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THE AM BULLETIN BOARD => Technical Forum => Topic started by: AB2EZ on January 16, 2008, 04:34:34 PM



Title: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 16, 2008, 04:34:34 PM
Hi!

I was chatting with Peter, K1PHG, today about the feasibility of using a toroidal transformer as a modulation transformer. Tim, WA1HLR, and I had also been chatting about this last week.

As I was talking with Peter, I realized that I had everything I needed to do a quick experiment:

I have an external modulator that I am using with my Ranger, which employs a backward connected Hammond output transformer (model 1629 SEA).

Separately, I have a small toroidal transformer that I recently purchased from Antek (http://stores.ebay.com/Antek-inc) that is the size of a supermarket donut (2 3/4 inches in diameter and 1 inch high), cost $8.00 plus $8.00 shipping, has a pair of 115 volt windings and a pair of 12 volt windings, is rated at 25VA at 50Hz, is specified as having been tested for dielectric breakdown at 3500 volts between the primary and secondary windings, and weighs 1.3 pounds.

http://cgi.ebay.com/12-12V-25VA-Toroid-Toroidal-Power-Transformer_W0QQitemZ370012573775QQihZ024QQcategoryZ73152QQcmdZViewItem

Note: this toroidal transformer employs a high permeability, solid core... and some of our intuition about transformers, based on traditional core materials/structures may not hold for this type of transformer. For example, because of the very high permeability... the number of turns on each winding is much lower than for a traditional transformer, the insulation is probably thicker and the capacitance between turns of the windings is probably less than it is in traditional transformers with the same turns ratios. The breakdown voltage is probably higher than we might expect (see the specification on the Antek web site) because of the thicker insulation. The leakage inductance is also lower than we might expect in traditional transformers whose core materials have lower permeabilities. There are no laminations to vibrate.


Using the 115 volt windings in series (230 volts), and the 12 volt windings in parallel, the turns ratio is 19.2:1. Therefore, the impedance transformation ratio is 367:1.

Using the 8 ohm output of my audio amplifier, the equivalent source impedance at the output of this transformer is less than 8 ohms x 367 ~ 2940 ohms... which is just fine for modulating my Ranger, because I am not concerned about the efficiency of power transfer from the audio amplifier to the Ranger.

Since the 230 volt rms winding corresponds to a peak voltage 230 x 1.414 = 325 volts... I felt that I could fully modulate the Ranger, when I run it at low power, without creating too much hysteresis loss in the transformer. [It actually worked fine (100% modulation) with 600 volts B+ (on the Ranger's 6146) as well, and the transformer stays perfectly cool with normal voice modulation.]

So... I disconnected the Hammond transformer, and inserted the Antek transformer in its place.

The result: it seems to work perfectly. Nice clean 100% sinusoidal modulation from 30Hz to well above 5 kHz, with the Ranger running at 40 watts (carrier level) output.

Amazing... this transformer is 1/10th the cost, 1/10th the weight, and much smaller in physical size than the Hammond transformer.

Antek makes transformers on toroidal cores capable of handling 800VA (32x the VA rating of the little one I am using). Furthermore, if you use a transformer with a turns ratio of 1:1 or 1:1.5 (etc.) you will have a traditional modulation transformer.

Important note: I am running the modulator in a modified Heising configuration, so there is no unbalanced DC going through the high impedance (output) winding of the transformer. I suspect that one would have transformer core saturation problems if one did not use a modified Heising configuration. [I did a quick check by switching my configuration from modified Heising to "standard"... and the output of my off-air monitor sounded very distorted]

Best regards
Stu


Title: Re: Inexpensive toroid transformer works as a modulation transformer!
Post by: WA1GFZ on January 16, 2008, 04:58:42 PM
Stu,
Early switchers used tape cores and the best ones were wound on cores with the thinner tapes. I'm pretty sure Variac cores are also tape cores and have the thicker tapes. The thinner cores gave the best high frequency performance. Who knows maybe a variac core is good enough. So you take a dead variac and count the turns for say 130 volts RMS across the whole winding. Now just scale the turns up and the wire size down. The only worry is the DC offset I bet a 20 amp variac could work pretty well at hundreds of watts and have plenty of room to wind wire.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: Steve - WB3HUZ on January 16, 2008, 06:18:34 PM
Very cool Stu. Great info. I've often wondered about toroidal transformers in modulator service. Just think how great they would work if they were actually designed and wound with audio in mind.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: ka3zlr on January 16, 2008, 06:48:16 PM
Very Nice Post Stu....good food for thought OM.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on January 16, 2008, 09:11:15 PM
Tape thickness of a tape core is thinner than E-I lamination so there are less eddy current losses. You still have to worry about dc offset saturation. no free lunch
A Toroid are harder to wind and isolating layers takes more work than an E-I bobbin.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: Opcom on January 16, 2008, 11:30:10 PM
variac ratings in their spec sheets indicate that many of them can be used at audio frequencies up to 4Kc.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 17, 2008, 08:50:22 AM
As an update:

