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Old 21st December 2005, 05:41 AM   #1
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Default center tap of heater winding

I am using a EZ81 rectifier tube for my line stage (based on pete millett;s low mu pre but usinf 6080) which is indirectly heated. The 6.3v tap of my transformer is center tap'd. Should I ground the center tap, not connect it, or something else?

If it matters, the gain tube is also indirectly heated, but is heated with a tap from a different transformer -- about which I have the same question, actually.

Thanks,

-d
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Old 21st December 2005, 10:10 AM   #2
dhaen is offline dhaen  Europe
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With the EZ81 winding, I think it hardly makes any difference where it is connected, but it shouldn't be left floating. The most reliable way would be to connect it to the cathode.

I see that on the Low mu pre , Pete has elevated the signal heaters with 2 resistors. If your other winding has a centre tap, then they should connect to this.
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Old 21st December 2005, 11:11 AM   #3
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I dunno if grounding it would be good.The cathode at B+ and the filament inside it at ground potential. -potential arcing?
The cathode would probably be a good bet.(assuming the winding insulation is up to it.)

The CT on the winding for the gain tube can be grounded,or connected to a voltage divider to elevate the filaments a few volts above ground,to help reduce hum.
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Old 21st December 2005, 05:36 PM   #4
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Thanks for the responses. I think I'll try the cathode first, and if that doesn't work and I have noise I'll move on to something else like grounding.

Why might there be arcing if the heater is at ground? The transformers are 125-0-125 leading to a B+ of about 140V

Quote:
Originally posted by dhaen
I see that on the Low mu pre , Pete has elevated the signal heaters with 2 resistors. If your other winding has a centre tap, then they should connect to this.

Quote:
Originally posted by DigitalJunkie
The CT on the winding for the gain tube can be grounded,or connected to a voltage divider to elevate the filaments a few volts above ground,to help reduce hum.
Pete said that the reason the heater is elevated is that the EL34 in the CCS can only take a limited voltage from plate to heater, and this is to lower that potential. In my case, I am using a solid state CCS and thus a much lower B+ which aleviates this problem.
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Old 21st December 2005, 05:40 PM   #5
dhaen is offline dhaen  Europe
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In that case there is little reason to elevate.
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