Hum in SVT power amp?

[martin manning]

Mark, have you refreshed all of the filters in the power supply?

[Mark]

Steven, I'm curious as to how the bias supply could be the problem, given that the output valves have been removed, as have the driver valves, I'll have a look at the earth point for the bias, but I'm not understanding how it could cause the fault yet.

[Mark]

Hi Martin, I haven't replaced all the filter caps but I have used newer caps either across various power supply nodes. At one point I created a new node and used the newer caps.

This is probably worth doing as the caps are the original caps since new. Though I don't like replacing parts unnecessarily.

[pompeiisneaks]

Well I could be wrong about the power supply node, but both the first stage and the cathodyne phase splitter are connected to the same power supply node and earth bus. I'd think lifting the plate resistor from the supply node would mean the hum would still be present on the first gain stage.

actually no, each filter capacitor also acts as a decoupling capacitor. So in theory the first premamp stage likely has it's own filter capacitor, etc. I don't know this amp off the top of my head but often there's a filter cap for the anodes, screens, PI, and then each stage before that. Therefore if the PI has noise from the plate resistor, it may be a bad filter cap for the PI section only. It's easy to test by temporarily jumpering an identical cap across it, to see if the noise dissipates, as the other cap will do the smoothing job the bad one isn't.

Of course, that's only if it IS the filter cap.

~Phil

[Mark]

Hi Phil

At one point I created a different node off and screen grid power supply using different caps and it made no difference to the noise/hum what so ever. I was surprised by this to say the least.

I'm very curious why the noise disappears when I lift the supply side of the plate resistor, and it doesn't disappear when I lift the cathode resistor. Most peculiar!

[tubeswell]

Have you considered the possibility that one of the filter cap ground returns is not connected due to a dead trace? (which could be causing the filter cap at that node to lose its decoupling function? Check the DC voltages on the HT supply rail to make sure that they are not all the same throughout the rail. And clip your R-meter on each filter cap ground lead, with the other meter probe going to the chassis ground, to check that these connections aren't open.

[Mark]

Thanks for the advice Tubeswell, I have checked the power supply earth wiring previously. At this point in time the cathode of the left triode in my diagram is earthed by a wire and not a trace. I will check again as I'm happy if it is a simple fix.

I have tried isolating the path of the noise/hum. As shown in the diagram, the bottom of the balance pot isn't attached to ground, so the valve shouldn't be able to operate. The phase splitter coupling caps have been lifted out of the circuit. The coupling cap to the cathodyne phase splitter has been lifted out of the circuit. The 20pF cap between both plates has also been removed.

I should note that the noise/hum only appears on the left triode and never on the right triode plate or cathode.

IMG_0981.JPG

[Mark]

I bit the dust on this one. I just couldn't find the problem, my brother got sick of the wait and asked me to assemble the amp. I didn't like the amp "beating" me but I was sick of working on the damned thing.