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Old 22nd December 2012, 09:47 PM   #1
Kenesu is offline Kenesu  Netherlands
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Default Creating artificial taps on transformers

Hello everybody! This is my first post, so please bear with me.

Elaborating on the title of this topic, I would like to know more about using resistors between the poles (a la voltage divider) of a transformer to create 'artificial' taps. Apparently people do it all the time to get center-tapped tube filament supplies, resulting in 3.15V across each resistor.
Then, wouldn't it be possible to obtain an artificial tap at any specified percentage of that side of the transformer, using different resistors, or existing taps?

To put this in context, I have scavenged some pentodes with matching output transformers from an old stereo; each channel featured a single-ended ECL86 (4 watts) and OT for a 5 Ohms speaker (not kidding), with an unused center tap on the primary. Now I want to run these pentodes in Ultra Linear operation, so would it be possible to connect two resistors between HT tap and center tap to get an artificial UL tap at 25%?
I figured that since the resistors would be in parallel with half of the primary impedance, using values of 1k would allow me to obtain the same anode resistance with an 8 Ohms speaker.
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Old 23rd December 2012, 12:33 AM   #2
JMFahey is offline JMFahey  Argentina
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Short answer, no.
Your 1K resistors would be in parallel with the load impedance, sucking a lot of power (in the area of 50%), bad business.
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Old 24th December 2012, 11:41 AM   #3
Kenesu is offline Kenesu  Netherlands
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Quote:
Originally Posted by JMFahey View Post
Short answer, no.
Your 1K resistors would be in parallel with the load impedance, sucking a lot of power (in the area of 50%), bad business.
Aw, too bad that I didn't take that into account. Guess I'll stick with 50% UL taps, perhaps with a 6 Ohm speaker.

Though some people have confirmed that the principle behind it is indeed a simple voltage divider, so one could technically obtain a tap at any point of a transformer side.
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Old 25th December 2012, 09:37 PM   #4
JMFahey is offline JMFahey  Argentina
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Yes, it *is* a voltage divider, and would work (with much higher value resistors), *but* the screens take a lot of current .
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