I tested this setup (using the toroidal transformer) in more detail this morning. I measured both the output of the toroidal transformer (driving the plate and screen of the 6146 in the Ranger via a modified Heising configuration... all done using the 9-pin connector on the back of the Ranger, with no mods to the Ranger itself), and the output of my off-air monitor.

Results:

Toroid input: a 30-9500 Hz sine wave audio waveform from the 8 ohm output of a 60 watt (rated) audio amplifier.

Toroid output: 100% modulating the B+ line of the Ranger with 400 volts peak (800 volts p-p) of audio.

Frequency response measured at the output of the toroid (i.e., the modulation on the B+ line): flat from 30Hz to 9500 Hz.

Frequency response measured at the output of the off-air monitor (which includes the effects of the Ranger's screen bypass capacitor on the high frequency response of the Ranger): no low frequency rolloff at 30 Hz, 3dB down at 6 kHz, 6dB down at 9500 Hz.

Best regards
Stu


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: TedN on January 17, 2008, 09:54:05 AM
Stu,

Thanks for the R&D!

After trying to dig up plate iron for a power supply, I decided to purchase two plate transformers from Antek. One of the two was non stock but John at Antek said he could have it in three weeks. Four weeks later both transformers arrived. Having some background in the manufacturing of toroidal transformers, they look well constructed. John left the impression that they will wind whatever You want at little or no additional cost, and a short lead time. 

"Amazing... this transformer is 1/10th the cost, 1/10th the weight, and much smaller in physical size than the Hammond transformer."

Unfortunately for Hammond this is true.

Depending on the application, toroid's can be a better design choice. No monkey business with the assembly of E I sections, You toss the core on the winder and build.

BTW toroidal inductors typically have a higher Q.

Anyway thanks for the research. As it seems, supplies for this wonderful hobby are slowly drying up, it's great to find better alternatives.

73's
Ted / KC9LKE

Also don't buy Antek on Epay. I have seen the bid price exceed the list price.
Antek has a website. http://toroid-transformer.com/


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on January 17, 2008, 10:51:35 AM
Not all that simple. You have to be sure there is enough voltage isolation between windings and each end of a winding. Wire usually has a breakdown of a couple hundred volts so start can't sit on finish or you get a flash over.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 17, 2008, 11:31:11 AM
Addendum

Much higher permeability of the core material (vs. traditional core materials) =>

A. Much fewer turns required [i.e., V = # of turns x d/dt (total flux), and, for a given H-field level, the flux is proportional to the permeability]

B  Much less leakage inductance [i.e., the H field is roughly the same inside the core and just outside the core; but, because of the high permeability of the core material, the B-field is much stronger in the core than in the small gap between the core and the wires wrapped around it. Flux is the integral of the B field over the cross-sectional area.  Thus, the higher the permeability of the core material, the higher the percentage of the total flux contained within the core]

Much fewer turns required =>

C. Much more room for spacing between turns, and less layers (or only one layer)

Much more spacing between turns and less (or no) layers =>

D. Much less capacitance to spoil high frequency performance
E. Much more room for thicker insulation on wires
F. No arcing between layers (if there are no layers)*

The limitation is that these transformers cannot support unbalanced DC, because they will magnetically saturate. Therefore, the use of modified Heising.

Stu

*Even the little transformer that I am using is specified to have been hi-pot tested at 3500 volts.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on January 17, 2008, 12:35:55 PM
Stu,
Tape cores will give you the highest perm. I wouldn't bother with iron or ferrite. Magnetics Co. sells tape cores. There is a lot of data on their site. I seem to remember the flux density is similar to an EI core so again there is no free lunch. Stacking 2 cores cuts the turns in 1/2
I would think magnetic path length. The advantage may be the longer magnetic path length will reduce H for a given number of turns allowing more DC offset so a big toroid could make it easier to handle DC offset.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: VE7 Kilohertz on January 17, 2008, 02:14:34 PM
Stu,

Nice bit of research!! Thanks!  I have some 300VA toroids here, dual 120VAC pri and dual 24V sec. These may also work.

