Output transformer push pull

[om kalsoum]

Hello everybody

i was just wondering, i know that by single ended amplifiers the output transformer is of an kind that can handle dc currents
without interfearing with the ac signal.
These air gap transformers don't get saturated in there core when dc is passing trough.

now for the transformer in push pull schematic the dc current is flowing in both directions in the primairy coils, don't the magnetic forces in the core cancel each other out.
and if so it is theoreticly possible to use a 2000 ohm centre taped 100v speaker transformer that was obviously not designed for dc currents.

now how i came to this crazy idee
i am building an power amplifier with 2 el86 in srpp
these put out a nice 5 watts and in the circuit is an output capacitor
so i am dc decopled from the amplifier so these transformers sound pretty nice

does anybody know something about the dc currents in primairy coils of transformers but keep in mind if current is passing trough 100v speaker transformers will the full output power be achieved

thank you verry much

[ray_moth]

I don't understand the relevance of 100 v when talking about OP transformers. However, what you say about the cancellation of magnetic fields in the primary is true, provided that the currents are equal, which they will be in Class A. As soon as you enter Class AB, the currents become asymmetrical and the core needs to be a bit more resistant to DC saturation. Some PP transformer makers, such as Lundahl, allow for this by providing an air-gao even in their PP designs.

[ilimzn]

Originally posted by om kalsoum
[B]...the transformer in push pull schematic the dc current is flowing in both directions in the primairy coils, don't the magnetic forces in the core cancel each other out.

Yes they do, if they are indeed equal, which will happen if the tubes are driven and biassed properly and (highly recomended) paired.

and if so it is theoreticly possible to use a 2000 ohm centre taped 100v speaker transformer that was obviously not designed for dc currents.

Yes, it is possible (if it is indeed center tapped), under the above conditions. It will, of course, be a 2k p-p impedance. The thing you have to be careful about is the maximum voltage applied, it is nto called a 100V transformer for nothing!

i am building an power amplifier with 2 el86 in srpp
these put out a nice 5 watts and in the circuit is an output capacitor so i am dc decopled from the amplifier so these transformers sound pretty nice

Of course, if you are uding SRPP, you still need an output capacitor. You could use the center tapped transformer as a PP output transformer in one of the standard push-pull topologies. There is a derivative of the SRPP which would do this also, but you would need a transformer with two separate windings, not center tapped (have a look at www.tubecad.com in one of the many SRPP discussions).

does anybody know something about the dc currents in primairy coils of transformers but keep in mind if current is passing trough 100v speaker transformers will the full output power be achieved

Assuming you don't overheat the transformer with excessive DC current, as long as the DC flux induced by the two oposing halves of the coil cancels out, you get whatever power rating the trafo is made for (keeping in mind that this is limited by how low/high in the frequency range you want to go, too).

Originally posted by ray_moth
I don't understand the relevance of 100 v when talking about OP transformers.

The '100V' is a standard designation for '100V line' audio transformers intended for PA installations.

However, what you say about the cancellation of magnetic fields in the primary is true, provided that the currents are equal, which they will be in Class A. As soon as you enter Class AB, the currents become asymmetrical and the core needs to be a bit more resistant to DC saturation. Some PP transformer makers, such as Lundahl, allow for this by providing an air-gao even in their PP designs.

Hm, wasn't this actually dependant only on how well the PP action is balanced? As long as the net DC current through a full cycle is zero, how does it matter if the amp works in class A or AB, current 'shape' notwithstanding as long as it is symetrical?

[om kalsoum]

Thank you for your reply well i even heard that normal ringcore transformers could be used instead of a very expansive pp transformer from lundahl of cource the sound quality is not to be compared but the option is there.

[Geoff H]

A good PP transformer will have a bifilar winding, that is 2 windings physically paralleled and connected in series. (some are quadfilar) That allows both halves of the primary to have the same inductance and resistance. 100v line transformers have one winding, with several taps, and may not have a true CT.

They can be used, however at the cost of distortion. There will be some DC component left unbalanced due to the difference in DC resistance. (the inner half may only have 1/2 the DCR of the outer 1/2)

Something else to consider. The transformer will have to dissipate power as a result of the DC component, thus it will operate at a higher temp than designed for. The power rating on a transformer is a function of the cross section area of the core. Anything over that will saturate the core. Thus a 30w transformer may be thermally rated at 3w RMS.

If you need to economise, have a look at guitar amp replacements. I think they'll do a better job than line transformers.

Regards,
Geoff.

[ilimzn]

Originally posted by om kalsoum
Thank you for your reply well i even heard that normal ringcore transformers could be used instead of a very expansive pp transformer from lundahl of cource the sound quality is not to be compared but the option is there.

Toroidal (ring core) transformers can actually give you surprisingly good results provided they are specified and used properly:
1) they will have a MUCH smaller power raing for audio frequencies (particulairly the low end) than the mains use rating, often 10 times lower. One of the reasons is that you will normally be using dual 115V mains primaries as the PP primary, which will usually see a much higher voltage - this means you have a 'shortage of turns'. Custom winding the transformer for the proper voltage and a low frequency (most winders can calculate all they need based on that) will give you much better results.
2) Take care of the voltage rating, the standard wire enamel used can withstand about 600V between adjecent windings. This is important since you will get best results usung mains transformers with a dual bifilary would 115V mains primary. It is highly recomended you never go over half of that rating, which will generally be satisfied with plate power supplies up to some 350V (perhaps more on autobias designs).
3) Toroids are FAR less imune to core saturation due to unbalanced DC flux in the core, in fact out of all transformer types they are the worst in this respect. Your amp circuit has to be designed with this in mind.

[Shoog]

You will get better performance from your SRPP by changing the top EL86 into a true pentode CCS. Most of the SRPP circuits use the same value resistor for the bottom cathode and the top cathode. This doesn't take the load into account. Of course this can be optmised for (if you know how) but it is simply better to turn the top into a CCS.

Toroidals would work well as your cap coupled SRPP transformer or your PP output transformer. I have done both. The EL86 works well at low voltages - so will be a good match to your toroids.

Shoog

[om kalsoum]

i think i am going to stick with my current design based on your answers
the schematic is in a attachment

the amp works great in this configuration

sorry for the schematics graphics i draw it in The Gimp a program like paint but better

thanks