Full Wave Bridge Using Center Tapped Transformer

[W5FH]

Just a few quick questions please -
     Have been looking at power transformers for a project and was studying the Pacific power transformer for the Fender 135 watt Silverface Twin Reverb; it has a 180-0-180 secondary at 500 mA and in my simple mind I was considering it for a full wave bridge. Was wondering, why did they center tap a winding when a bridge rectifier is to be used. So I pulled up schematic for the amp and was surprised when I saw that in the use of this transformer the center tap of the winding is connected to the mid-point of the two seriesed filter capacitors. Not being familiar with this type circuit I researched it and saw some previous posts here discussing it. In using this circuit with this transformer, with 180-0-180 secondary, the "high" output voltage is about 2.73 times the one-half secondary voltage (180 v x 2.73 = 491 VDC). The midpoint voltage (at series capacitor junction) would be one-half this voltage. I can see the usefulness of this type arrangement but why did Fender use this circuit when they are not utilizing the midpoint (1/2 value) voltage anywhere in amp? Is there an advantage, in using this circuit, of forcing the voltage to equalize across the two seriesed filter capacitors? Or is there other advantages? Eager to learn here. Thanks! 

[Latole]

Picture may help

[tubenit]

https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_135_schem.pdf

Can you PLEASE provide a link to where you found the information that the PT is 180-0-180?

My understanding is that a solid state rectifier would be 1.4 x 2ndry voltage?   So with 500v divided by 1.4 = around 355-0-355.

Take a look at the Hammond PT's for the Twin Reverb.

I am not saying you're incorrect. I'm simply puzzled about the source for the 180-0-180 info?

With respect, Tubenit

[HotBluePlates]

... why did they center tap a winding when a bridge rectifier is to be used. ... the center tap of the winding is connected to the mid-point of the two seriesed filter capacitors. ... Is there an advantage, in using this circuit, of forcing the voltage to equalize across the two seriesed filter capacitors? ...

If the series capacitors had exactly-equal capacitance, they would form an AC Voltage Divider that would evenly split the ripple voltage.  But electrolytic caps have tolerances like +100/-50% so they're never equal. 

We can use resistors across each cap in the series stack to help force equal-voltage across each cap.  We get the side-benefit of draining the filters when power is switched off.  But resistors, too, have tolerance.

If we connect the transformer center-tap to the midpoint of our series-caps, equal-voltage is enforced by the CT's position.  It can be used alone for equal-voltage across the series caps, but adding the resistors still gives us the advantage of draining the power supply when powered off.

Solid-state products would use such an arrangement for a bi-polar power supply, and ditch the resistors because they're not concerned with draining a low-voltage supply.

[sluckey]

My understanding is that a solid state rectifier would be 1.4 x 2ndry voltage?   So with 500v divided by 1.4 = around 355-0-355.

That's true for a conventional 2 diode rectifier. But 355-0-355 is the same as 710 all the way across the transformer. That would result in 994Vdc from a FWB!

But this transformer is 180-0-180 which is 360V all the way across the winding. Connect that to a FWB rectifier and you get 504Vdc.

Does that help?

[pdf64]

…
My understanding is that a solid state rectifier would be 1.4 x 2ndry voltage?   So with 500v divided by 1.4 = around 355-0-355.

Bear in mind that a full wave bridge FWB rectifier operates rather differently to the typical 2 phase, push pull full wave rectifier used in vintage amps, where the HT winding CT is connected to circuit common.

With a FWB rectifier, the whole HT winding conducts over the whole cycle. The unloaded DC output on the reservoir cap will be the VAC across the whole winding x 1.414.
So 509V from the 360VAC winding in the OP.

Whereas with a 2 phase, push pull rectifier, winding CT connected to circuit common, each half winding only conducts for half the cycle. The unloaded DC output on the reservoir cap will be the VAC across the whole winding x 0.7071. Or alternatively, the VAC of a half winding x 1.424

Take a look at the Hammond PT's for the Twin Reverb.

Look to the PTs above those, for the red knob twin; 337V centre tapped, at twice the current rating as the push pull rectified twin reverb types. The red knob twins use a FWB rectifier with the same arrangement as the 135W TR in the OP.
https://el34world.com/charts/Schematics/files/Fender/Fender_twin_94.pdf

For further info http://www.valvewizard.co.uk/bridge.html

[W5FH]

I surely thank each and every one of you for the nice replies and info here. I attached a link to the Pacific Audio Magnetics power transformer that I was studying. I have been impressed by their products in my internet studies; it was difficult to find my way to their actual store site. This sure looks like an excellent transformer.
    When I looked at the intended use of this transformer it intrigued me as I never have worked with a bridge rectifier using a center tapped winding going back to seriesed capacitor junction. Just to make sure I am fully understanding this circuit -- If I had filter capacitors of sufficient voltage rating and did not need to series two of them is there any other known reason for using this center tap arrangement? Is there a regulation or ripple reduction gain being capyued by using center tap back to seriesed capacitors? Thanks guys!!
 

[pdf64]

If I had filter capacitors of sufficient voltage rating and did not need to series two of them is there any other known reason for using this center tap arrangement?

No, in that case the CT offers no operational or functional benefit, it passes 0 load current.

[sluckey]

The ONLY purpose for connecting the CT to the junction of those series caps is to force the voltage to be equal across each cap. Nothing more.

If you had a single cap of sufficient voltage rating you would not use that center tap at all.

[W5FH]

Thank you all for enlightening me here and answering my questions. I am searching for just the right power transformer for a build that will use a pair of USA old stock Tungsol 6550's. My target is 500 VDC on these tubes (any opinions?). The build will have about 5 preamp tubes and push-pull 6550's on a super reverb size chassis in a custom Vibroverb 1x15 cabinet. The closest power transformer I have found is the Hammond 290D2X (rated at 710/610 VCT @ 300 mA). I feel that 300 mA. may be on the light side however this amp will not be pushed real hard in most situations. The other transformer I am looking at is the Edcor XPWR127 which is 690 VCT @ 400 mA which also has great filament specs and 60 volt bias tap. For the price this transformer looks like the best match to my needs but I am concerned that I may need to beef up the mounting tabs. And, again, the Pacific unit previously mentioned looks great but appears maybe a little more transformer than I need. If anyone has any time here I am all ears! Thanks.

[pdf64]

The HT current requirement is determined by the HT voltage and OT impedance.

[W5FH]

Yessir pdf64 -
   What is your opinion of 4.2K output transformer at 500 VDC on OT center tap for the 6550's pair? Maybe I should rephrase this - - I have a 4.2K 100 watt output transformer. What is your opinion of a good voltage target for a fairly clean ouput stage not necessarily looking for maximum output.
   Thanks!
 

[pdf64]

I assume you’re not looking to use a split rail HT, feeding the screen grids from a significantly lower HT supply than the OT CT? (Incidentally this is an application when the half HT voltage node of a CT and FWB rectifier is very handy.)

Or that the OT is a UL type?

The 440V design max limit for the screen grids needs consideration. If a regular pentode or UL output stage arrangement, that means the idle HT can’t be much above 450V.
That should achieve a real world 70-80W clean with a 4k2 OT, and avoid overstressing the valves.

[W5FH]

Many thanks pdf64-
    I was wondering about that screen grid voltage. My output transformer is an Edcor CXPP100 that does have the UL taps. I understand what you are saying in that if I went UL then my plate B+ will be constrained by the screen max voltage. Do you think I should use the UL screen taps and shoot for a B+ voltage (to OT center tap) of around 450 VDC idling? Or, should I use the split rail set-up rectifier and have a little highr B+ (around 500 VDC) and use the 1/2 point for screens? I know the screens will pull some current there, will that upset anything else in that type rectifier?

[pdf64]

The 450V idle with OT of 4k2 should work fine for UL or regular pentode.
If you set up a screen grid HT supply node, it’s easy to add a DPDT switch to change from UL to pentode operation.

[PRR]

... ... ... If I had filter capacitors of sufficient voltage rating and did not need to series two of them is there any other known reason for using this center tap arrangement?... ... ...

In the larger world: the FWB + CT is used in nearly ALL transistor amplifiers. The CT is "ground" and the hots become plus and minus. This allows the amp to drive a loudspeaker, without wasted DC, without an audio transformer or capacitor.

A VERY old example: