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G 15th May 2008 02:41 PM

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?

Tom Bavis 15th May 2008 03:44 PM

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.

zigzagflux 15th May 2008 07:35 PM

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?

Tom Bavis 15th May 2008 08:29 PM

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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