OT, the bigger the better? And which one to choose?

[jpfamps]

I have a question, so the signal power don't cause saturation, only the offset DC current cause saturation. Then why is it that the small core transformer saturate at low frequency? Better yet, what is limiting a 15W OT from using in a 50W amp?

Read Steve's post again.

"If the transformer is properly designed with enough iron to pass the lowest note on the guitar at full power without saturating (and I believe this was true for classic OTs) then saturation can only be caused by DC imbalance."

A lower power transformer using a small lamination will also use thinner wire which will have a lower current rating.

[Enzo]

Sorry, apparently I neglected to paste in the link to Peavey:

Peavey.com

[Steve Conner]

"Chapter 3 - Transtube" is the one with the magic beans. I agree with everything in there. The only question it leaves unanswered is how the output transformer of a PP amp would ever get saturated continuously in one direction. The DC imbalance thing is my own answer to that, based on my own research.

I think Peavey know about the DC imbalance, but they left it out either because they thought it was excessive detail that would confuse readers, or because they wanted to keep it secret.

My solution to modelling this is to go back to the transformer-driven solid-state topologies, but with proper current limiting and thermal compensation. The interstage transformer can be made to saturate and compress the signal in just the same way as a tube amp's OT. No mass-market amp maker would do this because of the extra cost of the transformer, so I'm happy to write about it.

[Alan0354]

I actually did a search and downloaded cpt 3 already.

[Alan0354]

Read Steve's post again.

"If the transformer is properly designed with enough iron to pass the lowest note on the guitar at full power without saturating (and I believe this was true for classic OTs) then saturation can only be caused by DC imbalance."

A lower power transformer using a small lamination will also use thinner wire which will have a lower current rating.

That's the whole question. I was told saturation only happened with DC imbalance, large signal will not cause saturation. Then you are talking about small core will saturate at low notes regardless of DC!!! That seems to have a contradiction.

Well regarding to small wires, let's just assume it is not so small that the resistance of the wire get in the way, it's the first question I am stressing.

[Austin]

So in summary, the larger size of the transformer adds inductance and current rating with longer and thicker wire and the smaller transformer may saturate on a low note of enough magnitude because they are happening slow enough to cause it to run out of inductance in between wave peaks. Is that about it?

[Steve Conner]

In a push-pull output stage, signal and imbalance are the same thing. Ponder what is the difference between a really low frequency signal (imbalance) and a DC imbalance. How low does a frequency have to be before it becomes DC?

I already introduced the concept of volt-seconds. The design of the transformer gives it a capacity of so many volt-seconds before saturation. A push-pull output transformer is designed to accommodate one half-cycle of "imbalance" at the lowest operating frequency and highest amplitude. The following half-cycle has the opposite polarity, so it "resets" the core, taking the flux through zero and close to saturation in the other direction.

So, to sum up, you could define a "DC imbalance" as a signal with more volt-seconds than the core can handle. Another way of saying the same thing: a signal with a frequency much lower than the lowest frequency the OT was designed for.

See the transformer EMF equation here: Transformer - Wikipedia, the free encyclopedia

[Alan0354]

I have to really stop and read cpt 5 of RDH4 before I post on this as I admit I don't know transformers.

Something just don't make sense for me. "Low frequency" is relative. In my world, 50MHz is low frequency!!! anything under 20KHz is super low frequency depends on what reference you are talking about. To every frequency, it can be consider low enough to be consider DC.

Bottom line, sounds like AC signal do saturate the core, it is just frequency dependent as my understanding all along. Just a large core allow the frequency to go lower before it saturate. So it is signal dependent. This is the whole concept of switching power supply. You switch at higher frequency which not only allow you to have much few turns, you can use a much smaller core with ferrite material. In fact I used this concept and told my engineer that design the switching supply to start with the highest frequency of switching possible to lower the number of turns and use the smallest possible core. We designed 5KV high power switching supply We increase the reliability by using real 30 gauge teflon insulated wire that is a lot bigger, but each wire can stand off 5KV+.