Refresh my memory..modified Heising is where the mod xfmr has a 1-5uF cap in series with the secondary to ground and the mod reactor feeds the B+ ..right?

Cheers

Paul


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: Todd, KA1KAQ on January 17, 2008, 02:27:09 PM
Despite the noise and fading here last night Stu, you sounded fine. Hope to hear you over the weekend for a better opportunity to hear it in action.



Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 17, 2008, 04:54:54 PM
Thanks guys... for the kind words of encouragement!

With respect to modified Heising...

Yes, the B+ is fed through a choke (to block the audio from being detoured into the power supply), and the audio is fed through a capacitor (to block the B+).

More specifically, it is important that the impedance of the choke at the lowest audio frequency of interest should be high enough so that it's impedance, in parallel with the plate resistance of the modulated tube (B+/the plate current), and in parallel with the impedance of the secondary of the transformer (associated with the magnetizing inductance of the transformer) should be greater than the effective source impedance of the modulator:

Example:

In my case, I have the following:

Heising choke: 20 Henries rated for 300ma.

Plate resistance: 600 volts/120ma ~ 5000 ohms

Magnetizing inductance (as reflected to the high impedance output of the transformer): estimated from the Antek specification sheet: 230 volts/.01A at 60Hz => 23000 ohms at 60Hz => 61 Henries (provided you don't saturate the transformer with unbalanced current or too much voltage)

Open Circuit Test (core loss test):
TEST CONDITION: Apply variable voltage to primary coil (115V terminals) from 115-140VAC at 60Hz. No load on secondary coils.
1. Primary V = 115VAC, Primary I = .01A or core loss is 1.0Watt.
2. Primary V = 140VAC, Primary I = .01A


Therefore, not yet including the effect of the capacitor (see below), and at the lowest frequency of interest to me... which is 30Hz...  I have 5000 ohms of resistance in parallel with approximately 30 Hz x 2 x pi x 15 Henries (i.e., 20H in parallel with 61H) ohms of reactance =

5000 ohms in parallel with j2827ohms

[Note that at 30Hz, more audio frequency current is flowing into the power supply or to ground through the reactances than is flowing into the plate of the tube. This is not necessarily the end of the world... but it does increase the burden on the audio amplifier as will be calculated next]

The transformer has a step down ratio of 230/12 = 19.2. Therefore the amplifier sees a load impedance that is 1/(19.2 x 19.2)  of the load on the secondary of the transformer. I.e.

Impedance seen by the audio amplifier looking into the 12 volt windings (in parallel) of the transformer =

[5000 ohms in parallel with j2827 ohms]/ (19.2 x 19.2) = 13.6 ohms in parallel with j7.7 ohms.

The audio amplifier that I am using is a modern amplifier that is specified to be capable of driving a load that is as small as 4 ohms. The fact that the load is mostly reactive makes me tempted to put a swamping resistor across the output of the amplifier... but I haven't done that.

Since the output impedance of my amplifier (4 ohms) is lower than the impedance of the load (down to 30Hz), the voltage that my amplifier develops across the load, for a fixed level of audio input to the amplifier, does not roll off for frequencies as low as 30Hz.

This is a well known aspect of the theory and practice of modulation (etc.)... if you drive a load from a relatively low impedance voltage source, the voltage developed across the load will not roll off (with frequency) until the load impedance drops below the source impedance.

From the perspective of the load (including the 6146 in the Ranger), the audio source impedance is 4 ohms x 19.2 x 19.2 = 1469 ohms.

With respect to the capacitor... there are some very enlightening discussions of what its value should be in the literature.

One school of thought is that the capacitance should be large enough to be invisible at the lowest audio frequency of interest. [Note: This will lead to a very low frequency, possibly high Q,  series resonance between the capacitor and the Heising choke (which are effectively in series, and across the transformer's secondary). This could create problems of oscillations at that very low frequency if there is enough of a feedback path at that frequency. An example of such a feedback path (for illustration) might be "talk-back" from the choke or the transformer into the microphone... and through the audio chain... if the audio chain passes such low frequencies.]

