Bucking Transformer wiring

[shaun]

Hiyo. I have a large PT from a Lowry organ that has a secondary winding that reads 13.5v with 120v going into the primary. The 13.5v once powered a bank of tubes but is no longer needed for a typical guitar amp (it also has 6.3v and a 5v secondaries).

I also have a higher than necessary voltage coming off the secondary, so I want to wire the 13.5v secondary in series with the primary, as it seems this will reduce the voltage of the primary and give me a lower secondary output.

Great. I can follow that concept. However, I must be rather stupid because, having looked at everything I can online, I simply can't figure out how to connect the leads. By counting fingers on both hands, I can tell there would be a bunch of leads involved - two primary leads with 120v going into the trans, and eight specific secondary leads going out (the secondary output leads going to the rectifier plates, the extraneous 13.5v leads, the 5v rec filament, and the 6.3v filament). But the only diagrams I've seen of wiring bucking transformers show four (five?) leads. Like I said, I must be stupid. I'm hoping it will be easy to enlighten me, but who knows... I'm probably reading the diagram wrong.

Here's one of the most informative examples I've been able to find on this subject - Thanks to Blueglow Electronics:

[kagliostro]

https://sound-au.com/articles/buck-xfmr.htm

Franco

[shaun]

Thank you Franco! I've seen this webpage, and the question it raises for someone with my limited knowledge is this: what do the "AC out" leads represent in the diagrams? Do they represent the Leads that would go to the rectifier plates in a guitar amp?

Also, the webpage doesn't seem to address the concept of using a spare secondary winding out of phase with the primary to reduce voltage.

I guess this stuff must seem easy to those who understand it, but while I have built a few amps, I get bizarrely confused over seemingly obvious stuff.

But I certainly appreciate the help.

[kagliostro]

Usually Bucking Transformer are used to lower a new line voltage
(a 120V line used to feed a 110V PT in an old amp or radio, as an example)

this as to preserve the apparatus from too high voltage levels

However you can use the same concept to tame the secondary of a PT that has a bit high voltage

Draw a simple scheme of your transformer (also a pencil one) and it will be easier to follow you and explain what you can do

Franco

[sluckey]

Try this...

[shaun]

Thank you Sluckey! That diagram is exactly what I needed.
My stupidity has a one percent less hold (although I'm not sure what ratio of brain winding that would be).

A final question on this topic for future reference - and perhaps I should have mentioned this sooner - is this: If I add a smaller transformer to buck the main PT, and I connect the secondary of the smaller transformer out of phase with the PT's primary, what do I do with the smaller transformer's primary winding leads - are they simply taped off?

Thanks again. I feel like a Hobbit seeking wisdom from the great Gandolf.

A self-reply: It seems when I look at Rob Robinette's version of a bucking transformer, he has good diagrams and he assembles everything in it's own box. I'm sure this is for safety's sake and is a very wise decision. Therefore, my thought that a bucking transformer could simply be added to the chassis is way off base.

[jjasilli]

I add my bucking T to the amp chassis if it fits; or to a sidewall, with a ground wire to the chassis.

A separate box for the bucking T is good if it's to be used with more than one amp.


EDIT:  I'm suspicious of Elliot's Fig. 4 "Proper way to Wire a Bucking T".  I think it provides no isolation, because it purposely converts the bucking T & the amp PT into one autotransformer.


I like sluckey's solution, because the 13V secondary may not have enough current handling capacity to use in the B+ supply.  By confining it to the primary side, it should have to handle only a small current.

[shaun]

Thanks for the feedback, jjasilli. That makes sense to me too. I guess he's designing a stand-alone version, but I'd like to build it into the chassis.

So, it raises the question of what to do with the primaries of the bucking transformer. If the buckling trans secondaries are connected to the PT primary in an out-of-phase configuration, do I simply tape off the bucking trans primaries?
Thanks.

[sluckey]

If I add a smaller transformer to buck the main PT, and I connect the secondary of the smaller transformer out of phase with the PT's primary, what do I do with the smaller transformer's primary winding leads - are they simply taped off?

The primary MUST be connected across Line and Neutral, after the power switch and fuse.

