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One secondary voltage and two biases
One secondary voltage and two biases
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Old 3rd June 2015, 03:02 PM   #1
max1979 is offline max1979  Italy
diyAudio Member
Join Date: Apr 2015
Default One secondary voltage and two biases

Hi all,

In my garage I have found a heavy transformer from 230V to 15V (with no load) - 2kg transformer with just one secondary, it looks like a 90/100W.
I have to test it using some resistance to be sure about the max current but I'd like to use it to feed the heaters of the pre phono I'm going to realise.
This is based on a ECC88 and a ECC83 for each channel: total heater current should be close to 600mA (12.6V).

Now, the problem is that the preamp is designed as to have a beta-follower (ECC88) and a mu-follower (ECC83) so I need to bias the heaters of the upper tubes.
Is it possible to have two different 12.6V outputs with two different bias voltage (upper and lower tubes) starting from the same transformer? Is it safe?
In case, where am I supposed to split the circuit? after the first cap? after a common choke?
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Old 4th June 2015, 08:56 AM   #2
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
In theory you could do something clever with capacitor coupling of the AC to the heaters, but I don't recommend it. Otherwise, you hit the problem of wanting one winding to sit at two quite different DC potentials.

Sell the transformer you have. Use the money to buy one with two secondaries.
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