I've just had a similar problem, the culprit was my lads fancy multicolour led lit keyboard...
Measuring transformers
Re: Measuring transformers
 Mike H
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Re: Measuring transformers
Looks pretty much like every other transformer I've checked the freq response of (including LTspice models)  the LF rolloff is due to the core being too small at that freq, the HF rolloff is simply because of the rising reactance of the windings, adding series impedance. Also there is interwinding capacitance, so you can get a lowpass LC filter effect as well. But I see nothing unusual about that plot except that the HF is quite high.
I never make the same mistake twice. Instead I prefer to fill my day with a variety of different mistakes
Re: Measuring transformers
Hi MikeMike H wrote: ↑Fri Aug 02, 2019 12:19 amLooks pretty much like every other transformer I've checked the freq response of (including LTspice models)  the LF rolloff is due to the core being too small at that freq, the HF rolloff is simply because of the rising reactance of the windings, adding series impedance. Also there is interwinding capacitance, so you can get a lowpass LC filter effect as well. But I see nothing unusual about that plot except that the HF is quite high.
I think the plot you're looking at is the rolloff of my sound card. It was a loopback test, the transformer wasn't connected.
This was the plot of the transformer  flat from 4Hz to 98kHz. I need to figure out how to extend the measurement below 4Hz to see where the LF limit is.....
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 Andy Grove IT frequency response and phase, no pads.jpg (17.03 KiB) Viewed 643 times
Re: Measuring transformers
You also should drive the transformer into its expected load and from its expected source impedance to see how it will behave in actual use.
Resistance isn't futile it's V / I.
Re: Measuring transformers
Yep, that’s next on the list. (As I said, I hope to measure everything that’s on that Jensen data sheet)
The source impedance of the soundcard is quoted as 94 ohms balanced, and I’ve done a quick test to verify that. So currently my setup is a long way from how I have been using these transformers (mainly driving them with 5687 and similar). One of the motivations for this measurement exercise is to understand how to get better performance from the transformer.
My thoughts are different valves, or parallel valves, or some kind of buffer or follower....
But first I need to establish exactly how low the impedance needs to be  or more accurately, where is a good compromise.....

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Re: Measuring transformers
Nice thread Max, looking forward to reading more
Re: Measuring transformers
As Nick suggested, here is some data showing the effect of source impedance. I haven't investigated the effect of load impedance yet, but the 6i6 is quoted as 52kohm which I guess is not too unrepresentative of the grids of the following stage as long as we stay away from A2. I will do some tests later at much lower impedance to simulate driving grid current.
I wanted to extend the test to lower frequencies but I found that I had to drop the sampling rate of the 6i6 down to 48kHz, which then meant an upper frequency of 24kHz. So what I have ended up with is two separate sets of curves  first here is the low frequency results from 1Hz to 20kHz.
The upper family of curves are the amplitude response, using the Yaxis graduated from +5dB to 20dB.
The lower curves are the phase response, using the other Yaxis graduated in degrees.
Red is 0 source impedance (I think the measurement method eliminates the impedance of the 6i6 because of the way the reference signal is derived)
Green is 680 + 680 ohms
Blue is 1200 + 1200 ohms
Magenta is 1800 + 1800 ohms (roughly representative of driving the IT with one half of a 5687 per phase). This looks to be about 1.5dB down at 3Hz
I wanted to extend the test to lower frequencies but I found that I had to drop the sampling rate of the 6i6 down to 48kHz, which then meant an upper frequency of 24kHz. So what I have ended up with is two separate sets of curves  first here is the low frequency results from 1Hz to 20kHz.
The upper family of curves are the amplitude response, using the Yaxis graduated from +5dB to 20dB.
The lower curves are the phase response, using the other Yaxis graduated in degrees.
Red is 0 source impedance (I think the measurement method eliminates the impedance of the 6i6 because of the way the reference signal is derived)
Green is 680 + 680 ohms
Blue is 1200 + 1200 ohms
Magenta is 1800 + 1800 ohms (roughly representative of driving the IT with one half of a 5687 per phase). This looks to be about 1.5dB down at 3Hz
Last edited by Max N on Thu Aug 08, 2019 10:59 pm, edited 1 time in total.
Re: Measuring transformers
And this is the higher frequency result, from 70Hz up to 90kHz.
Same colours/impedance as before.
