Raising filament winding above cathode with a EZ81 in the circuit OK? - diyAudio
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Old 15th May 2008, 02:41 PM   #1
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Default Raising filament winding above cathode with a EZ81 in the circuit OK?

If I were to build a preamp with a EZ81 in the PSU is it still alright if I raise the filament winding 10 - 20v above ground or will it cause problems for the EZ81?
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Old 15th May 2008, 03:44 PM   #2
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Well it won't help MUCH, but it lowers the heater-to-cathode voltage on the EZ81. Be sure to bypass the voltage divider to keep heater supply at AC ground.
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Old 15th May 2008, 07:35 PM   #3
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Which brings up a question I have been pondering over, which should also help G in his quest for elevating his heaters:

That has to do with selecting the size of the capacitor. Here's my assumption, correct me if I'm wrong:

Determine voltage divider. I usually throw out a total current of 1mA as an approximation. So if I have a 300V B+, and want to elevate the heaters to 50V, I would choose a 250K and 50K resistor set. Simple enough.

So, then, how to choose C ??? What I do is figure out the source impedance of my divider network. 250K in parallel with 50K is 41.6K. Then, I select the RC time constant for some number lower than 20Hz, say 5Hz. If RC= 5, then C = 120uF. 'lytic should be fine.

Is this rational thought?
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Old 15th May 2008, 08:29 PM   #4
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The capacitor has to be a low impedance for the noise sources, which are 60 Hz and up, and are relatively high impedance. Heater-cathode leakage, common-mode power-line noise from capacitance between primary and heater windings, etc. I think anything over 10 uF would be enough... but since it's a low voltage, even a 330 uF cap won't be very big...
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