[kagliostro]

..... the 13V secondary may not have enough current handling capacity to use in the B+ supply.  By confining it to the primary side, it should have to handle only a small current

Ciao JJasilli

I think you mean

..... the 13V secondary may not have enough voltage isolation handling capacity to use in the B+ supply.  By confining it to the primary side, it have to handle only a smaller voltage

Consider that in the primary of a PT there is always more current flow than in the secondary (talking about the HV winding)

and that the 13.5V winding, if I've understand correctly, is planned to supply some tube filaments, and filaments are high in mA consumption than B+, so, the problem can't be there

One difference to be considered (set aside the characteristics of the wire insulation) is that with this action the primary will drop more B+ than acting on the secondary due to the transformation ratio, yes, you'll have a bit less voltage on the 5V and 6.3V windings and if to be high is only B+ it isn't a wanted thing, but it will be not so much

Franco

[sluckey]

Hey guys, I don't know that my drawing will even work. I've never done that. That's why I said try it. Shouldn't cause too much harm. Your outlets have a 20A or 15A breaker, right?   

[jjasilli]

"the 13.5V winding, if I've understand correctly, is planned to supply some tube filaments, and filaments are high in mA consumption than B+, so, the problem can't be there"
Good point.


Hey guys, I don't know that my drawing will even work. I've never done that. That's why I said try it. Shouldn't cause too much harm. Your outlets have a 20A or 15A breaker, right?   
Good grief, I trusted you.


Also, according to Elliot, in his Conclusion, the 13V secondary cannot be used as a typical bucker in conjunction with any other secondary winding. I.e., if a secondary winding is used as a bucker, no other secondary winding may be used for any purpose.  Don't know if this impacts on sluckey's idea.

[kagliostro]

Ciao Steve

I think your drawing will work very fine

what of unexpected that can happen is that the 13.5v can be connected in phase instead of in counterphase and the voltage will increase instead of decrease, but if this happens reversing the wires connection  of the 13.5v winding will solve the problem

Franco

[sluckey]

I'm relying on shaun to confirm.   

[shaun]

Yes, as Kagliostro states, when wired in phase, the output vac increases. When wired out-of-phase, it decreases.

I've been experimenting with an old PT that has two secondary windings, and it seems to work quite well: when one of the secondaries is wired out-of-phase with the PT primary - as per Sluckey's diagram - there is a reduction in vac on all secondary leads. This reduction seems to act as a ratio, so when vac goes into the PT and through the 13v secondary, the output of all secondaries remains a percentage of the input voltage, according to the winding ratios of the 13v secondary to the primary. I believe the math is something like this: 120vac going into the PT minus 13vac = 107vac on the primary. But don't quote me on that.

Kagliostro, I believe you are right again when you suggest the 5v and 6.3 v windings will be effected. I'm not sure by how much yet, but my experiments will continue.

If I understand it correctly, according to Blueglow electronics' youtube video, the vac output of the secondary can be bucked, or decreased, by running the primary through any secondary, even the secondary going to the rectifier plates. Of course, winding ratios will determine any resulting output, so it seems considerable experimentation is required.

In my current conundrum, I'm hoping my 13v secondary will reduce the secondary output to the rectifier plates to an appropriate level - the Goldilocks zone, you might say - and it will probably also reduce the 5v and B+ at the the Rectifier, thus taming the PT's output generally. I'll post my results, although it could be a week or two because I'm still struggling with another amp conversion that has confounded me for weeks.

Thanks again to all.

[shaun]

jjasilli, I sense Elliot's description leaves a variety of interpretations open. I believe he means nothing should be run off the primary of the bucking transformer, but actually, it seems unclear what the exact interpretation could be.

[kagliostro]

Ciao Shaun

This reduction seems to act as a ratio, so when vac goes into the PT and through the 13v secondary, the output of all secondaries remains a percentage of the input voltage, according to the winding ratios of the 13v secondary to the primary.

the reduction is related to the winding ratio of the primary to the secondary

assuming you have a 100V primary and two secondary, one 10V and the other 100V

if you connect to the primary the 10V winding in counterphase with the primary this will lower the 100V secondary and a bit also the 10V secondary

so if you consider 100V (line) minus 10V (really they will be a bit less than 10V) you obtain on the 100V secondary winding something around 90V (a 100V primary PT with a 100V secondary has a transformation ratio of 1:1)

The exact result, as Sluckey say, is to see which will be the result of the experimentation

Franco

p.s.: Don't consider so much what I say, there are lot of guys here that are more skilled than I'm 

[PRR]

...If I understand it correctly, according to Blueglow electronics' youtube video, the vac output of the secondary can be bucked, or decreased, by running the primary through any secondary, even the secondary going to the rectifier plates. ....

NO!!

The bucking winding (in series with the wall wires) must NOT be *also* used for anything connected to the User-Side (such as the B+ rectifier)!!!

That would be very dangerous!!

