UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 120W PP EL34 amplifier re-born.

bikerhifinut · 13th Jan 2019, 10:34 pm

But It should be good at reducing electrostatically induced noise so as long as its got good contact with Chassis ground it isn't completely wasted effort.
You're using a toroidal mains transformer if I remember? You have a naturally low magnetic field surrounding it. Given a reasonable physical separation from the mains transformer(s) then the main source of hummy stuff is likely to be the AC heater wiring, power valves consuming between 1.5 to over 2A each can cause issues from the heater wiring. You already know about tightly twisted wires going to the bases and not having the wires causing a loop around the valve base where the wiring continues to the next valve. And good earth referencing, possibly using a humbucker pot rather than the 2 x 47 ohm resistor pseudo centre tap trick.
Now I got to thinking here, are you still using the EL34 heaters in series with 18.9V feed per 3 valves? The issue here is that although the current through the chain is kept at 1.5A, because the first Valve in the chain has the full 18.9V across it, is there an increase in the hum field? Or is it purely current related, in which case its a clever idea. Again heres a daft maybe idea.If you are only drawing 1.5A per wiring run at 18.9V then an LM338 regulator will comfortably handle that current and regulated DC on the heater wiring will virtually eliminate hum. It can be argued that it doesnt matter on a push pull output stage due to self cancellation, but if the heater wiring gets close to other components then I guess there could be some small hum pickup off the heaters? I may have misunderstood here Andy and you already are running DC heaters to your EL34 valves?

And it seems you have a satisfactory result now, so I was chucking a discussion topic on the heap as I am sure I may have some of my ideas muxed ip.

A.

13th Jan 2019, 10:34 pm	#57
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: 120W PP EL34 amplifier re-born. But It should be good at reducing electrostatically induced noise so as long as its got good contact with Chassis ground it isn't completely wasted effort. You're using a toroidal mains transformer if I remember? You have a naturally low magnetic field surrounding it. Given a reasonable physical separation from the mains transformer(s) then the main source of hummy stuff is likely to be the AC heater wiring, power valves consuming between 1.5 to over 2A each can cause issues from the heater wiring. You already know about tightly twisted wires going to the bases and not having the wires causing a loop around the valve base where the wiring continues to the next valve. And good earth referencing, possibly using a humbucker pot rather than the 2 x 47 ohm resistor pseudo centre tap trick. Now I got to thinking here, are you still using the EL34 heaters in series with 18.9V feed per 3 valves? The issue here is that although the current through the chain is kept at 1.5A, because the first Valve in the chain has the full 18.9V across it, is there an increase in the hum field? Or is it purely current related, in which case its a clever idea. Again heres a daft maybe idea.If you are only drawing 1.5A per wiring run at 18.9V then an LM338 regulator will comfortably handle that current and regulated DC on the heater wiring will virtually eliminate hum. It can be argued that it doesnt matter on a push pull output stage due to self cancellation, but if the heater wiring gets close to other components then I guess there could be some small hum pickup off the heaters? I may have misunderstood here Andy and you already are running DC heaters to your EL34 valves? And it seems you have a satisfactory result now, so I was chucking a discussion topic on the heap as I am sure I may have some of my ideas muxed ip. A.