OT with unknown secondary impedance

[Kagliostro]

A guy in an italian forum has an amp without speaker

the amp is unknown and maybe an old DIY Amp

which can be the way to know which is the right impedance of the speaker to be connected ?

I've a bit of an idea, about measure the winding ratio and then, assuming that the output impedance usually are 3.2 - 4 - 8 - 16 ohm, knowing the kind of power tubes and B+ go by approximation calculating which output impedance will match (using the winding ratio) to the primary impedance expected for those tubes

but I've never do something like this and would like to have your opinions/councils

Many Thanks

Kagliostro

[roberto]

Hi kagliostro, in which forum?

Put a 6.3Vac on each secondary tap, then check primary voltage.
Windings ratio will be Tp:Ts=Vp:Vs

Tp = turns (windings) on primary
Ts = turns (windings) on secondary
Vp = voltage ac on primary
Vs = voltage ac on secondary

Now you can calculate the impedance ratio between primary and secondary this way:

Zp:Zs=Tp^2:Ts^2


EG:
suppose to inject 6.3V on a secondary tap, and obtain 63V on primary.
Tp/Ts = 10
Zp/Zs = (Tp/Ts)^2 = 100

So a 16 ohm output is reflected into a 1600 Ohm primary impedance.

[rdjones]

Is no speaker present, and only a single secondary (no taps) ?

If the amp is in operating condition, you could send a test signal through the output to various load impedances and observe the level at which clipping occurs on a scope.
Lacking a complete analysis rig you could just measure the turns ratio in the usual way as outlined.

It helps to know the "style" the amp is designed after.
For example, early Marshall circuits have quite a bit higher primary impedance than later designs.
That said, for a particular tube type the range should still be less than 2:1 for the normally encountered B+ voltages.

rd

[Kagliostro]

@ Roberto

Ciao, sei tu Robi ? il link al tread è questo:

http://www.diyitalia.eu/forum/viewtopic ... 5&start=30

Bill89 aveva l'altoparlante ma l'ha gettato perchè ......... puzzava

il consiglio che ho dato io, come puoi vedere nel link, è proprio lo stesso tuo, solo che non conoscendo altri metodi, ed essendo un po' macchinoso da mettere in pratica, cercavo, appunto, altri suggerimenti

@ Rdjones

to use a signal generator and an oscope ........ I don't think the guy that has the problem has this equipment, but this seem to be a very good council, when I've finish to renovate my laboratory I must give a try - Many THANKS

Kagliostro

[roberto]

Hi Kagliostro,
yes, I'm the same Roberto you know.
I've lost that thread on the italian forum,
but I've seen you've done a great job!