Understanding schematics: transformers

[Octavian]

Hi all,

So I've build a pile of pedals and a simple little 1/2 watt tube amp (w/o transformer), and I've tinkered with some amps I own, but I'm getting to the point where I'm comfortable with the building process and safety precautions to try to build some more complex amps and effects. One thing that continues to stump me is how to understand transformers in schematics. Could someone please give me the quick idiot's guide to reading transformers in schematics?

TIA!

[Will.mendil]

was about to write a long text from a site I tried to remember, but I found this site which gives all the information on how to build transfomers. I don't expect you will build your own, but, you'll understand how to use them, read them etc...

behindthetone.com/johnfisher/ot1/ot.htm

[Metaldestroyer]

For power transformers, the positive of both the primary and secondary will usually be marked with a dot. A lead from the center is a tap, and if its in the center and unmarked it's usually a center tap--where B+ is applied in output transformers or a 0v reference in power transformers. For information regarding the wiring of power transformers, this is the best article I read when I was learning.

http://mnats.net/1176_reva-d_hairball_wiring_power.html

[GibsonGM]

^  Nice, Metal, except the dot indicates common phase relationships, not 'positive'....remember, a transformer is AC on both sides, in and out....so the concept of + and - doesn't really exist!   They are always changing. 

But, ends of a primary and secondary can have the same PHASE, meaning they are 'tracking' each other.  I.E., when one is the hot one, so is the other (marked) coil. 

Lots of good articles out there, as well as tutorials.  What I did when new, was to get a lower-voltage unit....12.6VAC or something...and have a go at making a small power supply. 

Then you learn that after the trafo comes the rectifier (what type? 1/2 wave? Full wave? Bridge?) and regulator, or just the filter network...don't forget the lousy equations whereby you can find out what your voltage will be after rectification!     Those equations are probably the most important part of choosing and using a transformer.

When building a tube circuit, it may be wise to copy some designs and use the designated transformer at first, since you know they sourced it properly (did the work for you so to speak).

[tubegeek]

Note that tube amps commonly use transformers with multiple secondaries - each will be isolated from the others and will transform the incoming wall voltage to a different output voltage. Typically you will have a 5V winding for a rectifier tube, a 6.3V winding for the audio tubes' heaters, and a high voltage winding, maybe 750 to 800 V, for the main supply.

The high voltage winding is often "center tapped" and shown as "375-0-375" - the same winding might be described as "750 V CT".

The reason for this is that it is much simpler to make a two-diode rectifier than a four-diode rectifier if you are using tube rectifiers, and that scheme works with a center-tapped secondary instead of a two-wire secondary (no center tap.)

In a schematic, you will have more curlicues for multiple secondaries and they will USUALLY be arranged near the common core symbol, two straight lines.

See any classic Fender amp schematic for examples.

[tca]

Here is some more reading by R.G.: http://www.geofex.com/article_folders/xformer_des/xformer.htm

Cheers.

[plexi12000]

just post a schematic......i wired tons of them

[amptramp]

One thing to add is that some schematics show a dot at each winding.  When current enters the dotted end of the primary, it leaves the dotted end of the secondary.  This shows up more often in switchmode power supplies as signal transformers have other means like pin numbers or wire colours to tell which way to connect the windings.

[Octavian]

just post a schematic......i wired tons of them

My biggest question is really how to know for sure which wires from the transformer correspond to which wires in the schematic. For the moment at least I'll just be following other people's drawings.

Something like this, I guess (via sophtamps):

[PRR]

> know for sure which wires from the transformer correspond to which wires in the schematic.

Find clues.

This plan says "600 Ohm to 8 Ohm". I do not know where to buy such a part. However it "should" come with a lead-out sheet, saying which side (2 wires) is 600 and which side (2 wires) is 8.

In simple large-ratio cases, you can also try your Ohm Meter. The audio impedance is NOT the DC resistance. Audio impedance is generally about 10 times higher than DC winding resistance. The "10" is very variable, 5 to 20 maybe. However look for one side around 60 Ohms and another side about 1 Ohm. Maybe it is 45 Ohms or 90 Ohms, but that's surely your "600". Since most Ohm Meters don't do 1 Ohm well, just look for anything way-low (almost like you just touched the meter-probes together).

When you get into real-Voltage amplifiers the primary may be "5,000 Ohms". Same thing: one side will be hundreds of Ohms DC, the other (speaker) side will be near 1 Ohm.

[tubegeek]

PRR covered the output transformer.

For the power transformer, you should have for that transformer a set of two leads that measure some small resistance to each other. You should have a set of three that you can measure three pairs on as well. There should be no measurement between any of the leads across the two sets!

Set of two: call them A and B.
Set of two: this is your mains input, should be black and white or black and black.
A to B: some small resistance. Probably slightly larger than the C, D, E windings.

Set of three: call them C, D and E.


IMPORTANT!!  A to C or D or E: open circuit. B to C or D or E : open circuit. IMPORTANT!!

Set of three: you will measure approximately twice the resistance between one pair out of the three than between the other two pairs. The lead that is NOT part of the higher-resistance pair is the center tap. It may be color coded differently from the two others in the set of three.

Example:
C to D, D to E, E to C: small resistances, one test about twice as big as the other two tests. Let's say CD: 1 ohm, DE: 1 ohm, EC: 2 ohms. Then the center tap is D.

