Sanity check - back to back transformers and CT - diyAudio
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Old 1st March 2013, 12:32 AM   #1
00940 is offline 00940  Belgium
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Default Sanity check - back to back transformers and CT

Hi everyone,

Would the following be ok: to use a 10-0-10, 40va, center tapped transformer to power a ss buffer (with a bipolar supply) and then connect a 9-0-9, 18va transformer on one half of the first transformer secondary (10vac), in order to get a B+ of about 120 to 150vdc for a tube gain stage ?

It's not so much the back to back transformers issue that worries me (I know about the poor regulation, possible saturation issues and necessary derating), but the unbalance in current draw for the first transformer. Taking the supply for the heater from the opposite side of the CT would kind of (over) balance of things...

A schematic being worth a thousand words:

Click the image to open in full size.

Thank you for any comment or suggestion on this
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Old 1st March 2013, 03:52 AM   #2
DUG is offline DUG  Canada
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Imbalance may show up as added 60Hz component to 120Hz ripple in DC supplies.

If you are going to regulate then it is not an issue.

Just remember that the 18V, 18VA transformer, when run on 10V is now a 10VA transformer because of the current ratings.

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Old 1st March 2013, 09:01 AM   #3
00940 is offline 00940  Belgium
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Allright then, thank you. :-)
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Old 1st March 2013, 09:40 AM   #4
AndrewT is online now AndrewT  Scotland
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Connect the 18Vac transformer to the 20Vac output. It will be close enough to not overheat.
Now split the former primaries to give your 110/120Vac for the B+
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Old 1st March 2013, 11:59 AM   #5
DUG is offline DUG  Canada
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Quote:
Originally Posted by AndrewT View Post
Connect the 18Vac transformer to the 20Vac output. It will be close enough to not overheat.
Now split the former primaries to give your 110/120Vac for the B+
I would suggest that you test to see if the transformer is able to handle the overvoltage without saturation issues.

Place a small resistor in series with the 18V winding...1 ohm should do.

Apply 10Vac and measure the voltage across the 1R...should be small.

Apply 20Vac and measure the voltage across the 1R and then review the results.

If the transformer does not have any issues with the 20Vac then the voltage in the 20Vac case will be twice the 10Vac case.

If the current has more than doubled then you will have to review what the number is and go from there.

IMHO

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Old 1st March 2013, 01:36 PM   #6
00940 is offline 00940  Belgium
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I've tested the setup with 10vac into 18vac, a bridge, a cap and a 15ma load (11k resistor). I've let it run for 2 hours. No problem at all indeed. I get 155vdc for the B+ (wich means about 110vac, I don't trust the ac measurements of my dmm).

As for running 20v into 18, I'll try it tomorrow. There is a 125v tap on the primary winding of the second transformer. It cannot be used as CT but I could tap it for b+, for about 175vdc I guess.
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