Another school of thought, that is particularly applicable to the case where the Heising choke's inductance is equal to the magnetizing inductance of the modulation transformer (which is not the case here), is that you should choose the capacitor to resonate with the Heising reactor at the audio frequency where the reactance of the inductor equals the plate resistance of the tube being modulated (Wow! That's a mouthful!). This will ensure that at the series resonant frequency, the Q of the resonant circuit is low (because of the loading associated with the plate resistance). It will also extend (somewhat) the low frequency rolloff of the total impedance across the secondary of the transformer. It's interesting to note that when the Heising choke's inductance equals the magnetizing inductance of the modulation transformer, the load on the transformer at the resonant frequency will be purely resistive, and equal to the plate resistance of the r.f. circuit being modulated.

If I were to use the above design rule... even though the inductance of my Heising choke (20H) is 3x lower than the magnetizing inductance of my modulation transformer (~60H)... then I would proceed as follows.

The frequency at which the Heising inductor's reactance equals the plate resistance is given by: 20 Henries x 2 x pi x f = 5000 ohms.

Therefore f= 40Hz.

If I wanted the capacitor to resonate with the inductor at f=40Hz, then I would need a capacitor whose impedance at 40 Hz is 5000 ohms. Thus I would need a capacitor whose value is 0.8uF. If I used a value of 1uF, then the resonant frequency would be a little lower... but this would have very little effect on the behavior of the circuit.

All of the measurements I described in my earlier posts in this thread were made with a capacitor having a value of 2.5uF, rather than the 1uF value calculated above.

[Note: I tried this, subsequent to my earlier posts, on January 19, 2008, by using four (4) 4.7uF capacitors in series, with a 100,000 ohm balancing resistor across each capacitor... to get a total value of a little more than 1uF.

The impact of using a 1uF capacitor in this application (where the magnetizing inductance of the transformer is 3x the inductance of the Heising choke) is that the load on the secondary of the transformer looks like the plate resistance (5000 ohms) in parallel with a capacitive reactance (instead of an inductive reactance, as would be the case if the capacitor were chosen to be larger) at frequencies near 40 Hz. The audio power amplifier seems to be happier with a resistive load in parallel with a capacitive load at frequencies near 40Hz... rather than a resistive load in parallel with an inductive load.... but there is some low frequency roll off in the modulation at frequencies between 50Hz and 100Hz that wasn't present when I used a 2.5 uF capacitor. The modulation frequency response is down around 3dB at 50Hz, and down about 6dB at 30Hz when I use a 1uF capacitor. It is essentially flat down to 30Hz when I use a 2.5uF capacitor.]

If you want to minimize the voltage between the capacitor and ground, the capacitor should be placed between the transformer's secondary and ground.

WARNING: The peak voltage between winding of the Heising choke (if it is a single unit, rather than multiple units in series) and its case (if the case is grounded) will equal the peak voltage on the plate of the modulated tube(s)... i.e., more than 2x the B+.

WARNING: The voltage across the capacitor, and also across the secondary of the modulation transformer can be several times the B+, under certain conditions, as described immediately below.

There is an important (to take into account) series resonance effect that will occur in a modified Heising configuration at low audio modulation frequencies. I.e., 60H of transformer magnetizing inductance in series with a 1uF capacitor forms a series resonant circuit at 20 Hz. Therefore, you should assume that the voltage across the capacitor, and the voltage across the secondary of the modulation transformer, may each be several times as large as the modulation being superimposed on the B+ (plus the B+ itself, in the case of the capacitor)... particularly if one tries to modulate the transmitter with a high percentage of modulation at very low audio frequencies. Given the effects of: the plate resistance of modulated the r.f. tube,  the series resistance of the transformer's secondary, plus the loading effect of the audio source across the primary of the modulation transformer (which acts like a load on the secondary of the transformer), and with a suitable bleeder resistor across the capacitor, one can keep the Q of this series resonant circuit low enough to keep the associated voltages within reason. For example, if the Q of the series combination at the series resonant frequency is 5, then (with 100% modulation with a sine wave at that frequency) the peak voltage across the capacitor, when it is series resonating with the magnetizing inductance of the transformer may be more than 5x the B+; and similarly for the voltage across the secondary of the transformer.

Given the above, it is a good idea to not apply a 100% sine wave modulating signal whose frequency is near the series resonant frequency discussed immediately above!

