measuring primary L creates migraine headache

[imaradiostar]

I haven't read the entire RDH4 chapter on transformers but I'm tired of thinking about it and I have class in 6 hours so I thought I'd post and Steve can answer since it's probably about time for breakfast across the pond!

I'm fortunate to work in a lab with some really neat equipment so I brought 3 output transformers to work and spent some time on the HP 4192a low frequency impedance analyzer. The goal of my testing was to determine the low frequency response of some transformers.

I know that the reflected primary Z and L can be used to estimate the low end roll-off of a given transformer. To that effect I've measured the transformers. I can't find anything in RDH4 that says what frequency I should be measuring at!

example, a largish potted 5k:8/16 output TF from "audio development company" as measured on the HP4192A:

10Hz, 29.5H
20Hz, 24.3H
30Hz, 21.2H
40Hz, 19.3H
50Hz, 18.0H
60Hz, 17.0H
70Hz, 16.0H
80Hz, 15.2H
90Hz, 14.6H
100Hz, 14.1H
200Hz, 10.8H
500Hz, 9.3H
1kHz, 9.6H

You get the point. I have similar lists for the other two transformers. I'm not sure which one to plug into the formulas...and I started getting sick of it...engineering job plus full time school plus life means I can't do all of this thinking at the moment. Then it occurred to me...

I could connect my variac to a large power transformer (say an 800VCT unit) to produce a "plate signal" of 60Hz and connect it across the transformer primary and have a fixed resistance (load bank) connected to the transformer secondary. Measure the voltages in and out and turn up the juice a little at a time. When I hit the point where the secondary voltage isn't increasing I know the transformer has hit the wall, so to speak. When the Vsec is 50% of the expected value, that's the power level for -3dB, right?

Anyway, it's late and this post is eternal. Thanks, goodnight. Or good morning.

jamie

[mooreamps]

I haven't read the entire RDH4 chapter on transformers but I'm tired of thinking about it and I have class in 6 hours so I thought I'd post and Steve can answer since it's probably about time for breakfast across the pond!

I'm fortunate to work in a lab with some really neat equipment so I brought 3 output transformers to work and spent some time on the HP 4192a low frequency impedance analyzer. The goal of my testing was to determine the low frequency response of some transformers.

I know that the reflected primary Z and L can be used to estimate the low end roll-off of a given transformer. To that effect I've measured the transformers. I can't find anything in RDH4 that says what frequency I should be measuring at!

example, a largish potted 5k:8/16 output TF from "audio development company" as measured on the HP4192A:

10Hz, 29.5H
20Hz, 24.3H
30Hz, 21.2H
40Hz, 19.3H
50Hz, 18.0H
60Hz, 17.0H
70Hz, 16.0H
80Hz, 15.2H
90Hz, 14.6H
100Hz, 14.1H
200Hz, 10.8H
500Hz, 9.3H
1kHz, 9.6H

You get the point. I have similar lists for the other two transformers. I'm not sure which one to plug into the formulas...and I started getting sick of it...engineering job plus full time school plus life means I can't do all of this thinking at the moment. Then it occurred to me...

I could connect my variac to a large power transformer (say an 800VCT unit) to produce a "plate signal" of 60Hz and connect it across the transformer primary and have a fixed resistance (load bank) connected to the transformer secondary. Measure the voltages in and out and turn up the juice a little at a time. When I hit the point where the secondary voltage isn't increasing I know the transformer has hit the wall, so to speak. When the Vsec is 50% of the expected value, that's the power level for -3dB, right?

Anyway, it's late and this post is eternal. Thanks, goodnight. Or good morning.

jamie

You bring up a very good subject.... I would not try to measure the inductance of an output transformer at 60 cycles. The measurement is not valid since it is out of range with the instrument. For audio frequency measurements I take, for a guitar amp, not for a home theater amp, I choose 440 cycles, because this is the lower A string on the guitar.


