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Old 11th Jan 2019, 8:20 pm   #52
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Re: 120W PP EL34 amplifier re-born.

daft question Andy,
Wheres the earth return from the input socket running?
You show the screen earthed at the grid but not at the input so I have to assume that you have an earth return somewhere.
Theoretically earthing a screen at one end shouldnt create a hum loop, and the convention is usually to earth the screen at the input end rather than the output end although It wont really make a difference I think.
Now then, you should have your signal earth return from the socket to as near as possible to the earthed end of your input valve ie the bottom of the cathode resistor for a direct path. Otherwise the return path could be anywhere along the zero volt rail?
I'd use a bit of twin core screened microphone cable, connect one inner conductor to the screen at the socket outer (Earthy end) and then connect the 2 inner wires up at the valve grid and ground respectively if that makes sense to you? If you havent got twin core, just use the screen as your earth return from socket to input valve, it might be enough. And make sure there are no other earth runs between input socket and inputs. Use insulated phono sockets as if its got electrical continuity with the chassis you are asking for trouble using the chassis as an earth return.
It sounds a bit like the mild hum issue i had with my EL34 stereo power amp which I cured by re wiring the earth runs from the preamp section as a single wire to the star earth rather than two separate ones back which had created an extarnal hum loop via the connecting cables. Still baffles me.
I can't remember but are they monos or a stereo amp with separate PSU still?
I generally don't use a DC blocking capacitor on the input grids as It's not really necessary unless you know that your source has a DC offset on it. All my preamps have output coupling caps or are designed for zero DC offset so I dispense with the DC blocking cap. None of my commercial Valve amps use an input cap either. It doesnt do any harm though other than altering the LF response. I'm wondering if you're picking up noise via the capacitor too.
Cheers.

Andy.

Last edited by bikerhifinut; 11th Jan 2019 at 8:40 pm.
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