Best regards
Stu



Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: Steve - WB3HUZ on January 17, 2008, 05:21:55 PM
Typica values found in 250-1000 watt BC rigs are 40-60 Henry choke and 1-2 uF blocking cap. It's usually better to go with a larger amount of inductance and a smaller value of capacitance to avoid overshoot.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on January 17, 2008, 05:34:59 PM
I notice the Tom Vu 813 put the ckoke across secondary in series with the cap. This could limit the voltage across the cap since it is floating at B+. I'm not smart enough to know which configuration is best.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 17, 2008, 05:52:13 PM
Steve

Hi!

I agree that if I wanted to build a technology-faithful replica of a traditional broadcast transmitter, that I would follow the design rules that were used by the engineers who built broadcast transmitters in that era. They certainly knew what they were doing!

Some of us are trying to built plate modulated transmitters that are, in a manner of speaking, a hybrid between traditional designs and modern day technologies.

For example, we might be willing to use a modulator that is capable of delivering a lot more audio power than is required to modulate the rf tube(s)... in trade for the ability to use a modulator whose effective output impedance is lower than the plate resistance of the modulated rf stage. Perhaps not cost-effective if you are going to make a lot of these... but its just a hobby. In the grand scheme of things, building a home-brew a plate modulated vacuum tube transmitter serves no business purpose.  :)

By using a lower impedance modulating source, with plenty of reserve audio power... I don't need to worry about the reactance of my Heising choke being lower than the plate resistance of my Ranger's output stage at 30Hz.

I do agree that if I had a few more 10H 300mA Hammond chokes... I might use four in series, rather than two, just for the 'heck" of it.

As another example, we may want our transmitters to handle (with good linearity and modest roll-off) frequencies that are lower or higher than the frequencies that were handled in traditional broadcast transmitters (for no good reason, other than wanting our transmitters to have that capability).

As another example, we may be using RF decks that have a different plate resistance from that which was used in traditional broadcast transmitters. For example, just for fun, I might want to use a 4-1000A at 6000 volts and 150 mA, instead of a pair of 4-400A's at 2000 volts and 450 mA. I think at 4-1000A is a really neat looking tube.

Therefore, I think it is helpful to go back to the theory, rather than relying on the design rules that were based on assumptions which might not be valid for our objectives (which are to have fun).

Best regards
Stu


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: kc2ifr on January 17, 2008, 06:08:46 PM
Stu,
Thanks a bunch.....as usual u look at  "REAL WORLD" conditions.
The nay sayers are also noted.......
Bill


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: KA8WTK on January 17, 2008, 06:16:17 PM
While doing a little web surfing today I found this article. It talks about micro-gapped toroids that are resistant to DC saturation. ( Idc < 2A)

http://www.cmi-ferrite.com/News/Papers/PCIM1098-1.pdf

Makes you wonder if you could take a large toroid, saw it in half and glue it back together as a core for a modulation transformer.

Bill


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on January 18, 2008, 08:43:00 AM
I've seen toroid sold as 1/2 circle so you would band them. Hyperseal is another option and it is made the same just a different shape making it easier to wind a bobbin. Hey guys how about a microwave transformer core???


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: Steve - WB3HUZ on January 18, 2008, 03:13:36 PM
Quote
Therefore, I think it is helpful to go back to the theory, rather than relying on the design rules that were based on assumptions which might not be valid for our objectives (which are to have fun).


I was going back to the theory. It shows that using a larger cap produces overshoot. It has little or nothing to do with frequency response requirements.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WBear2GCR on January 18, 2008, 10:27:33 PM
I've seen toroid sold as 1/2 circle so you would band them. Hyperseal is another option and it is made the same just a different shape making it easier to wind a bobbin. Hey guys how about a microwave transformer core???

Are you thinking of "C cores"?

Dunno of any toroidal transformer coils that can be cut in half...

I know about "Hypersil" which is a high silicon content alloy...?
Then there is Permalloy, which is high nickel content.






      _-_-bear


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WBear2GCR on January 18, 2008, 10:49:01 PM
At the risk of raining slightly on the parade...

The experiment that Stu did does appear to work sufficiently well.
However, Stu's experiment was using a very small (low VA) transformer.

My estimation is that it makes it to 5kHz ok, and that is quite good enough for what we are doing. It may go higher in freq, but it did not appear linear higher up watching the output of my IF section on my scope - but then many ham transmitters run out of gas there too! Not a problem.