-g

[Steve Conner]

Simple answer: Use the lowest frequency you intend to pass through the thing. (And ideally about the same amplitude, too, which no network analyzer that I know of will do.)

Complex answer: An iron-cored inductor is a nasty, non-linear thing with self-capacitance, core loss, and so on. Any test instrument that tries to analyze it as if it were "just an inductor" will have to fall short somehow. Look over in the Pickup Makers forum, they have the same problems measuring the inductance of pickups.

[Wilder Amplification]

For audio frequency measurements I take, for a guitar amp, not for a home theater amp, I choose 440 cycles, because this is the lower A string on the guitar.


-g

No..."440Hz A" may be the 4th harmonic of the low A string, but the fundamental of the low A string is 110Hz. 5th fret on the high E/10th fret on the B string would be an A 440Hz fundamental.

[imaradiostar]

So what I'm really hearing here is I need to put the thing in an amp and see what it really does. "It seems so...simple!" -Bill Murray in "What about Bob?"

I still think it would be interesting to hook it up to a large transformer, dummy load and a variac and plot voltage out versus voltage in and determine how it responds as voltage goes up. I guess by determining what it does with a 60Hz sine wave I can find a pretty decent representation of the guitar response. Logically I could halve that power level for bass use, extending the response to 30Hz.

I have this nasty habit of building things on paper or in the computer and never really following through and listening to them. I am always trying to use the parts I have the best way possible which means too much mental exercise and not enough building. I need to get over that.

jamie

[Steve Conner]

That's a classic movie! Whenever I'm having a bad day at work (most days lately since we got into FPGAs) I often think of Bob and his "Baby Steps".

Voltage out vs. voltage in isn't how it works. If you want to see what the transformer is doing, you need to put some resistance in series with the voltage, so you can see what current it's drawing.

I've tested OPTs in the past by leaving the primary open, and backfeeding the secondary through an 8 ohm resistor (or 16 or 4 or whatever the OT's rated impedance is) from a big audio power amp. I do this so I can use a lower frequency than the line frequency: 10Hz or whatever. With 50 or 60Hz, a decent OT would need so much voltage to saturate that it would probably break down its primary insulation first.

If you're really neurotic you can add an integrator and a scope in X-Y mode to this rig, to plot the B-H loop.

[imaradiostar]

That's a classic movie! Whenever I'm having a bad day at work (most days lately since we got into FPGAs) I often think of Bob and his "Baby Steps".

Voltage out vs. voltage in isn't how it works. If you want to see what the transformer is doing, you need to put some resistance in series with the voltage, so you can see what current it's drawing.

I've tested OPTs in the past by leaving the primary open, and backfeeding the secondary through an 8 ohm resistor (or 16 or 4 or whatever the OT's rated impedance is) from a big audio power amp. I do this so I can use a lower frequency than the line frequency: 10Hz or whatever. With 50 or 60Hz, a decent OT would need so much voltage to saturate that it would probably break down its primary insulation first.

If you're really neurotic you can add an integrator and a scope in X-Y mode to this rig, to plot the B-H loop.

So I don't want to load the secondary if I feed it backwards? (ooops...I mean I don't want to load the primary which is now the secondary because it's being used backwards)

I have a LOT of large 50 watt 300 ohm resistors right now. Could I theoretically connect a big PT to the variac and connect my resistors in series between the big PT's HT windings and the output TF's primary windings to simulate the plate impedance of the output tubes? I suppose I could measure the voltage drop across the big resistors and see what was happening.

For the ideal big picture (and without having a big clean power amp at the moment) I suppose I could measure supply voltage (output of a variac'd big plate PT), primary voltage (on the OT side of the dropping resistor), and secondary voltage into a suitable load. I could also vary the series primary resistor to represent different plate impedances. This way I'd know how the tubes interact with the inductance of the OT and have an idea of when the voltage ratio of the output TF started to break down.