You can always put a small measured AC voltage into one winding, say 5 to 20 V, and see what you measure out of other windings. Note that if you put 20V into one of the half-secondaries of the transformer above, you will have something more than 120V on the primary so be careful.

Also, any time you mess with mains wiring at all, be careful.

Also, be careful.

[sajy_ho]

Hey man,I've built the schematic you posted a while back. It sounded way awesome that I wish I never gave it to my friend!
As for the Output transformer I used a cheap 75V line matching transformer. I don't remember exactly which taps I used but I remember I connected it to get 800R at the primary, because the datasheet suggests 800R load for the 12k5.( I suspect Stephane stated 600R primary impedance because that's what he had around at the time!)
I didn't include the power transformer, instead I used a 12V/1.5A for the heaters and used a voltage doubler for the B+. This is  the one I used: http://forum.musikding.de/cpg/albums/userpics/17486/positive-voltage-doubler-lt1054.gif

[Octavian]

Thanks for the feedback everyone.

So, here are some more questions / clarifications:

- For amps and whatnot, is it correct that normally we deal with only power and output transformers? The power transformer changes the voltage and the output transformer changes the resistance going to the speaker?

- Does a transformer hold any charge after being disconnected form power (like a capacitor) - do they need to be drained when the power is turned off?

- I've noticed that some transformers have nice sleek black enclosures, and some don't. Are these enclosures available separately (for aesthetics)? Are they necessary, or is it a safety thing? Is it safe to touch the exterior of a transformer without (or with, I suppose) an enclosure?

And one non-transformer related question:

- R7 is marked as 1M 40%. 40%?!?!? that means R7 could be anywhere from 600k-1.4m? That seems like a huge range!

[davent]

The 40% for R7-  i'm guessing, refers to the slope of the pot, i've seen reference to 10% & 30% audio pots before, linear would be 50%, resistance at mid point of travel, and like i said earlier, i'm guessing.
dave

[PRR]

> For amps and whatnot, is it correct that normally we deal with only power and output transformers?

Usually. (Some DIs use transformers to break ground. There's a few effects which use "interstage" transformers.)

> The power transformer changes the voltage and the output transformer changes the resistance going to the speaker?

They both do both: voltage and current (which is resistance). In "Power" transformers we usually speak about Voltage and Current. For Audio transformers we usually speak about the Impedance (which is just the ratio of voltage to current).

> Does a transformer hold any charge after being disconnected form power (like a capacitor) - do they need to be drained when the power is turned off?

They "kick". The stored flux typically comes-out about as fast as the lowest bass frequency they can handle. Very small fraction of a second. But don't be holding a bare lead *when* the power goes off. The "kick" can be 10 times the DC voltage.

> Are these enclosures available separately (for aesthetics)?

Rarely. Enclosures can range from a little plastic to keep you from poking the paper insulation, to steel end-shells, to total shell, and to totally sealed "cans" filled with insulating oil or exotic chemicals.

> Is it safe to touch the exterior of a transformer without (or with, I suppose) an enclosure?

A happy transformer is touch-safe. Except the connections. I would not worry about 12V work. High-volt transformers usually have leads which run inside the box. However I did recently see a transformer working at 250V but with exposed bare terminals. The builder did insulate the terminals after the picture was taken.

Un-happy transformers are rare but can happen, especially if the chassis has had a bad life, or has been damaged. Transformers are a lot of thin insulation. Always ground the iron core! Don't mess with the paper/plastic wrap if it has been damaged.

> R7 is marked as 1M 40%.

This is almost certainly a "simulation". To compute voltages the simulator needs to know where the pots are set. These are set at 40% ("4" on a 0-10 knob). That's probably some default value though-- there's no sim model for that tube.

There *are* also % numbers on pot tapers. But a simulator would not care (does not affect the electrical action, just the spread across the pot knob range).

[amptramp]

Thanks for the feedback everyone.

So, here are some more questions / clarifications:

- For amps and whatnot, is it correct that normally we deal with only power and output transformers? The power transformer changes the voltage and the output transformer changes the resistance going to the speaker?

Normally that is all there is on a tube amp and there is usually no output transformer on a solid-state amp.  Occasionally an amp may have an isolated input with a small signal transformer but that is rare.  I also have a Universal Radio radio from 1938 that is my daughter's favourite guitar amp and it has simple circuitry - a 6J5 feeding two 6F6G outputs through an audio interstage transformer that provides phase splitting.  For a guitar amp, this is actually a good topology because you do not need the low frequencies but it does prevent the use of feedback beyond the output stage itself.

- Does a transformer hold any charge after being disconnected form power (like a capacitor) - do they need to be drained when the power is turned off?

They will give a voltage spike when current is interrupted but they do not store charge for the long term.

- I've noticed that some transformers have nice sleek black enclosures, and some don't. Are these enclosures available separately (for aesthetics)? Are they necessary, or is it a safety thing? Is it safe to touch the exterior of a transformer without (or with, I suppose) an enclosure?

This enclosure is usually designed to keep the magnetic field completely contained within the enclosure for purists seeking the greatest suppression of effects from stray magnetic fields (like the power transformer coupling into the output transformer).  I have not seen them available separately.  If the transformer core is grounded, it is safe to touch but the terminals are not.

And one non-transformer related question:

- R7 is marked as 1M 40%. 40%?!?!? that means R7 could be anywhere from 600k-1.4m? That seems like a huge range!

Hmm... Paul beat me to it as usual.