[vampwizzard]

...If I understand it correctly, according to Blueglow electronics' youtube video, the vac output of the secondary can be bucked, or decreased, by running the primary through any secondary, even the secondary going to the rectifier plates. ....

NO!!

The bucking winding (in series with the wall wires) must NOT be *also* used for anything connected to the User-Side (such as the B+ rectifier)!!!

That would be very dangerous!!

100% concur. Unless you're a harmonic drive controller manufacturer who's intent on blowing up their small section of Milwaukee, don't do that. In the prior life had a customer do this.. attempted to recycle medium voltage in a feedback loop.. they couldn't figure out why it literally blew up my harmonic filter protection.

That being said. If you put that same secondary voltage under load you will flicker and oscillate your incoming power supply. We should all agree thats a nono for a tube amp.

[PRR]

> it literally blew up my harmonic filter protection.

Not the issue here.

Connecting user-side circuits to wall side circuit is DEADLY. The user can end up "figuring out" why he's dead. At best, why the amp gives vicious shocks.

[vampwizzard]

none of its a problem until you mix the lines without bypass. in my case this shunted through protection devices to catastrophic failure, in the amp scenario the person grounding themselves would suffer catastrophic failure. Certainly no laughing matter.. very dangerous, its a really big mistake and not obvious to many people. people forget they arent isolated anymore. 

[PRR]

This is not a drive controller.

Musicians should not have to "remember" they are not isolated.

Hot-chassis amps, and failed "death caps", zapped a lot of musicians before safety rules and practices were upgraded.

Windings connected to live wires from the wall (remember either or both can be live!) must NOT be also connected to the amplifier circuits.

[vampwizzard]

100% agree on all points.

[shaun]

Yes, that makes sense. Thank you PRR!

[jjasilli]

Is sluckey's wiring safe? There is primary AC on the secondary side, though no "direct" connection to the B+ & heater secondary windings.

[vampwizzard]

Yes. The only connection on that particular secondary winding is back to the primary. No other connections should be made to that particular secondary.

[PRR]

Is sluckey's wiring safe? There is primary AC on the secondary side

A transformer does not have "sides". It has multiple *windings*. Yes, they are usually in two groups. And there may be some added insulation between the "wall power" winding(s) and the "inside power" winding(s). Using the 12V heater as a line-bucker is not what was intended. On a church-organ PT I would not worry about it, they were generously over-built. (If the preacher is teaching hell-fire, and smoke comes out of the organ, the congregation panics.)

Have we considered what the bucker will do for the 6.3V winding?

[shooter]

the congregation panics .)

what's the country song about kids letting a squirrel loose 

[sluckey]

This...

[jjasilli]

Have we considered what the bucker will do for the 6.3V winding?

Yes, by experience that 12V bucking trannies are ok; 13.5V not much different.  But anyway:

120/6.3 = 106.5/x; 120x = 5.59125; x= 5.59125V. Heaters should have at least 5V; 5.2V leaves room for error.  All is OK.  If this old PT was expecting 117VAC or lower, then secondary voltages will be higher with today's 120V+ wall supply.

[tubeswell]

I'm late to post - here's bucking boosting schematics from Merlin's Designing Power Supplies' book

[shaun]

Thanks, tubeswell! This diagram seems the most sensible and easy to understand, especially as they use the secondary windings to buck or boost other secondary windings. So the primary never goes over to the secondary side, and that would appear to be the safest approach.

[shooter]

especially as they use the secondary windings to buck or boost

Of an AUX transformer that has phasing indication, otherwise you're doing the 50/50/90% thing guessing 

[shaun]

Thanks to everyone for the good guidance on bucking a PT. The image notes its success.
The dark stripes on the page are invisible to the naked eye, but they're caused by the LCD tubes over my workbench.

[sluckey]

How much voltage do you measure across the 6.3 winding?

[shaun]

Drats! I'd like to say the heater voltages were perfect, but no; the Rec is only 4.8vac, and the 6V6s only 6.0.

I guess the lesson here is to match the amp to the transformer, not the other way around, as Sluckey suggested. I've spend way too much time trying to fit square pegs into round holes lately, probably because I'm a cheapskate. Ultimately, it's probably much better to spend the $$ and work with the appropriate parts than to try and cobble unusual stuff together. But maybe that's quite a good lesson to learn in my first year of building. And in the process of all that screwing around, I've learned a lot about why things DON'T go together. Primarily, it's because this seems to happen: 

Thanks for all the input on this, good people of EL34 world. Much appreciated.

