Back to Back 6-0-6Vac Xfmrs

[jeff]

Hello,
I'm thinking of using two 120Vac to 6-0-6Vac transformers back to back to make a power supply, and have a few questions.
The two Xfmrs I have are rated at 1.2A and .45A.
1) Connect the 1.2A to the switch, fuse, and power cord
2) Connect the 6-0-6 wires to the .45A Xfmr
3) Connect the mains wires of the 2nd .45A to my rectifier.


Correct?


I have a few questions.
A) When connecting the 6-0-6 wires do I connect BOTH transformers 6-0-6V C.T. to ground, or just one?
    if one, which one? The first Xfmr's secondary or the second transformers secondary?
B) What will this 'new' Xfmr rating be?
     Since it's a 6-0-6Vac I assume I'll be heating the tubes series(12V@150ma) so, for example, I use 4 tubes, I would be .6A out of the Xfmr's 1.2A rating leaving me 120V@"what" for B+?


In other words, the more tubes I heat, the lower B+ current available from the second Xfmr, and vise versa?

[tubeswell]

yes,

I gather you're wiring the heaters for 12VAC (with each 6V output tube pair in series).

And provided that you're aware that the total current draw on the 1.2A PT will be the sum of the load seen by both PTs. (i.e. if the 0.45A PT is loaded to maximum, then the 1.2A PT could only support 0.75A* for heaters.


(*which is okay for say 2 x 6V6 in series (drawing 0.45A) + a couple of 12AX7s wired for 12V operation (drawing 0.15A each) - however, having said that, these will not draw anywhere near 0.45A from the other PT, so there's probably a bit more wiggle room)

[jeff]

yes,

I gather you're wiring the heaters for 12VAC (with each 6V output tube pair in series).

And provided that you're aware that the total current draw on the 1.2A PT will be the sum of the load seen by both PTs. (i.e. if the 0.45A PT is loaded to maximum, then the 1.2A PT could only support 0.75A* for heaters.

Ok, thanks, I dig that.
But say I did do that. I'm heating tubes, pulling .75A for heaters, I'm connecting the .45A backwards to get 120V, I use a voltage doubler to get ≈264V, What's the mA rating of my B+?


In other words:
I connect a 120V to 6-0-6V@.45A normally to the wall, I can pull .45A A-ok.
I connect the same Xtfm backwards to a 6-0-6V supply, I can only pull .045A, or 45mA?
Is that correct?


A transformer rated 12V(6-0-6V)@.45A forward equals a transformer rated 120V@.045A backwards?
12V X .45A = 120V X .045A = Wattage of Xfmr

[glass54]

Hi Jeff
I have done this many times (shortage of suitable Transformers for Preamps in Aussie) over the last25 years.
So say you are using tube heaters in series (12A*7) requiring 150mA per tube @ 12V AC nominal.
If you have 2 tubes, heater current total is 300mA.
Now say you have a Transformer 120V:12V AC rated at 1A secondary. Your Total VA rating is 12VA.
Your heater consumption is 0.3A, therefore 0.3x12=3.6VA (You still have 8.4VA available to feed your reversed second transformer 
Now for your second transformer 120V:12V rated at 0.45A ie 5.4VA (based on 12V and 0.45A) At Full output, your 120V maximum current will be 5.4/120=0.045A
Your first transformer connected with heaters and second (HT transformer) consume 0.75A at 12V (300mA heaters and 0.45A for second transformer assuming full load)
At this rate, you are safely within main (First transformer) ratings with a little room for losses, so transformer should be very happy.
Trust this helps.
Kind regards
Mirek

[jeff]

Nice, thanks
So I can pull 45mA at 120V.
If I build a voltage doubler rect to get 264V does that drop to half the current available?
In other words, with a full wave rect, I'm looking at ≈168V@45mA. Using a voltage doubler I'd get 264V@22.5mA, or would it still be 264@45mA?


Not sure if using a voltage doubler halves the available current or not.


Also.
Both 12V are 6-0-6V(two yellows and a black 12V c.t.). Yellows go to yellows, but, do I want to ground BOTH Xtrm's C.T.s or ONLY the first Xtrms C.T.?


