[NJARC] A little radio theory question ...

[Al Klase]

Hi Al,

Defective audio transformers in battery sets are an epidemic.  They are 
wound with a lot of very fine wire, say #40 (0.00314"), and the years 
have not been kind to them.    Rewinding one of these things is a 
complete freak show.  You'd really have to want to do it, or have a lot 
of money to pay someone to do the job.

I'll try to address this without getting too geeky.

I measured a random 1920's transformer:   Primary resistance = 550 Ohms. 
( that would be about 500 feet of # 40 wire.)  Secondary resistance = 
1800 ohms.  Turns radio = 1:3 ( Measured by applying 1 volt p-p at 1 KHz 
to the primary, and observing 3 volts p-p on the open secondary with a 
'scope.)  Note that, at least in this case, the P:S resistance ratio is 
consistent with the turns ratio.

*The important characteristics of a transformer are:*

*Turns ratio:*  E.g., 1:3  That means that 1 volt in will be 
/*transformed*/ to 3 volts out. The impedance ration is the square of 
the turns ratio, e.g., 1:9. So it will /*transform*/ a 10K ohm source 
into a 90K ohm output impedance.

*Primary inductance:*  E.g., 7.2 Henry. This establishes the 
low-frequency response of the transformer.  In this case, 7.2 H at 200 
Hz = 9048 ohms.  That's a reasonable plate load for an '01A tube.

Inductance calculator; 
http://www.66pacific.com/calculators/inductive-reactance-calculator.aspx

*More below:*

On 7/20/2018 12:49 PM, Al Schapira wrote:
>
> ... concerning interstage audio transformers in TRF sets.
>
> Say you have a UX-200 detector stage transformer coupled to the 
> following UX-201A stage.
> Typically, the transformer ratio is 1:3 or 1:4 (primary:secondary) for 
> an impedance ratio step up of 9 or 16 to one.
>
> Now, the plate resistance spec for the UX-200 is 30k and the UX-201A 
> grid is otherwise open and the 201 is usually biased at -4.5 volts. So
> 1) How critical is the turns ratio?
*Not really critical.*  The turns ratio represents additional free 
gain.  An '01A has an amplification factor of 8. I.e., a 1-volt input 
produces an 8-volt output, the the transformer multiples this by three. 
   A voltage gain of 3 is +9.5 dB, significant, but it's not a big 
deal.  If  the gain is only 2, that's +6 dB, or if it 4 the gain would 
be 12 dB.  You're unlikely to notice these differences by ear.  
Transformers of this sort, with turns rations higher than 1:4 - 1:5 are 
difficult (expensive) to build.
> 2) How critical are DC resistances in the primary and in the secondary?
*Again, not very*.  I be suspicious of a transformer that had a primary 
resistance significantly less than 500 ohms.  It likely would not have 
enough primary inductance to support acceptable low-frequency response.
> 3) What are typical values for the DC resistances in the primary and 
> secondary windings?
Primary: on the order of 500 ohms.  Secondary:  on the order of of 2000 
ohms.
>
> You guessed it. I have three TRF sets with open transformers. Anyone 
> have any to spare?
> c
Because the gain provided by a single transformer is not that great, you 
can get away with bypassing a dead XFMR with a capacitor and a 
resistor(s) across the open winding(s).  Not elegant, but the set will play.



SEE: http://antiqueradios.com/chrs/journal/transformers.html

The solid-state hack discussed here has promise: 
http://antiqueradios.com/forums/viewtopic.php?f=12&t=341526
If your not a purist.

Hope this helps everyone,
Al



-- 
Al Klase – N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/mailman/private/njarc/attachments/20180722/643f95fa/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: nloincoooliojkbi.png
Type: image/png
Size: 78275 bytes
Desc: not available
URL: <http://mailman.qth.net/mailman/private/njarc/attachments/20180722/643f95fa/attachment-0001.png>