Another Bill Murray fave that we watched just a little while ago- "Groundhog Day." Man I love it! When I worked at a corporate telecom job that I hated we used to watch Office Space at work. One of my bosses would imitate Lumburg's "yeeeeahhhh..." and he got to be a little too good at it. Creepy.

jamie

[imaradiostar]

wait, I'm stupid. I just fully grasped what is going on in a PP tube amp- though I've read it a million times it just clicked.

If you had output tubes of a theoretically perfectly low impedance, they could overcome the primary inductance, essentially removing it from the frequency response calculation. If I had a BIG pt connected to my primary I'd essentially be doing the same thing. I'd be going "wow, that's funny, this thing is flat down to 20Hz at 320 watts." when that would be physically impossible for the core size.

The thing I still have a difficult time understanding is the parallel/square nature of an output- as in the mathematical difference between say a 4k plate to plate load (that's 1K per side, right?- 4000/8=500, sqrt500=22.36, 22.36/2=11.18, 11.18squared=1k) and a similar tube in a SE class A amp running at say- 4k impedance with relatively similar voltages. Is it because the "resting" tube is swinging the other way with the aid of the inductive load? Anyway, that is a bit off topic I suppose.

jamie

[Steve Conner]

Maybe... something like that :P

The thing to understand is that the voltage ratio never really breaks down. It only breaks down in so far as the transformer draws excessive current when it saturates, and that causes an IR drop in the primary. But in a good transformer it's not that noticeable.

At 60Hz you'll need to hit a hi-fi OT with three times its rated voltage before you see any saturation. That could well blow it.

I once got a Fender 300PS OT that I wanted to test the insulation on. I hooked the speaker winding up to a variac on my bench at work, and cranked it up until I had 300W at 8 ohms' worth of voltage, which stepped up to 800V RMS between the open primary ends. I left it running all afternoon, and it was fine.

[imaradiostar]

Ok, so I don't want to hit it with ridiculous amounts of voltage- unless I connect a big resistor in series then I can get a feel for how the primary inductance interacts with the impedance of the tubes. This would be with an unloaded secondary and an artificially high amount of series primary resistance so I could see the effect sooner.

It's dang confusing because amplitude and frequency play into the picture- obviously if a TF is flat to 20hz at 20 watts out it doesn't mean it'll necessarily be flat at 40 watts out.

Would a 4k:8ohm OT that's down -6dB at 40Hz be good down to 20Hz if it were used as a 2k:4ohm OT and -6dB at 80Hz if it were used as an 8k:16 OT? (given equal wattage and appropriate drive impedance in each scenario)

jamie

[Steve Conner]

Would a 4k:8ohm OT that's down -6dB at 40Hz be good down to 20Hz if it were used as a 2k:4ohm OT and -6dB at 80Hz if it were used as an 8k:16 OT? (given equal wattage and appropriate drive impedance in each scenario)

Yes, that's it exactly!

[Bruce / Mission Amps]

...

I once got a Fender 300PS OT that I wanted to test the insulation on. I hooked the speaker winding up to a variac on my bench at work, and cranked it up until I had 300W at 8 ohms' worth of voltage, which stepped up to 800V RMS between the open primary ends. I left it running all afternoon, and it was fine.

I've done the same thing but would frequently (actually more often then not) blow the damn primary fuse of my Variac! I just couldn't put very much voltage (more then 25-35vac, depending on the OT) into the low DCR/Zed of the secondary on the OT before it would blow the fuse.

[Wilder Amplification]

I've done the same thing but would frequently (actually more often then not) blow the damn primary fuse of my Variac! I just couldn't put very much voltage (more then 25-35vac, depending on the OT) into the low DCR/Zed of the secondary on the OT before it would blow the fuse.

Well you figure a typical 100 watt OT on the 16 ohm tap with a 16 ohm load sees about 40 volts @ 2.5 Amps across it at 100 watts out. Not sure what value fuse is in your variac but the numbers make sense.