Thanks
Jeff

[sluckey]

Only ground the first Xtrms C.T.

[Merlin]

I have a few questions.
A) When connecting the 6-0-6 wires do I connect BOTH transformers 6-0-6V C.T. to ground, or just one?

Only the first CT should be grounded.

B) What will this 'new' Xfmr rating be?
     Since it's a 6-0-6Vac I assume I'll be heating the tubes series(12V@150ma) so, for example, I use 4 tubes, I would be .6A out of the Xfmr's 1.2A rating leaving me 120V@"what" for B+?

First transformer is rated for 12V*1.2A = 15VA.
Three heater consume 7.2W. You will also lose 3 to 5VA in magnetising the second transformer, leaving maybe 4VA for your B+.
Back-to-back is quite lossy so you won't actually get 120Vac from the second transformer, more like 100Vac depending on the type of transformers you're using. After voltage doubling that's about 280Vdc.
As for current, 4VA/100V = 40mArms. You're using a voltage doubler which effectively halves this figure, and with a typical power factor of 0.65, that translates to perhaps 40mA*0.5*0.65 = 13mAdc.
That may be less than you expected, but still more than enough for three 12AX7s!

[kagliostro]

May be you can be interested to see this



it was planned for other voltages but I think you can be interested



May be something like this ?


EDIT: THE FOLLOWING SCHEMATIC IS DRAWN INCORRECTLY, SEE FURTHER POSTS



There are two limitations

. the current disposable after the doubler will be 1/2 of the current disposable before it

. The DC current disposable for the Heaters is 0.6 of the AC current

Franco

[jeff]

Great, lots of good info.
Been gone so long almost forgot how awesome this site is.
Thank you everyone

I have another question.
Does a voltage doubler yield DC voltage that is 2.8 X the AC or 2.2 X AC?
       I'm only familiar with a doubler that involves two diodes and two caps(working as a half wave stacked on top of a have wave) the rect on top of Franco's schematic. It looks like the bottom rect(with caps) is a full wave stacked on top another full wave....?
Any one have voltage doubler schemos handy to post?


Thanks
Jeff.

[jeff]

Franco,
Thanks for the schematics.
Is the 120VAC Doubler on the 3rd sch. Reversed? Input/output flipped?
Wouldn't that short the AC/reverse bias the caps every other half cycle?

[Merlin]

Does a voltage doubler yield DC voltage that is 2.8 X the AC or 2.2 X AC?

2.8 off load, but it will be fairly saggy under load.

Franco's schematics have an error: the 12V CT of the second transformer must be left unconnected.

[jeff]

Does a voltage doubler yield DC voltage that is 2.8 X the AC or 2.2 X AC?

2.8 off load, but it will be fairly saggy under load.

Franco's schematics have an error: the 12V CT of the second transformer must be left unconnected.

Also, is the doubler on the 120V on the 3rd schm. Reversed. Input/output? Just 'double' checking.

[glass54]

Hi Jeff
Please see attached power supply that I designed and used on a Bass Preamp (1RU) back in 2003.
I decided on a DC Heater supply (+12.5V) and the Transformers were 240V:12V AC (Australian Standard)
Tube selection included 2 off 12AX7.
The Preamp is still going
I am also including a basic Voltage doubler (because you have 120V AC). This would replace the bridge rectifier BR1_PS.
Regards
Mirek

[Merlin]

Also, is the doubler on the 120V on the 3rd schm. Reversed. Input/output? Just 'double' checking.

Well spotted, yes it's all bass ackwards. I don't know where Franco is getting his schematics, but they can't be trusted.

[jeff]

Hi Jeff
Please see attached power supply that I designed and used on a Bass Preamp (1RU) back in 2003.
I decided on a DC Heater supply (+12.5V) and the Transformers were 240V:12V AC (Australian Standard)
Tube selection included 2 off 12AX7.
The Preamp is still going
I am also including a basic Voltage doubler (because you have 120V AC). This would replace the bridge rectifier BR1_PS.
Regards
Mirek

Perfect
Thanks

[jeff]

Also, is the doubler on the 120V on the 3rd schm. Reversed. Input/output? Just 'double' checking.

Well spotted, yes it's all bass ackwards. I don't know where Franco is getting his schematics, but they can't be trusted.

K, just checkin'
I think it was modded from the 240V schem, cut n paste just got it rotated 90° in haste. Simple mistake, although, it should be fixed for the next guy that sees it but doesn't keep reading past the schem.

[kagliostro]

Yes, I apologize, the Upper "bridge" was drawn wrongly (quick copy and paste, too quick)




About the fact the CT of the first Transformer is not to be connected to the other CT I don't know

I'm not able    to find a reason 




Franco

[jeff]

No harm, no foul. Simple cut paste mistake, but got me questioning myself.
Peace.

[kagliostro]

No problem also for me 


Only I would like to know about the problem that incomes connecting the CT of the two 6-0-6 volt windings 


Franco

[Merlin]

Only I would like to know about the problem that incomes connecting the CT of the two 6-0-6 volt windings 

Because if the two 6V windings on the second transformer are not absolutely identical in number of turns, and/or not matched to idential drive voltages, the two windings on the second trasformer will 'fight' each other. If there is even one turn difference you effectively create a shorted turn which will create heat.
By leaving the CT unconnected you completely avoid this problem.

[kagliostro]

Well, this I can understand, a transformer of not perfect quality can introduce problems and disconnecting the CT is easier than to look for a perfect transformer

I think that a transformer that performs what you say will be a very bad quality one, but why to assume the risk ?

Franco

[jeff]

OK, one more question
Say you've got a 120V-12V primary. Can you hook that up to a 120V-6V? Probably a stupid idea, but since a transformer is a turns ratio, by putting 12V on the 2nd Xfrm's 6V taps you would get 240V out.


Bad Idea to exceed the output voltage's rating?
In other words, if you have a 120V-6V Xfmr and
put 6V in you get 120V out
put 3V in you get 60V out
Put 12V in... You'd get 240V out.... But.... You've exceeded the rating it was designed for.


Dumb question, I know. Just curious.
I think I remember some tiny Xfrms people we're feeding the 9V side with 12V, or something like that, to get more than 120V out.

[kagliostro]

Bad idea because of core saturation

You can do 6v -> 12v but not 12v -> 6v

---

One thing you can do is

               ->6v/120v
120v/6v                  | 240v
               ->6v/120v

To do that you require a big 120v/6v to be used as first transformer, connect in parallel the 6v winding of the "output" transformers (must be the same kind) and connect in series the 120v winding of the "output" transformer

Better it will be if you can use a 6v/220v transformer as "output" transformers

----


Talking about doublers, you can use 2.6 (or 2.5) instead of 2.8 as multiplier, this will give you a more near to the real obtained voltage


The other thing to remember is that the e-caps must be large enough, a small e-cap will result in a very pour output voltage regulation


If I remember correctly a thumb rule say 1000uf for each A of current

Franco

[jeff]

K,
Sounded like a bad idea, but thought I'd ask...for science.
I guess what I could do is get a U.S. 120V to 12V and a U.K. 240V to 12V.(12V to 240V)
Imma use what I got though, add a voltage doubler.


Thanks

[Merlin]

When you only need a few mA for preamp tubes you can also do stuff like this, which only needs one transformer.

[jeff]

Thank you everybody for the info.
One more question.
Could I use a 12V AC wall wort as the primary. If so, how would I ground the device/chassis?
With two Xfmrs I'd take the three prong cord, hook the white n black to the Xfmr, then ground the green to the chassis.
With a wall wort, I won't have a ground.
I have seen tube pedals that use wall worts, how is the chassis grounded?

[kagliostro]

Doug share the info about a tube pedal he build

give a look there

Franco

[jeff]

OK, thanks

[Merlin]

With a wall wort, I won't have a ground.
I have seen tube pedals that use wall worts, how is the chassis grounded?

The chassis will be connected to the ground or 'sleeve' pin of the audio jack sockets. It has no connection to your house ground through the wall wart.