Now I know what this Tranny can do

[Tarzan]

Hi,
A Happy New Year to you all and a lot of projects for 2009.

Many power supply tansformers are available and all can be used for what they are intended to do.
But, and here it comes...
It must be clear that this is done on trafo's where no technical data is available.
If any body can give figures for trafo's used in excisting amps, I will be glad and happy to receive it.
But I looked around and could find none.

How can a trafo (wich ever one) where no data is available like VA power, max current for each secundary etc be tested?
We do not want to know the primary side. That's assumed to be known.

How much can there be squeezed out of a transformer.
Take for instance a simple one:
The trafo for a Fender Champ, labeled 125A35A.
Assume we apply a load (made by adjustable resistors) to each secundary and apply a primary voltage.
We now adjust the loads until the secundary voltage drops by 10%.
We now the voltage and current, hence the power, the trafo can deliver.
Can we assume that this is a real situation and use these found values as a basis to design an amp around this trafo?
Is 10% good or do we even have to go further, 15 or 20%?

That's it...

Thanks,
Tarzan

[Tarzan]

Funny, there is no reaction?
103 vieuws in a few days!!!

It seems that finding out the properties of a psu trafo is not important.

Knowing what material is used in a resistor and measuring the sonic properties of them (and not to forget caps) is VERY important.
Tweeking preamp stages to the extremes and almost dying for a NOS ECC83 from Telefunken is important.
Calculating the possible riple with choke x or y is important.
Finding a good output transformer and doing calculations for it is important.

But knowing if the power supply trafo is ok; No no.
I have the feeling that all possible tweeking and calculations starts AFTER the PSU trafo.

So we have to pick a trafo from a vendor and assuming it's OK.
Even if it's delivering to much or even barely enough;

"we do not calculate these thingies; we concentrate on resistors, silicon or no silicon cabling. Use of pcb or hand wired. Orange Cap, Blue Caps or Keystone Caps...)"

Well, what do I have to think.
Don't know.
I continue to follow my path and do some measurements on trafos.

Tarzan

[Tom Bavis]

Use a transformer within its ratings - no worries. PSU Designer program will give you a close estimate of DC output.

If you have a transformer and don't know ratings, there are various ways to estimate maximum output. Voltage drop and power loss can be calculated from winding resistance. And naturally you'll use it well below that to assure that it runs cool...

If the transformer output voltage is dropping 10% under load, it is also dissipating 10% of the power that it's delivering. Can a 50VA transformer dissipate 5W? Probably. Can a 500VA transformer dissipate 50W? NO WAY!

[Tarzan]

Oeps, The tranny number had to be 125P1B; The 125A35A is the Output Tranny...

Not that is makes a difference to the "vast number of reactions"

Thanks Tom for your reply.
Indeed an ESTIMATE can be made by the said calculations.
But that dosn't garantee that these voltages and/or curents are available at full load.
And that's the whole intend of my question.
And voltages do sag with 10% or more. Even in an Ampeg bass amp with 6 x KT88.

I have to write again that we like to start with a clear knowledge of the trafo we like to use or have.
Using a trafo without details of the voltages and currents it can deliver is not the question.

To clarify:
Trafo's are available in three ways.
1. New and with technical data.
2. Used with known data.
3. Used but no technical data except that is used in a Fender, Marshall or whatever.