Output transformer rating

[beatnik]

I have a few output transformers with limited information available so I would like to find out how to calculate the maximum current rating in the primary winding so I can determine which tube and circuit design the transformer could be used with.

I have done a bit of research about formulas and such but I don't think I got this right.

For example let's take an output transformer with a 7:1 ratio (30K/600) and a maximum level of +23dBm as specified in the datasheet

23dBm into 600ohm = 10.94V rms

Secondary current = 10.94V / 600ohm = 18.2mA

Primary current = Secondary current / 7 = 2.6mA

If the output transformer primary is center tapped that would mean 1.3mA each primary winding ? It seems very little given the physical size of the transformer.

I suppose knowing the wire gauge of the primary could give an idea of the maximum current rating, but that is obviously impossible to know without taking the transformer apart.

Alternatively, could the current rating be determined using the winding DC resistances and the turns ratio ? I have some transformers with only that information available.

 

[beatnik]

Measured DC resistance 720 ohm on each primary side.

Came up with a different calculation than above - and obviously a different result.

+23dBm = 10.94V (from online calculator)

10.94V * 7 = 76.58V on primary side

76.58V / 2 = 38.29V on each primary (it's a push pull transformer)

38.29V / 720 ohm = 53mA on each primary


Does this make any sense to you ?

 

[emrr]

PP class A 6L6 transformers burning 50ish mA/side usually measure under 200R, FWIW. I would expect your transformer, using the highly accurate method of "gut feeling" is probably spec'd from something in the 5-10mA/side range, doubtful more.

 

[ruffrecords]

You do not say if it is a single ended or push pull transformer. However, 2.6mA rms is about 7.5mA peak to peak so a single ended output stage would need at least half that amount of quiescent current so a more likely value would be 5mA which does not seem unreasonable for an output stage that only has to produce 200mW of power. Note that the primary voltage for +23dBu on the secondary is 76.58V rms which means the stage needs to be able to swing 76.58 * 2.828 volts = 216 volts.

Cheers

Ian

 

[CJ]

you can cross check that with a dbm calculator,

https://www.daycounter.com/Calculators/Decibels-Calculator.phtml

23 dbm into 30,000 ohms

calculator spits out 0.200 watts and 77.4 volts,

now it is a simple power formula Watts = IE


0.200/77.4 = 0.00258 amps or 2.6 ma

for kicks you can calculate voltage drop across 720 ohms = 1.87 V

efficiency 77.4 - 1.87 = 75.53

75.53/77.4 = 97.6% efficient + core loss,

How hot will the primary get?

0.0026^2 x 720 = 4.8 mw so no heat.

core size and power:

general estimates

Mic/Line In uses 1/4" tongue
12AX7a Innerstage uses 3/8" 1/2" tongue
12AX7a Line Out uses 3/8" to 1/2" tongue
6V6 Single Ended uses 1/2" to 5/8" tongue
Melcor/API Opamp uses 5/8" to 3/4" tongue
2N3055 (DC involved) uses 3/4" to 7/8" tongue
EL 84 push pull uses 3/4" to 1" tongue
6V6 push pull uses 7/8" to 1" tongue
EL 34 and 6L6 push pull uses 1" to 1 1/4" tongue
EL 34 and 6L6 quad push pull uses 1 3/8" tongue
6550 KT 88 quad uses 1 1/2" to 1 3/4" tongue maybe over stacked
KT 120 uses 1 7/8" tongue

tongue size will depend on material and type of lam ie F lam, UI, EI, L , 50/50 nickel, 80 Ni, non grain or grain orient, 80 Ni for mic/line input , innerstage and line out, 50/50 for tube out (line level) grain and non grain for a power or output XFMR although some jokers are using nickel for OPT's

divide numbers to get catalog sizes

1/4 = EI 25
3/8 = EI 375 or EI 38
1/2 = EI 50
5/8 = EI 625
3/4 = EI 75
7/8 = EI 87
1 = EI 100
1.25 = EI 125
1 3/8 = EI 137
1 1/2 = EI 150
1 3/4 = EI 175
1 7/8 = EI 187

metric is different

 

[beatnik]

Many thanks for the insights so far.
The transformer I was referring to is a vintage UTC HA-114, which is a push pull type.
I have attached the available information in the manufacturers catalog.
CJ maybe you have encountered this transformer in one of your dissections ?

I am a bit confused as to whether the datasheet max. level of +23dBm is referred to the primary or the secondary.
On other datasheets by Sowter, Lundahl, Cinemag, I see the max. level linked to a specific frequency and THD measurement, expressed in dBu.
Am I wrong to assume this level is normally referred to the secondary ?

I have been trying to apply CJ maths to another push pull output transformer I have, a Peerless S-448-Q but things don't seem to line up properly.
In this case the maximum primary current rating is available on the datasheet 15mA, as well as the maximum operating level +18dBm
I have measured the DC resistance 422 ohm on one primary side and 433 ohm on the other.

18 dbm into 20,000 ohms

calculator spits out 0.063 watts and 35.5 volts,

now it is a simple power formula Watts = IE

0.063/35.5 = 0.00177 amps or 1.7 ma

This is not matching the published max primary DC rating of 15mA


BTW, if an inductance measurement would be any helpful to calculate the current rating, I have an LCR meter available.

 

[beatnik]

Note that the primary voltage for +23dBu on the secondary is 76.58V rms which means the stage needs to be able to swing 76.58 * 2.828 volts = 216 volts

You mean a peak to peak AC voltage from plate to plate ?

 

[ruffrecords]

dBm and dBu are different. dBm is a power relative to 1 milliwatt at some specific impedance. For rf guys this impedance is 50 ohms, for us audio guys it is 600 ohms. Whatever the impedance, 23dBm means 200 milliwatts. The voltage required to put 200 milliwatts into a load depends on the impedance of the load. If the load is 600 ohms then the voltage for 1mW is 0.775 volts and this voltage is defined as 0dBu. In the old days, the load was always 600 ohms so dBm and dBu become the same. Nowadays we tend to have 10K bridging loads so dm is not relevant and we use dBu instead.

Transformer manufacturers are always sloppy in their specifications and it can be quite difficult to determine whether levels are referred to primary or secondary. Of course, we are interested in secondary levels.

Distortion in transformers is strongly related to frequency being worst at lower frequencies. For this reason some manufacturers will quote distortion at some level at some low frequency (often 50Hz but it ought to be 20Hz). It is safe to assume that distortion at this level will be lower at all higher frequencies.

The transformer you calculated has a 20K centre tapped winding so each plate sees 10K. This will double the rms current to 3.4mA which is nearly 5mA peak. Pro audio PP output stages are invariably class A so you will need a quiescent significantly larger than this to minimise tube distortion. 10mA would not be a bad choice which leaves a comfortable margin below the max spec.

Cheers

Ian