PT center tap & bridge rectifier question

Meteorman

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Working up a Pro Jr following Hoffman's layout.
Was hoping to use an old Hammond 270AX PT I had laying around.
This transformer has a center tap -- the Pro Jr uses a bridge rectifier and a transformer with no center tap.
My reading suggests that grounding the center tap inappropriately can cause a lot of magic smoke to leave the transformer, and other parts as well.
Is it OK to just cap off the CT, or is this transformer doomed to fail in the Pro Jr circuit ?
thanks
/mike

upload_2021-10-23_10-42-42.png
 

2L man

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For bridge rectifier PT output voltage needs to be half what it is for CT full wave rectifier where two diodes are used.
 

Ten Over

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In other words, sounds like, this PT cannot easily be integrated into the Pro Jr circuit.
It can easily be wired for a Pro Jr and it has enough heater amperage. However, the plate amperage is a little skinny at 58mA. If it were me, I'd go ahead and use it. If you want to go for it, I can draw out how to wire it.
 

2L man

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Math for 480V 58mA rated output for Center Tab DC use:

First power calculation: 480V x 0.058A = 28VA.

Then DC voltage: Pi x 240VAC = 338VDC

Then DC current: 28VA : 338V = 82mA

CT DC current capacity comes 1,41 (Pi) higher than what full output voltage current rating is and it is because CT Full Wave Rectifier take current from output coil half only each half waves and coil can cool 50% the time.

And in practive current flow shorter time when filter capacitors charge and pesk current can come higher than 82mA.
 
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bebopbrain

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The 270AX provides barely half the DC current of the stock transformer. I would just buy the proper transformer so it will last forever.

https://www.hammfg.com/electronics/transformers/rectifier

Compare formulas for Full Wave Capacitor Input Load and Full Wave Bridge Capacitor Input Load.

Stock transformer: Fender 41782, Hammond 290XXA
secondary 260 VAC
secondary 170mA IAC
VDC peak = 1.41 x 260 = 366 VDC
IDC = 0.62 * 170mA = 105mA

Replacement: Hammond 270AX,
secondary 480 VAC
secondary 58mA IAC
VDC peak = 0.71 x 480 = 341 VDC
IDC = 1.0 * 58mA = 58mA
 

King Fan

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Lots of good thinking here. FWIW my soggy cocktail napkin says the Pro Jr. current draw is > 80mA?
 




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