There appears to be disagreement regarding the impedence of the Sowter O/P Tx's Steve s has loaned to me. The original owner, Tony Moore tells me he ordered them to be 3.5K. Sowter have apparently advised Kenny these are 2K. As I have them here, how do I measure their impedence?
They have wiring for HT, Anode, Common, 4 ohm and 8 ohm.
Thanks in anticipation of another lesson to be learnt.
Measuring Impedence on O/P Transformers
#2
Hi Greg,
This is a good article.
http://www.radioremembered.org/outimp.htm
Although I would use a sig gen rather than a Variac and the mains.
regards,
Andrew
This is a good article.
http://www.radioremembered.org/outimp.htm
Although I would use a sig gen rather than a Variac and the mains.
regards,
Andrew
Analogue, the lost world that lies between 0 and 1.
#3
that's a good article.
Although the frequency used for the nominal impedance measurements will have a bearing on the result, surely?
the reactance (X) is 2 x Pi x freq x the inductance in henries. the Impedance Z = square root of R squared + X squared
So at 60Hz the impedance will be a good deal higher than at 20Hz.
I've had a look at Sowter's site, and the inductance values for some of the transformers are between 15 and 20 Henries.
so the reactance at 60Hz for a 15H transformer is 5.6kohms
at 20Hz it's nearer 1.9kohms.
Or have I grabbed the wrong end of the stick again?
Although the frequency used for the nominal impedance measurements will have a bearing on the result, surely?
the reactance (X) is 2 x Pi x freq x the inductance in henries. the Impedance Z = square root of R squared + X squared
So at 60Hz the impedance will be a good deal higher than at 20Hz.
I've had a look at Sowter's site, and the inductance values for some of the transformers are between 15 and 20 Henries.
so the reactance at 60Hz for a 15H transformer is 5.6kohms
at 20Hz it's nearer 1.9kohms.
Or have I grabbed the wrong end of the stick again?
#4
Nope, you're correct, it will drop off at lower frequency, that's why I said use a sig gen; that way you can use several freq, 1kH is a good a starting point as any.
Andrew
Andrew
Analogue, the lost world that lies between 0 and 1.
#5
Not in the way you are thinking. The equation for the reactive impedance of a inductor will as you say vary with frequency. But in reality, you are not measuring the impedance of the transformer, you are measuring the turns ratio, which should be constant across the audio band (crap output TX if not) as its set by the ratio of primary and secondary turns of wire. Having worked out the turns ratio, you can then calculate the impedance the valves will see by seeing what primary impedance the turns ratio will produce with the secondary loaded by a 8R resistor (loudspeaker).Although the frequency used for the nominal impedance measurements will have a bearing on the result, surely?
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#6
If you use 50Hz and the TX ain't all that great and you are using a digital volt meter's RMS feature to measure the Vin versus Vout, to get the turns ratio, then you may get odd results at the lower frequency due to the fact there's not enough inductance, the sine wave will distort. In the worst case it will become a triangluar shape, this will confuse the RMS reading of the voltage. That's why 1Khz is better and a scope is even better.
Most TX should be good to 40Hz, if you look at the pictures of Neal's amp measured at Owston you'll see the distortion increasing below 40Hz. So on a decent TX 60Hz US mains should be fine, as should 50Hz, but soemhow in my head 1Khz seems a better bet. The flip side of this however is that a cheap DVM may not give an accurate RMS reading at non mains frequencies - a scope really is better.
So Nick is right the turns ration doesn't change, but you are right reactance is a function of freq.
Most TX should be good to 40Hz, if you look at the pictures of Neal's amp measured at Owston you'll see the distortion increasing below 40Hz. So on a decent TX 60Hz US mains should be fine, as should 50Hz, but soemhow in my head 1Khz seems a better bet. The flip side of this however is that a cheap DVM may not give an accurate RMS reading at non mains frequencies - a scope really is better.
So Nick is right the turns ration doesn't change, but you are right reactance is a function of freq.
Analogue, the lost world that lies between 0 and 1.
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Mike H
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#7
To measure the actual impedance, you need to measure the AC Volts and the AC current (which could be microamps) of the primary, then you can get its impedance by Ohms law.
The old quick and simple Wireless World method was use a 5V AC heater winding (50 Hz), this Voltage is a constant (but make sure what it is exactly first!) put it across primary with load resistor on secondary, measure the AC mA or uA (requires multimeter that can do it, and in series with the 5V AC).
Do the V/I, bob's your aunt fanny
NB: originally this WW method was cited for determining the incremental inductance of an OPT (with no load).
HTH
The old quick and simple Wireless World method was use a 5V AC heater winding (50 Hz), this Voltage is a constant (but make sure what it is exactly first!) put it across primary with load resistor on secondary, measure the AC mA or uA (requires multimeter that can do it, and in series with the 5V AC).
Do the V/I, bob's your aunt fanny
NB: originally this WW method was cited for determining the incremental inductance of an OPT (with no load).
HTH
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
#8
Just caught up with this, when i bought the transformers i was suprised to find out they where 2k primary, one of the many reasons i had little use for them. To find after greg had taken them they where 3.5 k was a surprise, as i had been sent the specs a few years ago and they stated 2k, I would have expected a different model number to the standard spec, i had planned to check the turns ratio when they are returned, since all this focus on them, and 3.5k is much more usefull a transformer to me. I do wonder now is sowter just supplied their standard transformer for the job or really did wind to tonys spec? Be interesting to find out...
Steve
Steve
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
#9
This thread may not be being that much use to Greg, AFAIKS he wants to know what impedance the transformer will present to the output valve. All the talk of measuring primary inductance is good, but not much help.
Simple way, connect output of filament transformer (6.3v) to primary, secondary open. Measure with multimeter voltage across primary, measure voltage across secondary.
Turns ratio will be:
Rturns = Vpri/Vsec
The impedance ratio will be the turns ratio squared.
Rimp = Rturns * Rturns
The primary will present the valve whatever is connected to the secondary multiplied by the impedance ratio.
So, if input voltage was 6.3v, output voltage 0.3v
Turns ratio = 21
Impedance ratio = 441
With 8 ohm load (on 8 ohm tap), primary impedance 3528 ohm.
For a audio transformer or any use, 50Hz will be fine, and the chance of the meter reading 50Hz accurately if greater than any loss in the transformer. And as you are measuring the ratio, the only losses worth worrying about are the core saturating.
Simple way, connect output of filament transformer (6.3v) to primary, secondary open. Measure with multimeter voltage across primary, measure voltage across secondary.
Turns ratio will be:
Rturns = Vpri/Vsec
The impedance ratio will be the turns ratio squared.
Rimp = Rturns * Rturns
The primary will present the valve whatever is connected to the secondary multiplied by the impedance ratio.
So, if input voltage was 6.3v, output voltage 0.3v
Turns ratio = 21
Impedance ratio = 441
With 8 ohm load (on 8 ohm tap), primary impedance 3528 ohm.
For a audio transformer or any use, 50Hz will be fine, and the chance of the meter reading 50Hz accurately if greater than any loss in the transformer. And as you are measuring the ratio, the only losses worth worrying about are the core saturating.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#11
Just give me a shout if you want me to measure this for you!
James.
James.
#13
James has kindly tested Steve S's O/P Transformers and we can confirm they are indeed 2K impedance. See details below.
- Attachments
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- Steve S's TX plot.pdf
- (62.49 KiB) Downloaded 446 times
Last edited by Greg on Tue Jul 30, 2013 9:45 pm, edited 2 times in total.
#15
Thanks greg, they are as i thought... It would have been better for my uses had they been 3.5 but never mind...
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number