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Old 23rd January 2009, 02:47 PM   #1
wicked1 is offline wicked1  United States
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Default Basic Transformer hookup wiring

Hi,
Ive got a very basic transformer wiring question. I'm doing some work on the amp I built a year ago and just want to double check everything before I button it up. I'm sure I've got this right, but it's been a year since I thought about this stuff, so better to ask now and be done w/ it...

On the output transformer schematic there are the two spirals representing the primary and secondary starting on the top, ending on the bottom, primary on left secondary on right.

The top lead on the primary and secondary have *'s next to them. The top lead on the 2ndary is clearly common/ground, as the next 3 leads are 4, 8, 16ohm.
On the primary side which lead would go to the powersupply, and which to the tube?

thanks
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Old 23rd January 2009, 09:04 PM   #2
wicked1 is offline wicked1  United States
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So, I thought maybe I was just being really stupid, and spent all afternoon looking at schematics, and I still don't understand how to do it. Does it not matter?

Some of the schematics I see have the output transformer wired w/ the common on the secondary being across from the power input to the transformer. (lead connecting to power supply)
Such as, the tubelab simple se, http://www.tubelab.com/AssemblyManua...ematic_SSE.htm

and this one
http://community.webshots.com/photo/...62123407ZaJGuO

Then there are other schematics that show the transformer w/ the secondary common being across from the power output of the transformer (lead connecting to the tube)

http://www.enjoythemusic.com/magazin.../trex/fig1.jpg

which should it be???
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Old 23rd January 2009, 09:56 PM   #3
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Hey

In an amp with no global feedback and with no screen taps it does not really matter which way you hook it up. You will however probably see a difference in effective shunt capacitance that could effect bandwith if you are cutting it close. Normally lowest effective capacitance is obtained when connecting the anode to the innermost winding and B+ to the outer.


/Olof
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Old 24th January 2009, 12:21 AM   #4
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Quote:
So, I thought maybe I was just being really stupid, and spent all afternoon looking at schematics, and I still don't understand how to do it. Does it not matter? .....In an amp with no global feedback and with no screen taps it does not really matter which way you hook it up.
It is true that in a non feedback amp without UL taps, it doesn't make much difference. If the amp takes feedback from the transformer secondary and uses it anywhere in the amp, it does matter.

If the transformer is wired one way the feedback will be negative reducing the distortion and output impedance. If reversed the feedback becomes positive increasing the distortion and possibly causing the amp to oscillate. The real confusion lies in the fact that there is no standard method for identifying the absolute polarity of the transformer windings without testing it. The Simple SE schematic is drawn the way it is because that is what works for the majority of transformers that I have tested. Often it is necessary to try it both ways and pick the connection that offers the less total amplifier gain (lowest volume for a given volume control setting), or doesn't oscillate (make rude sounds). This ensures negative feedback.

If the transformer has a UL tap, and it is used, it should be wired up the way the transformer manufacturer intended. Again there seems to be no standard color code for this. With odball transformers, again I have just tried it both ways, and picked what I thought was the best sound. It is not as obvious as the secondary - primary polarity.
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