• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

How to test interstage transformers?

I'am experimenting with interstage transformers for the first time and I'am getting strange results. at first I was using a simple amp with a D3A. Now I have made this test setup.

This is how I test:

To the left is a generator with a Ri of 8K and to the left is a load of 8K and the audio analyser is parallel with the load.

What is going wrong?


I have done the same measurement with a source impedance of 1K and a load of 100K.

Nearly the same result.

At first I was using the D3A with the interstage to drive a GM70 tube also with this results. So I'am doing something fundamentally wrong or there is something wrong with the transformer.

The transformer is made by a respected Dutch transformer maker. No need to mention names. He is willing to investigate the matter but I have to be sure that it is not my stupidity first:)
On some EDCOR interstage trannies I get poorer response if I hook one end (pri or sec) the 'wrong' way. As if the windings fight each other if they are not hooked up in the proper phase relative to each other. Could this be the case here?

If the tranny u got is for 8k load, isn't it only one side u should load with 8kohms? Since it reflects the impedance. If u load it with 8kohms on both sides, isn't the effective impedance 4kohms? Which actually should give u an even better frequency response, but at the cost of signal attenuation.

My guess is that something aint no right:)

Helpful eh?
Yes, to amplify this a little; The AC grounds should be connected such that the windings don't fight each other through capacitive coupling. Your diagram doesn't show the polarity or the ground connection between the generator and analyzer. If it's a bifilar IT and your amp or test circuit uses an inverting connection (AC grounds at opposite ends of the winding) then this could be the trouble.


The audio analyzer is having a balanced input so in theory after reversing the secondary there should be no difference in the measurement. But to be certain I have tried this nonetheless. No difference here.

But Pieter may have a point regarding the bifilar winding. Maybe here is something gone wrong with the assembly of the trannie. I will look into this possibility.

For now thanks everyone for the input,