Custom OT Impedance Question

[R.G.]

I don't mean to draw this out, but I haven't once been able to access "dyna-lead.com", nor to read any statement made by the OP regarding "designed from the ground up", I have only seen such a statement in the Dynamic Amps Co. blurb, who make a Dynalead amp (no hyphon) in Arizona (OP is from MI). I can see what irked you, but it seems it was further compounded by mistaken identity.

'S'OK. I find that I probably have a number of my own hats to eat. Apparently I didn't find "dyna-lead" and rolled instantly to "dynalead", which popped up the Dynamic Amps Co (MI) page, and was off and running.

So: to dynaleadamps, I apologize. I confused you with the Dynamic Amps Co. on the basis of your names, and my prattling about "engineered from the ground up" was entirely unjustified.

I do think you and the Dynamic Amps Co. have some phone calls to make to each other, though.

Again, sorry for my ill-based reply.

[Enzo]

Well I'm no engineer, but I do like railroad trains.

What threw me off here was ordering a transformer of X impedance and then wondering what would happen when we change the circuit around it. Transformers have no impedances, just ratios. The specified load on the output presents the specified primary impedance to the tubes. The transformer has no idea in the world how many tubes are there. It is up to the designer to connect an aggregate tube impedance of proper amount to the OT primary.

[Steve Conner]

Transformers have no impedances, just ratios. The specified load on the output presents the specified primary impedance to the tubes.

Well, they don't, but they do. An ideal transformer doesn't care, but the non-idealities of a real transformer mean that it can only accept a range of impedances.

In the case of an audio transformer: Magnetizing inductance limits the low end. The further upwards you stray from the design impedance, the more bass you lose, and the more distorted the bass gets.

Leakage inductance limits the high end, and the further down you go from the design impedance, the more treble you lose.

DCR of the windings limits the efficiency, and again, that gets worse the further down you go.

In short, the transformer will only meet the manufacturer's spec for bandwidth, distortion, insertion loss and power handling when it's used at the impedance it was designed for. If the spec was overkill for your application to start with, that may not be a problem.

I remember you using the analogy of a car gearbox. To a first approximation, they only have ratios, but they also have limits. One designed for a 2-stroke racing motorcycle probably wouldn't appreciate the torque of a truck engine.

[Enzo]

Yes, everything within its limits. But I guess the point I had in mnd was that just calling it some impedance doesn't make it so. Stick a 32 ohm speaker on there from an SVT8x10 cab and the primary won't be right. Or as in the above case, double up the power tubes on the primary and now the secondary won't match up.

[MWJB]

I think we're going round in circles now, the OP stated at the outset that anticipated secondary loads would be 4/8/16ohms. Knowing this, and his intention regarding primary impedances, the OT in question did effectively "have a primary Z". All he asked was what would be the effect of doubling up power tubes on resulting impedances.

He seems to have accidentally invented a perpetual motion machine...and before you say it, the irony of me perpetuating that motion isn't lost on me ;-)

[R.G.]

He seems to have accidentally invented a perpetual motion machine...and before you say it, the irony of me perpetuating that motion isn't lost on me ;-)

I think it's likely that all perpetual motion machines have irony cores.

[Enzo]

Most transformers have irony cores.


E, I, E, I....Oh.

[J M Fahey]

Well, I still remember this quite fresh post http://music-electronics-forum.com/t22131/ which , coupled to this SE transformer one, make me have some doubts.
There's an old saying "don't throw good money over bad one" or something like that.
Why get a couple expensive KT120 "just not to waste this transformer"?
I'd *love* to see it, would you be so kind as to post a picture?
Preferably with something by it to get the scale, say a pack of cigarettes or a guitar.
290 mA ! 40W class A !! Wow !! It's easy to get copper for that, but ... the iron?.
Won't be so indiscrete as to ask how much it cost, but (hint , hint), if you happen to say so, I won't cover my ears either.

[dynaleadamps]

No worries. After cooling my jets I realized that I would probably feel the exact same way if I was in your position.

I don't purport myself to be an expert, and maybe that is one of the problems that arise from being able to buy and build a simple amp kit and then become addicted to building. I definitlely put the horse before the cart in some ways.

I have spent the last year and a half reading on theory and design, and prototyping and tweaking. I service and repair elevators, so I do have a working knowledge of AC and DC circuitry and am aware of how easily someone can get killed if just one wire is misplaced, or one solder joint isn't properly flowed.

I simply found a deep passion in building tube guitar amps and after completing 3 designs (that are of course based on any number of other designs) I feel that the amps I sell are somewhat unique in tone, hence the slogan.

So all in all, no apologies are necessary, because when it comes down to it RG is right. I should have much more knowledge and experience in order to safely build and sell these devices. The good news is that I have't sold a one! I just get to keep experimenting with 'gothic ring' layouts and turret layouts!

[dynaleadamps]

Oh, and I am changing the name. The confusion is totally unfair to Dynamic Amplifiers, who looks like they make interesting amps. That's what I get for not doing a thorough name search.

Thanks again for all your help, Gents ...

[loudthud]

Kevin O'Connor's book, Principles of Power covers design equasions for SE and PP amps and gives many examples including a couple of 25W SE amps.

[imaradiostar]

Loudthud, are they push-pull watts or SE watts?

In all seriousness- with that kind of iron it might be fun to get some KT120's or maybe gu50's or whatever they're called on Ebay and go to town.

I wish I could afford to buy all of Kevin's books. He seems to be quite prolific.

jamie

[loudthud]

Kevin's books are some of the best money I ever spent. I orderd them on a Thursday night, paid for expedited shipping, and they arrived in Texas on Monday afternoon. The Principles of Power book is only $44 and it focuses on power amps and (mostly) Hammond Transformers. Most of the theory is covered in his other books but it contains design examples that you won't find elsewhere.

Since the book was written, Hammond changed the specs on their power transformers and has several new units in the 300 series available that members of this forum should find useful.

[deci belle]

just wanted to subscribe to this thread cuz it's getting over my head real fast!

[J M Fahey]

I had almost forgotten this thread.
Ok, cross "almost"
As of one question whose answer got lost in the hurricane:

All he asked was what would be the effect of doubling up power tubes on resulting impedances.

the *very* short answer is : *none*.
The 4/8/16 to 2K5/4K/6K ratio remains exactly the same, no matter which or how many tubes you connect at the primary side.
Simple as that.