[kagliostro]

4.8V instead of 5V and 6V instead of 6.3V ?

Good, skillful enlisted

You can go on with your project    your voltages are satisfying

Franco

[shaun]

Really? Well okay then, I'll give it a try. Thanks kagliostro. I guess I should learn what the limits are for those voltages.

[kagliostro]

I forgot .....

Those voltages are very good if you measured it with a load

if you measured it unloaded, you must repeat the measure under load

Not all tubes but many has indicated on the datasheet the heater voltage tollerance

Franco

[jjasilli]

4.8V instead of 5V and 6V instead of 6.3V ?

Good, skillful enlisted

You can go on with your project    your voltages are satisfying

Franco

EDIT:  Despite my prior calculations that reduced heater voltage should be OK, the real world says otherwise -- assuming your meter is accurate.  Merlin says 5V is the minimum with 5.2V a safe practical minimum for 6.3V heaters.  Maybe 4.8 is OK, but you're below spec.  Another solution is a small supplemental filament tranny for either the preamp or the power tubes.  The reduced current draw on the main PT may get its filament voltage up to a better level.

[kagliostro]

Ciao JJasilli

I think he measure

4.8V referring to the (5V) Tube Rectifier Heater Winding

and

6V referring to the (6.3V) Other Tube Heater Winding

--

BTW: I think that the winding that now measures 13.5V was a 12.6V winding when the transformer was feed with 110V instead of 120V as now, so also the 5V and 6.3V winding will measure in excess if feed with the actual 120V line, this will be the reason also if the voltage is dropped (with the 13.5V connected in series with the primary) the other voltages remain near the necessary voltage

Franco

[PRR]

> Merlin says 5V is the minimum with 5.2V a safe practical minimum for 6.3V heaters.

Certainly true for small tubes, which have ample emission for the small currents we run.

"Power" tubes are more critical. 6V6 sheet says 5.8-6.8V. I would not blink at 5.7V. But 5.2V is significant under-heating and on a Power tube that may matter.

If he's got 6.0V, now and when all heaters and plates are juiced, or at least 5.8V, it's OK.

Also note that 6V6 is one of the early "for car radio" tubes. Old "6V cars" would go up to 7V roaring on the highway and under 6V with the engine off. Engine off they would start to sound crappy in 10 or 20 minutes as the battery drained. And they were not expected to run thousands of hours either way like a home radio might.

[jjasilli]

Whoops, I mis-read Shaun's post: 6V is fine (and close to the calculated prediction).

[shaun]

Wonderful encyclopedic knowledge. Thanks. Those measurements were unloaded, so they'd drop when loaded. I wouldn't know how much until I configured the tubes, but possibly too low. I've also realized there a hum in the PT, so I have to either fix that or decide if I want a hummer amp.

So much to learn. Thanks.

[Bash]

Sorry for reviving an old thread.

I've been reading up on bucking transformers, but there is one thing that I could not find.

Usually the examples that are given just reduce the input a little (e.g. 120 V -> 110 V).
Would it be safe to use it to cut the voltage into half?

I've got a Yamaha JA1701a rotating speaker, salvaged from an old organ (these are the same units as used in David Gilmour's RA-200 amp). Based on the schematic I've figured out what all the wires do, including changing the speed with a simple pot.
However, the unit requires a 100 Volts.

Currently I am using it with a small Hammond transformer that transforms my mains Voltage from 230 to either 132 or 66 Volts.
The 130 V is clearly too much (I get all kinds of nasty crackling noises when I try that for a few seconds).
The units works on 66 Volts, but it is clearly underpowered. Sometimes it won't start spinning, unless I push it first, and the the acceleration is rather slow.

Would it be save to use this transformer as a bucking transformer and reduce the 230 Volts with 132 to end up with 98 Volts?

I can't see why not, but better be safe than sorry.



BTW, this is the transformer I'm talking about.
https://www.hammfg.com/files/parts/pdf/186E120.pdf

[sluckey]

Should work. However, the secondary is only rated for 1/2 amp. What is the current requirement for the unit you would like to power?

[Bash]

Could not find information on that, but when it is spinning the current draw fluctuates between 30 and 65 mA. So it should be safe.

[sluckey]

Could not find information on that, but when it is spinning the current draw fluctuates between 30 and 65 mA. So it should be safe.

65mA does not sound reasonable for a unit with a 2 speed motor in it. Don't you have something that can measure AC current?

[Bash]

I did.
I used my multimeter for this, set to AC current.

That was the read-out I got.

[sluckey]

How did you have the meter connected?