I have a tube amp that I've built which uses 2x 60FX5 tubes in single ended stereo configuration with a 250mA 120:120 mains power transformer. I use a rectifier circuit with a solid state DC regulator for B+ and it is solid and clean.
While I do use AC on the heaters, it is not the source of the hum. If I use a second transformer it is silent as can be. I can connect the heaters to my 120V DC B+ source and it is also quiet but it exceeds the power rating of the transformer. My problem appears to be that the 120V secondary rings like a bell every time a diode cuts off.
I have managed to get rid of most of my hum by using .1uF caps across each diode in the bridge in addition to 2.2uf and 22 ohms across the transformer secondary in parallel to the tube heaters but it isn't enough. By the time I increase the capacitor values enough to get rid of the hum I've exceeded the current limit of the transformer.
Is there a diode type that can replace the 1N4007s I use in my bridge that would help? Possibly soft recovery or go the opposite direction and use ultrafast?
While I do use AC on the heaters, it is not the source of the hum. If I use a second transformer it is silent as can be. I can connect the heaters to my 120V DC B+ source and it is also quiet but it exceeds the power rating of the transformer. My problem appears to be that the 120V secondary rings like a bell every time a diode cuts off.
I have managed to get rid of most of my hum by using .1uF caps across each diode in the bridge in addition to 2.2uf and 22 ohms across the transformer secondary in parallel to the tube heaters but it isn't enough. By the time I increase the capacitor values enough to get rid of the hum I've exceeded the current limit of the transformer.
Is there a diode type that can replace the 1N4007s I use in my bridge that would help? Possibly soft recovery or go the opposite direction and use ultrafast?
Physically they are both quiet, they are well resined. The ringing is electrical and much higher frequency than mains which is how it couples to the cathode in the tube. Swapping the transformer doesn't solve the problem, only using a separate secondary to give clean 60hz AC works.
I got ya. Was a bit confused on the way the 2nd tranny was being used. Not as a different PS tranny. So if the B+ is clean, and the heater AC isn't the source of the hum, where's it coming into the audio path or output transformer?
Clean AC doesn't induce hum but the high frequency ringing of the transformer induces a buzz into the cathode. It isn't much but it doesn't have to be as these pentodes are high gain and meant to run full output power at 3V input.
Some people are enthusiastic about snubbing their power transformer secondaries, so there is zero ringing.
There is a thread here on diyAudio that shows what to do and how to do it; the thread title includes the word Quasimodo. If you read the first page or two of the .pdf tutorial {an attachment to Quasimodo post #1} you can get an idea of what it is and how it works.
There is a thread here on diyAudio that shows what to do and how to do it; the thread title includes the word Quasimodo. If you read the first page or two of the .pdf tutorial {an attachment to Quasimodo post #1} you can get an idea of what it is and how it works.
I had problems with a valve audio mixer with the rectifier being very noisy.
I tried snubbing it but that just added an AC signal on top of rectified DC !
In the end I just used the correct Schottky diodes and all was well.
If you are using a 3-wire power cord and also using your chassis as a ground, try lifting the safety ground off the chassis to see what you get. Are any of your signal shields (coax) chassis grounded?
There are fast, soft recovery diodes that switch clean without introducing ringing. I would start there first.
Generally, diodes for high frequency applications like SMPSs have soft but fast recovery characteristics.
What are Fast Recovery Diodes (FRD)? | Semiconductor | SHINDENGEN ELECTRIC MFG.CO.,LTD
BTW, when you say 'hum', are you really meaning 50/60 cycle hum? Or is it more a rattling, buzzing sound?
Jan
Generally, diodes for high frequency applications like SMPSs have soft but fast recovery characteristics.
What are Fast Recovery Diodes (FRD)? | Semiconductor | SHINDENGEN ELECTRIC MFG.CO.,LTD
BTW, when you say 'hum', are you really meaning 50/60 cycle hum? Or is it more a rattling, buzzing sound?
Jan
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I'm highly confident that it isn't a ground loop issue as running the heaters on DC yielded an extremely quiet output.
It's much more of a buzz I suppose than a hum. I thought I had a scope photo of the un-snubbed transformer ringing but I can't seem to find it so I'll try to get a photo this weekend. I have some UF4007s that I can try in a bridge now too.
Cathode current is ~45mA per tube into 60R and 50uF. Thinking about this more though, I recall seeing the hum in the high impedance grid circuit which does have coax and is grounded only on the potentiometer side at ground. I'll verify this statement though.
It's much more of a buzz I suppose than a hum. I thought I had a scope photo of the un-snubbed transformer ringing but I can't seem to find it so I'll try to get a photo this weekend. I have some UF4007s that I can try in a bridge now too.
Cathode current is ~45mA per tube into 60R and 50uF. Thinking about this more though, I recall seeing the hum in the high impedance grid circuit which does have coax and is grounded only on the potentiometer side at ground. I'll verify this statement though.
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