ANTIQUE-RADIO-LAB

Antique Radio Forum for Collectors antique-radio-lab.forumotion.com
 
HomeHome  RegisterRegister  FAQFAQ  SearchSearch  Log inLog in  

Share | 
 

 Speaker output specs

View previous topic View next topic Go down 
AuthorMessage
copain



Join date : 2011-12-28

PostSubject: Speaker output specs   2013-11-07, 3:35 pm

What are the specs--primary and secondary DC resistances--for choosing a speaker output transformer that will match a 6F6 to a speaker with a 10-ohm voice coil?
Back to top Go down
Cliff Jones
Site Administrator
Site Administrator
avatar

Join date : 2010-11-22

PostSubject: Re: Speaker output specs   2013-11-08, 12:45 am

Here's a good tutorial on measuring impedance of a transformer.
[You must be registered and logged in to see this link.]
I looked up in my tube manual the load resistance and its around 7K-8Kohm on the primary side.

to measure the DC resistance of an impedance you multiply the impedance by 1.25.


I don't think that the secondary is that critical between 8 ohms and 10 ohms.
It is more important to consider power. If you are replacing a bad audio transformer that used a 10 ohm speaker its not that critical. Use one for an 8 ohm speaker. The speaker itself changes impedance with frequency anyway.

you could use a Variac on the primary and adjust for 10 volts ac and measure the open secondary voltage, then insert a variable resistor on the secondary and adjust the resistance till it reads a voltage that is 1/2 of the open secondary voltage. Then measure that resistance of the resistor and it will be the DC resistance of the secondary. The resistor will not change its resistance in an AC signal whilst a speaker or transformer will change impedance.

The standard method of measuring impedance's in an audio circuit is by using a 1KHZ audio signal. If you have an audio generator then you could use it, rather than a Variac. Just measure the signal generator voltage output to the primary (hooked up of course) and then do the same method (as previous) using a variable resistor on the secondary.

_________________
I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!
Back to top Go down
copain



Join date : 2011-12-28

PostSubject: Re: Speaker output specs   2013-11-08, 10:57 am

"[T]o measure the DC resistance of an impedance you multiply the impedance by 1.25."

OK, 7000 X 1.25 = 8,750.  So, I look for an output transformer whose primary reads 8,750 ohms on
an ohmmeter?  Impossible; something is wrong here.

Like all other hobbyists, I'm sure, I am looking for a quick way to choose a substitute output transformer.  

A very quick way is to check the schematic, if available, and see what the resistance is between the plate of the output tube and B+.

Or, often, the schematic will indicate the DC resistance of the  transformer and one's problem is solved.

Or, find a schematic of a unit which is pretty much the same as the one under repair and check as above.

Or, simply try a replacement, listen carefully, and you might just be surprised how normal the audio sounds.

When all else fails you can buy a universal matching transformer; they are not prohibitively expensive.  

All the above is far more direct and simple than trying to follow the theoretical material you provide, especially the Web site you link to, which my tired old eyes cannot cope with--very small print on a dark background.

Incidentally, a far more clear and simple explanation of impedance matching-etc. is given in Marcus and Levy's Elements of Radio Servicing.

To conclude happily, I found a transformer in my junk box; its primary measures 533 ohms and it works perfectly as a replacement.
Back to top Go down
Cliff Jones
Site Administrator
Site Administrator
avatar

Join date : 2010-11-22

PostSubject: Solved   2013-11-08, 6:29 pm

[You must be registered and logged in to see this link.] wrote:
"[T]o measure the DC resistance of an impedance you multiply the impedance by 1.25."

That is a general rule of thumb when you don't know the DC resistance.

OK, 7000 X 1.25 = 8,750.  So, I look for an output transformer whose primary reads 8,750 ohms on
an ohmmeter?  Impossible; something is wrong here.

Why is that impossible? Most output transformers are in the range of 1500-12000 Load ohms on the primary. The plate resistance is around 80,000 ohms.

Like all other hobbyists, I'm sure, I am looking for a quick way to choose a substitute output transformer.  

You didn't pose the question as being a quick simple answer needed , you asked for specifications, which imply specific details in the answer.

A very quick way is to check the schematic, if available, and see what the resistance is between the plate of the output tube and B+.

You didn't say you did or didn't have a schematic, because you gave a comment without what reference materal would be helpful to your research.
I did say the information I obtained was from a tube manual (which is customary for questions of tube circuitry)and a reference for you to follow.


Or, often, the schematic will indicate the DC resistance of the  transformer and one's problem is solved.

That is common knowledge, but Not that simple of a solution if you don't have those notes in the schematic.

or, find a schematic of a unit which is pretty much the same as the one under repair and check as above.

That too is common knowledge needless to say.

Or, simply try a replacement, listen carefully, and you might just be surprised how normal the audio sounds.

I did say that in differnt terms--> "I don't think that the secondary is that critical between 8 ohms and 10 ohms.
It is more important to consider power. If you are replacing a bad audio transformer that used a 10 ohm speaker its not that critical. Use one for an 8 ohm speaker. The speaker itself changes impedance with frequency anyway."




When all else fails you can buy a universal matching transformer; they are not prohibitively expensive.

Yes if you already know that, but what if you don't? A lot of poeple havn't deeloped the ability to understand questions without some previous knowledge of what is being sought after.

All the above is far more direct and simple than trying to follow the theoretical material you provide, especially the Web site you link to, which my tired old eyes cannot cope with--very small print on a dark background.

Then you should have stated such in your question rather than make such a broad all encompasing question.

Incidentally, a far more clear and simple explanation of impedance matching-etc. is given in Marcus and Levy's Elements of Radio Servicing.

Yes I to have that book and several hundred besides, so, since you have that book why did you ask in the first place?

To conclude happily, I found a transformer in my junk box; its primary measures 533 ohms and it works perfectly as a replacement.
Wonderful, So since you solved your own questions this topic is now solved and locked.

_________________
I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!
Back to top Go down
 
Speaker output specs
View previous topic View next topic Back to top 
Page 1 of 1
 Similar topics
-
» Sadr criticized the Christmas Bonus Speaker and his Deputies and deemed "not for justice has nothing"
» This Is Why Speaker John Boehner Resigned? discern
» Plans and Specifications
» PNP tailor-fit specs for Mike Arroyo choppers
» Modification of technical specification

Permissions in this forum:You cannot reply to topics in this forum
ANTIQUE-RADIO-LAB :: Radio Repair Bench :: Electronic Formulas, Theory, and applications-
Jump to: