DIY Output Transformer?

[Stuggi]

I've read online that designing your own output trannies is next to impossible due to the advanced technowizardry that goes on in them.

However, now I found a source that states that it's as simple as just figuring out the impedance ratio y, and then voltage ratio x = y^2. Does this mean that I could wind my own super-duper-mega-deluxe output trannie for my pseudo-champ with 4-8-16 ohm taps instead of just the standard 3.2 ohm tap (refering to the 1750C Hammond replacement transformer with a 7000 Ohm primary) or would I just shoot myself in the foot? And have I understood this part of the design aspect correctly, that possible speaker impedances are just an aspect of what windings your output transformer has?

[Stuggi]

I also noticed that the negative feedback also needs to go somewhere. Do one just keep it hooked up to the 4 ohm tap, or does it need to be hooked up to whatever tap the speaker is connected to?

[celeste]

Not impossible, but a major challenge. For some help, try downloading OPT assistant at http://www.dissident-audio.com/OPT_da/Page.html .

Impedance ratios are easy, frequency response it a different matter what with all the parasitics to control.

To me the really hard part is getting core lam's in fewer then 1000 lots.

[Stuggi]

Maybe then it's easier just to find something premade... But as for frequency response etc, how can it be that some people use power transformers as output transformers? Do they just try and hope for the best or what? :D

[<jbc>]

http://shop.griederbauteile.ch/index.php?cPath=600_639

Grieder Bauteil stocks E-I lams and spools for scratch building trafos.

[celeste]

Originally Posted by Stuggi

Maybe then it's easier just to find something premade... But as for frequency response etc, how can it be that some people use power transformers as output transformers? Do they just try and hope for the best or what? :D

Toroid PT make ok wide band OT's. For a guitar amp, wideband is not really needed, so in some situations an EI core can work ok. They are designed to work at 50 or 60hz, so the low end is not a problem, but they tend to run the cores pretty high on the BH curve so you might have to over spec the VA/watt rating to reduce core distortions. The high end is where things get questionable. The lack of interleaving in the windings of a PT means that capacitive coupling is very low and the high end falls off early. Because we are not really interested in anything above about 5khz, and we often restrict the high end in the curcuit, we can make the PT work as a OT by increasing the high end gain to overcome the poor response of the high end, often this can be done by just turning the treble control up.

So they can work, but are a compromise.

[milracing]

I was wondering about automotive ignition coils. Two sets of windings and some metal. I have no idea if they would be useful for anything like this but they are a couple coils of wire w/ some metal. Modern ones turn ~13v DC into ~80,000 volts...... DC though so I dont know. Easy to find and mass produced though....

[celeste]

Originally Posted by milracing

I was wondering about automotive ignition coils. Two sets of windings and some metal. I have no idea if they would be useful for anything like this but they are a couple coils of wire w/ some metal. Modern ones turn ~13v DC into ~80,000 volts...... DC though so I dont know. Easy to find and mass produced though....

They form an autoformer, so have no isolation between the two sets of winding, so have no DC blocking effect. In theory, you can use them as SE ot's, but you have full B+ on the speaker VC, not the safest thing to do. They could be used in parafeed topology, but kind of pointless to go to the trouble of parafeed and then use something that cheap.

[JohnnyCrash]

I've used the custom wound Heyboers Turretboards.com has for Epiphone Valve Juniors ($54) for a few 5 watt builds.