Not especially informative because of the upper frequency limit of the soundcard. The 1k8 + 1k8 (magenta curve) is starting to dip at 90kHz, about 0.5dB compared to the response at 3kHz. I will have to use a signal generator and scope to get a 3dB point for each source impedance.
Next step I will drop the load impedance to simulate A2 operation of the following stage......
Same colours/impedance as before.
Not especially informative because of the upper frequency limit of the soundcard. The 1k8 + 1k8 (magenta curve) is starting to dip at 90kHz, about 0.5dB compared to the response at 3kHz. I will have to use a signal generator and scope to get a 3dB point for each source impedance.
Next step I will drop the load impedance to simulate A2 operation of the following stage......
Re: Measuring transformers
Here is the effect of load impedance. Source impedance is the same for both red & blue measurements  about 1K8 plus 1K8 or 3K6 balanced.
The red line is loaded by the audio interface (52K), which gives about a 1dB drop.
The blue line is loaded with 3K6. The source impedance and load impedance act as a potential divider, so we lose half our signal, hence we are a further 6dB down (roughly)
Actually the bandwidth of the transformer itself is still pretty good, within 1dB from 4Hz to 90kHz.
So if we wanted to drive a purely resistive load of 3K6, we would be in pretty good shape. But what if we wanted to drive a pushpull pair of output tube grids into A2....?
I've chosen these impedances to be somewhat representative of that situation  say a pair of 5687 driving a pair of 211s into A2. The problem then of course is that for most of the voltage swing, we would have a load impedance of maybe 100K (depending on the grid circuit resistance of the output stage). As we swing into positive grid volts, the grid starts to conduct. I seem to remember that when I looked at 211 datasheets for grid current vs volts, the grid drew about 25mA at +50V, equivalent to about 2KOhms.
So we have a load which transitions from 100K to 2K (I'm sure I could rig up a dummy load to simulate this with some resistors and a diode but I need to think about exactly how to do it). We end up with a 1dB loss for most of the signal swing, transitioning into a 6 or 7dB loss. The end result will be distortion  I can visualise what it will look like, the peaks on one side of the swing will be flattened. The more we drive into A2, the more flattening.
Anyway, this is all a bit of a digression, because actually this is just the effect of grid current and not really a characteristic of the transformer. The transformer itself seems happy enough driving a 2K resistive load.
However, if I want to use this IT to drive a pair of output valves into A2, I will need a low source impedance. Any suggestions on how to do that?
I could parallel a pair of 5687, but that will only get me down to 1K.
I have some E810F and some E55L which I could run as triodes to get me lower than the 5687.
Or I could use some kind of follower or buffer......
The red line is loaded by the audio interface (52K), which gives about a 1dB drop.
The blue line is loaded with 3K6. The source impedance and load impedance act as a potential divider, so we lose half our signal, hence we are a further 6dB down (roughly)
Actually the bandwidth of the transformer itself is still pretty good, within 1dB from 4Hz to 90kHz.
So if we wanted to drive a purely resistive load of 3K6, we would be in pretty good shape. But what if we wanted to drive a pushpull pair of output tube grids into A2....?
I've chosen these impedances to be somewhat representative of that situation  say a pair of 5687 driving a pair of 211s into A2. The problem then of course is that for most of the voltage swing, we would have a load impedance of maybe 100K (depending on the grid circuit resistance of the output stage). As we swing into positive grid volts, the grid starts to conduct. I seem to remember that when I looked at 211 datasheets for grid current vs volts, the grid drew about 25mA at +50V, equivalent to about 2KOhms.
So we have a load which transitions from 100K to 2K (I'm sure I could rig up a dummy load to simulate this with some resistors and a diode but I need to think about exactly how to do it). We end up with a 1dB loss for most of the signal swing, transitioning into a 6 or 7dB loss. The end result will be distortion  I can visualise what it will look like, the peaks on one side of the swing will be flattened. The more we drive into A2, the more flattening.
Anyway, this is all a bit of a digression, because actually this is just the effect of grid current and not really a characteristic of the transformer. The transformer itself seems happy enough driving a 2K resistive load.
However, if I want to use this IT to drive a pair of output valves into A2, I will need a low source impedance. Any suggestions on how to do that?
I could parallel a pair of 5687, but that will only get me down to 1K.
I have some E810F and some E55L which I could run as triodes to get me lower than the 5687.
Or I could use some kind of follower or buffer......
Re: Measuring transformers
In the meantime, back to measuring the ITs. The next thing I want to investigate is the effect of DC imbalance...I haven't figured out how I'm going to do that yet.......