[pdf64]

The JMI era Woden transformer AC30 is 5 sections: 1/4 primary, 1/2 secondary, 1/2 primary, 1/2 secondary, 1/4 primary. The two outside section make one half of the primary and the middle section make the other half of the primary. This way you can get the two halves of the primary well balance for all parameters only using one wire gauge for the primary, which simplifies the winding procedure...
The Tiny Terror uses a similar 5 section design, as do the Drake transformers used in Marshalls from the late 60s onwards...

Does anyone have any insight as to how the various secondary taps may commonly be accommodated by the above (or similar) sectional winding arrangements?
eg assuming the series combination of the secondary sections was intended for 16 ohms (and one winding end remains the output 'common'), would an 8 ohm tap only use a small section of one section, and a 4 ohm tap not use one of the sections at all?

[Gregg]

If two sections of the primary in series are intended for 16 Ohm then the same sections wired in parallel will be good for 4 Ohm (that is if they have the same number of turns which is not difficult to check externally). If you need an 8 Ohm tap then you should add some more turns to the paralleled 4 Ohm sections assuming you have access to the OT internals, enough space between the coil and the iron and if you know the number of turns of the 16/4 ohm secondaries. The additional turns wire thickness will be easier to estimate knowing the OT rated power.

[Mike Sulzer]

If two sections of the primary in series are intended for 16 Ohm then the same sections wired in parallel will be good for 4 Ohm (that is if they have the same number of turns which is not difficult to check externally). If you need an 8 Ohm tap then you should add some more turns to the paralleled 4 Ohm sections assuming you have access to the OT internals, enough space between the coil and the iron and if you know the number of turns of the 16/4 ohm secondaries. The additional turns wire thickness will be easier to estimate knowing the OT rated power.

I assume we are discussing an output transformer for a tube amplifier; that is, primary impedance in the kilo-ohm range, secondary in the 4 to 16 ohm range. You should not try to change the secondary impedance by changing two primary windings from series to parallel. This changes the primary impedance; that is, it lowers the inductance by a factor of four, and can give you inadequate bass response even on a guitar amp. Might increase plate dissipation on low notes as well.

[Gregg]

I will have to disagree. I'm not talking here about mismatching the secondary like connecting a 16 Ohm speaker to a 4 Ohm secondary or a 4 Ohm to the 16 Ohm one. I was talking about rewiring the 16 Ohm secondary for 4 Ohm operation and connecting a 4 Ohm speaker to it. In this case there's no "loss" of inductance and or change of the primary impedance.

[Mike Sulzer]

I will have to disagree. I'm not talking here about mismatching the secondary like connecting a 16 Ohm speaker to a 4 Ohm secondary or a 4 Ohm to the 16 Ohm one. I was talking about rewiring the 16 Ohm secondary for 4 Ohm operation and connecting a 4 Ohm speaker to it. In this case there's no "loss" of inductance and or change of the primary impedance.

Maybe I misunderstand, but I think you are reducing the number of primary turns by a factor of two. If so, then you are reducing the inductance, and in doing so, you would make a transformer that should be used with four tubes in parallel on each side of the push pull.

[Gregg]

It's not like that. The primary is not affected by this rewiring. We don't touch it at all. It will have the same number of turns at the end of this procedure.
What we're doing is changing the turns ratio to accommodate for a 4 Ohm speaker while keeping the primary impedance unaffected.
For example if you have 1200 turns of primary (all sections) and 60 turns for the 4 Ohm secondary then you need 60*sqrt(2)=85 for the 8 Ohm and 60*2=120 turns for the 16 Ohm secondaries. So instead of using 2 same number of turns sections in series for 16 Ohm we can wire them in parallel (assuming you have access to the connections between those) in order to use a 4 Ohm speaker. And since the 16 Ohm secondary wire usually is thinner than the 4 Ohm one by paralleling them you are increasing the wire diameter accordingly.

[pdf64]

If two sections of the primary in series are intended for 16 Ohm then the same sections wired in parallel will be good for 4 Ohm

I think that 'primary' in the above may have been a typo, and that 'secondary' was the intended word; it all makes sense then?

But with regard to my query, if (as is commonly the case for MI amps) secondary taps are used (rather than the secondary winding connections re-configured), if the secondary is in 2 sections then the 4 ohm tap would seem to make no use of one of the sections?
Which may affect the characteristics of the transformer quite significantly.