getting the specs on a transformer

[pango]

I'm trying to reverse-engineer an old amp. I have the schematic, but it's rudimentary in the sense that some components are labelled without their values.

Most of it I think I can muddle through. But the transformers (two, one labelled isolation and another connected directly to the frame of the speaker, which I assume is the output trans) are not listed with any info on the schematic.

Any suggestions on how I might know which transformers to buy to replace them?

Thanks

 

[Joel]

The output tube type and operating point should point you in the right direction. Also the size of the iron is another clue.

 

[cerrem]

A few quick and dirty methods can be used here...
First note the ohms of the speaker it uses....lets say it's 8 ohms...
With the amp OFF and speaker DISCONNECTED...Get some aligator clips and apply the mains 120V @60Hz across the output transformer primary winding....don't panic, this is perfectly fine...
If you feel safer to use a variac for this and slowly raise the voltage, then do that....
You basically want to get the AC voltage ratio from Primary to Secondary....Once you get that you SQUARE it and now you have the impedance ratio.....Take that and multiply by your speaker impedance ...and there you will have your plate load....

Chris

 

[blacklight]

It may be a electrodynamic speaker. In that case the transformer fixed to the speaker is there to provide a magnetic field instead of a permanent magnet. This is also called a field coil speaker. They're otfen found inside old radios. It may even serve another purpose as a choke for the power supply.

 

[pango]

Thanks for the advice guys. I can try to respond a bit to your suggestions/comments.

1. I no longer have the old speaker. It was pretty damaged, and not worth re-coning, so I disconnected the trans from it and tossed it. I did measure it with a DMM and it was about 8ohms (I know that's not the most reliable way to measure imped, but it's all I could do). I now have the amp running through the trans, to an 8 ohm speaker with no problems. Not saying that proves anything, just explaining the current configuration.

2. regarding the check with 120v, that doesn't sound too hard. Some questions though.

a. would I need to take the trans completely out of the circuit? (i.e., disconnect primary & secondary leads first). I can see where I'd want to disconnect the speaker of course, but what about the primary side?

b. it seems that I should be able to use any voltage and the transformer will either step it up/down by the same ratio, or is that not correct? In particular, why do I have to apply 120v? Couldn't I just apply, say, 9v and see what the output ratio is, and then do the calculations you suggested?


3. the tubes in the amp are 18GD6A, 60FXS, 36AM3

thanks again for the help