]

I notice too that 50hz is present on (B) even with standby off.

Something wrong here. Look carefully for a wiring error in the power supply.

[bluescaster]

Hi, i found the error, a wire under the board . My friend began the job with a different layout that me.

Now the amp sound great !

AB763 kind of tone with 5E3 kind of attack, very cool !

Thank you for the help.

Kind regards.

[Structo]

congrats on finding the problem.