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Transformer Question (Read 2729 times)
Verified Member

Posts: 1
Transformer Question
11/08/11 at 17:47:40
Good Day ~

Let me start by saying that for all intents and purposes, I'm an electronics noob.  I understand a chunck of the basics but I've got a LONG way to go.  I've built a ZKIT1 that sounded fantastic right off, and  I'm hooked.

I'm now planning on building the same circuit with point-to-point soldering and tube rectification via a 5U4.  Essentially it's the SE84C I guess.

My question is regarding the B+ connection to the power transformer.  The schematic I'm looking at shows the B+ connected to the center tap of the 5 V secondary.  The transformer I'm planning on using is the Edcor XPWR008-120 which only has a center tap on the high voltage secondary.  Can I connect the circuit to this center tap instead?  Since there are no other center taps available to connect to directly to ground, will I encounter other problems if I do this?

This is undoubtedly a foolish question with an obvious answer.  It's part of my learning process I guess.

Thanks in advance for your help.

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Seasoned Member

pair of dual 18

Posts: 298
Re: Transformer Question
Reply #1 - 11/09/11 at 18:14:25

DO NOT connect B+ to the center tap of the high voltage secondary...that center tap must go to ground.  Without that connection you get a 600v half wave rectifier instead of a 300v full wave rectifier which would result in smoke and damaged parts.

Connect the two plates of the 5U4 to the two ends of the high voltage secondary winding.  Connect the center tap to ground.  Connect the  two ends of the 5vac winding to the two heater pins on the 5U4.  Connect B+ to either heater pin (but NOT both).  

That said, I should ask why you want to replace two perfectly good 1N4007 silicon diodes with a troublesome voltage regulator tube?  You will drop your B+ by about 50v which will result in lower output power.  Also, if you are building from scratch, be sure to put in the CCE mod (.1uf 400v cap from pin 6 to the cathode of each output tube).  If it were me building this, I'd use the 1N4007 diodes instead of the 5U4, use the 5vac for the heater of the input tube, then parallel the current output tubes with a couple more output tubes (using the 6.3 vac for all the output tube heaters).   This would increase output power if you install 4 output tubes or give you "normal" power if you just use 2 output tubes.  This is DIY after all - you might as well do something a little different.

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