5U4 Warmup Hum

[ejp]

The mod I am going to try is to add negative feedback from the output transformer's 8-ohm output back to the stage before the output tube.

There’s not enough gain there for the NFB to do much good. Run it back to the input stage.

 

[edwardjurich]

The added negative feedback is intended to reduce the output source resistance a bit. Considering that the typical speaker system has one or two percent distortion at lower volume levels, increasing as amplifier power increases, there is no point in trying to get amplifier distortion ultra low, vacuum tube or solid state. I probably will not have time to try any amplifier mods for a while. I'm busy getting a chassis ready for an experimental amplifier using 6SN7s as output tubes. When I do, I'll start a new post about it.

 

[6A3sUMMER]

Get a 4 wire Ohmmeter, and check the cold filament resistances. I bet the rectifier that hums has a cold filament that is about 1/2 the Ohms of the other rectifiers cold filament Ohms.

 

[egellings]

As a kid, I started working with vacuum tube audio in the 1950s' and never really stopped. After all these years, I have a weird 5U4 that baffles me. I seem to have a 5U4 rectifier that puts out a brief hum just as the filament starts to glow, but before any DC is produced. It is a PSVANE classic series 5U4 I bought for the classic look. I have never had a rectifier tube produce a brief hum before any DC output appears. How it gets to the speaker before any tubes conduct baffles me. It is a very low level hum barely audible. If I plug in another 5U4, there is no brief hum before DC output. Both output transformers are three inches away from the power transformer. I doubt there is any inductive coupling going on between the transformers. The 5 volt filament winding does have a center tap from which I pull the DC output. Filter cap at the 5U4 is 33uF at 500V. After a 6HY choke there is a 100uF at 500V cap. After the choke, there are 120 ohm power resistors that feed each channel B+ with a 470uF at 500 volt cap. None of that should matter since I can plug in a different 5U4 and no brief hum. As far as I can tell, the PSVANE 5U4 is doing no harm so I put it back in.

If the tube is gassy, that might cause a slight hum until the tube starts conducting full current. Just a guess on my part.

 

[6A3sUMMER]

Has this not yet been solved??? Wow!