Just a quick question :)

[Bernardduur]

Hey all!

Just a quick question I thought of today which I could not find that easy on the net. On an output transformer, SE, on the prim side, it notes 7.5kohm; sec are 4 and 8 ohm. If I should measure the prim side with a DMM, should I find the exact same value or is it more a loading issue and should I measure something completely different?

Thanks!

I'm debugging a friends Valve Jr which suddenly went down in volume and output. When I measure the output transformer I get a really low value (approx 125 ohm). I just want to be sure if the OT is causing the problem!

[Wilder Amplification]

Hey all!

Just a quick question I thought of today which I could not find that easy on the net. On an output transformer, SE, on the prim side, it notes 7.5kohm; sec are 4 and 8 ohm. If I should measure the prim side with a DMM, should I find the exact same value or is it more a loading issue and should I measure something completely different?

Thanks!

I'm debugging a friends Valve Jr which suddenly went down in volume and output. When I measure the output transformer I get a really low value (approx 125 ohm). I just want to be sure if the OT is causing the problem!

No. You will read the DC resistance of the coils, which will be MUCH lower.

Impedance ratings on any output transformer are "reflected". This means that when connecting a 4 or an 8 ohm load to their respective taps, it will reflect a 7.5K ohm load to the primary. In order to find the impedance ratings you'll have to feed an AC voltage to the primary, then measure the voltage out on one of the secondary taps.

You would then divide the input voltage by the measured output voltage on the secondary to get the voltage/turns ratio.

Then you would square that number (i.e. multiply it by itself) to get the impedance ratio.

Then you multiply the impedance ratio by the ohms rating by the ohms rating of the secondary tap you measured the secondary voltage on to get the reflected impedance on the primary side.

On this particular transformer, if you apply 120VAC to the primary, you should measure about 3.9 volts on the 8 ohm tap and about 2.7-2.8V on the 4 ohm tap -

120V / 3.92V = 30.61

30.61 x 30.61 = 936.9

936.9 x 8 Ohms = 7.495K

120V / 2.7V = 43.3

43.3 x 43.3 = 1875

1875 x 4 Ohm = 7.5K

[Bernardduur]

Ah yes, makes sense! My thoughts were in the right direction

Just tested the OT and at both I get 1V AC on 230V AC prim. Clear result!