Rob Flinn
Well-known member
I always reference my heater ground to chassis ground & have never experienced any problems. In fact I had more problems when I haven`t referenced it to ground.
heater PSUs
- Thread starter Rob Flinn
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Help Support GroupDIY Audio Forum:
To avoid problems with electrostatically induced noise from heater to cathode, the heaters are best referred to ground. But this does not necessarily mean that they need be at ground potential - often it is desireable to lift the reference point for the heater to overcome limited heater/cathode maximum voltage specs when the cathode is high above ground.
Jakob E.
Jakob E.
It should be referenced to something, to be sure the heater/cathode voltage isn't exceeded. But if you're making a preamp, I would reference it to a higher voltage than ground - something like 50V DC or a higher voltage if the circuit includes cathode followers etc.
You can see an example here: http://stiftsbogtrykkeriet.dk/~mcs/MicPre/micpre_v2.gif
Best regards,
Mikkel C. Simonsen
You can see an example here: http://stiftsbogtrykkeriet.dk/~mcs/MicPre/micpre_v2.gif
Best regards,
Mikkel C. Simonsen
Rob Flinn
Well-known member
MCS & Gyraf
Yes, you`re right sometimes it is better to reference it above ground as in certain parts of the Fairchild 670. I think in this case it is the valves that are in the regulator circuit that are referenced at 1/2 the HT voltage.
However in most circuits you come across they are referenced to ground.
Yes, you`re right sometimes it is better to reference it above ground as in certain parts of the Fairchild 670. I think in this case it is the valves that are in the regulator circuit that are referenced at 1/2 the HT voltage.
However in most circuits you come across they are referenced to ground.
rafafredd
Well-known member
MCS, what´s the reason for that interesting diode + caps bridge?
[quote author="rafafredd"]what´s the reason for that interesting diode + caps bridge?[/quote]
Look closer... it's a regular diode bridge with snubbing caps - just drawn differently than what most of us are used to!
Peace,
Al.
Look closer... it's a regular diode bridge with snubbing caps - just drawn differently than what most of us are used to!
Peace,
Al.
Rob Flinn
Well-known member
Or use a valve rectifier, then whether this switching noise makes a difference or not, it won`t be a problem to you !
> reference it above ground as in certain parts of the Fairchild 670. I think in this case it is the valves that are in the regulator circuit that are referenced at 1/2 the HT voltage.
That's because the cathode of the power tube is up around +300V, and the volt-amp tube not much below (maybe 150V?). So the only safe place for the heaters is up in that area, 150V-300V, so neither tube exceeds its heater-cathode voltage rating (100V-200V).
Same thing can apply with cathode followers, which often have cathodes up over 100V. If the heater insulation is only good for 100V, then the heater winding has to float well above common ground. If you want to try to run cathode-followers and semi-grounded-cathode stages on the same heater winding, split the difference: 50V-100V on the heater winding.
There is also a fashion for not having a solid voltage reference on the heater winding. Theory is that heater leakage will make the winding find its own voltage mid-way between all heaters, and if one heater shorts to cathode the system will keep working. This seems fishy to me: it works when it works, but can fail in odd ways. Also you almost always have to hang a 0.22uFd or so to ground to supress audio and RF leakage wandering around the heater circuits, so a heater-cathode short probably will sicken the amplifier.
When all your cathodes are semi-grounded: grounded heaters will work, and if the heater insulation is perfect they work very well. But 0.1% of tubes will have small insulation leakage. If the heater is held positive of cathode, the resulting hum is less than with a grounded or negative heater winding. So many mass-produced amps bias the heater up around +50V. In most cases, grounding makes no difference, but once in a while you find a tube that buzzes with grounded heater but is clean with +50V bias. Obviously in mass production it is better to add $0.12 of parts to every amp than to test every amp and tube-roll the occasional leaker.
That's because the cathode of the power tube is up around +300V, and the volt-amp tube not much below (maybe 150V?). So the only safe place for the heaters is up in that area, 150V-300V, so neither tube exceeds its heater-cathode voltage rating (100V-200V).
Same thing can apply with cathode followers, which often have cathodes up over 100V. If the heater insulation is only good for 100V, then the heater winding has to float well above common ground. If you want to try to run cathode-followers and semi-grounded-cathode stages on the same heater winding, split the difference: 50V-100V on the heater winding.
There is also a fashion for not having a solid voltage reference on the heater winding. Theory is that heater leakage will make the winding find its own voltage mid-way between all heaters, and if one heater shorts to cathode the system will keep working. This seems fishy to me: it works when it works, but can fail in odd ways. Also you almost always have to hang a 0.22uFd or so to ground to supress audio and RF leakage wandering around the heater circuits, so a heater-cathode short probably will sicken the amplifier.
When all your cathodes are semi-grounded: grounded heaters will work, and if the heater insulation is perfect they work very well. But 0.1% of tubes will have small insulation leakage. If the heater is held positive of cathode, the resulting hum is less than with a grounded or negative heater winding. So many mass-produced amps bias the heater up around +50V. In most cases, grounding makes no difference, but once in a while you find a tube that buzzes with grounded heater but is clean with +50V bias. Obviously in mass production it is better to add $0.12 of parts to every amp than to test every amp and tube-roll the occasional leaker.
Rob Flinn
Well-known member
I often wondered abou this with shunt regulated push pull or cascode configeration, where one cathode could be a quite a high voltage & the other could be at a significantly lower voltage. When using somthing like an ECC88, both valves are in the same bottle.