The limitation on using power toroids as transformers for audio comes when you try to use larger VA ones!

In general the limitation on transformers for audio is from Leakage Inductance, and Interwinding Capacitance. The LF limit is strictly a function of having an adequate number of turns and an adequate amount of inductance. That's usually not a big problem on power iron.

As your transformer size goes up, the two parameters above tend to get worse quickly. I've measured quite a few toroidal power transformers for frequency response, and they are not good. This includes some 125VA size and some 700VA size that I recall clearly.

One can usually judge the quality of a transformer by loading it properly, and then looking for the "bump" in response on the high frequency side. In the case of a good audio output or interstage transformer, one looks for the "bump" to be at 2X the max freq that you want to pass or better. That puts the bump up in the ultrasonic for a high quality transformer, somewhere around > 70kHz.,for a darn good output transformer.

There's no reason not to use a transformer that gets you only as far as the one Stu is using... but I'll be very surprised if larger units will get you past 2500Hz before phase shift sets in, and the response starts to droop.

This is why mod iron is different than power iron. And why mod iron is wound very differently than power iron. And also why building high power mod iron was very difficult and took many years and iterations before good and useful designs were found. In general, the higher the power, the harder it is to make a transformer remain broadbanded and flat out to and past 20kHz. (In this case we're looking for flat and smooth at least to 5kHz.)

For some interesting reading on output transformers, it is worth while to read what is in Radiotron, and also look up Partridge and his original work on designing output iron (similar issues to mod iron) - also there are quite a few neat articles on high power mod iron designs online and in the mags...

Just be cautious about grabbing higher VA power transformers and assuming that they will work as well as the little one in Stu's experiment.

                  _-_-WBear2GCR


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on January 18, 2008, 11:05:28 PM
If you cut a tape core in half you will have a mess but I have seen them many times come in two pieces. I've used cores like that to induce transients on a long cable. Audio transformers are an art.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 19, 2008, 09:22:35 AM
Bear

Hi!

Re-measured this morning using a stand-alone audio sine wave generator as input to the audio "power" amplifier that drives the ferrite transformer:

Test conditions:

Capacitance in series with the transformer: 2.5 uF

Audio signal source: A sinusoidal audio output signal from a GW GAG-810 audio frequency signal generator is used as input to a Samson Servo 120a audio power amplifier (rated 60 watts output into 8 ohms). The output of this audio frequency signal generator is displayed as the 1st trace on a dual-trace scope

Input-to-output chain: Audio output from GW GAG-810 audio frequency signal generator => input of Samson Servo 120a => output of Samson Servo 120a => 12 volt windings (in parallel) of 25VA toroidal transformer => 230 volt output winding (two 115 volt windings in series) of 25 VA toroidal transformer => modified Heising input of Johnson Ranger (Heising choke = 20H rated for 300 mA, coupling capacitor = 2.5 uF) => >90% modulated RF => REA off-air pickup/detector => 2nd input of dual-trace scope, in parallel with REA modulation monitor input.

Scope settings: Both inputs are AC-coupled. Scope is a 100MHz Tektronix TDS3012B. Traces are amplitude-overlayed when 1kHz audio is applied... by adjusting the fine scale setting of trace 2. Then, no further adjustments of the trace scale settings are made. The sweep rate is adjusted to accomodate (display) the audio frequency being used. Triggering is from trace 1. There is a time offset (delay) between trace 1 and trace 2 at observed low frequencies, corresponding to phase shift.The time offset at 1kHz is negligible.

Results:

From 25Hz to roughly 6500 Hz, the amplitudes of the traces continue to track within a few percent of variation. The audio frequency was adjusted smoothly, and there were no bumps in the tracking.

The output deviates from the input by less than 3dB (less than a 70% reduction in output amplitude) at 10kHz

There is about 30 degrees of offset between the input and the output at 50Hz, with the output sine wave leading the input sine wave.

I also applied a 125Hz square wave audio input signal... and I observed a droop decay time of 4 milliseconds (to the 1/e point) on the output waveform of the REA pickup/detector (trace 2). Note that the input waveform (trace 1) had a droop decay time of 12 milliseconds.


Best regards
Stu






Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WBear2GCR on January 19, 2008, 09:31:09 AM
Yeah Stu!

FB!

I think I'd prefer a resistive load for spec testing, but the ultimate test is still in the rig.

The only caveat I have is that it is unlikely - not impossible, just unlikely - that larger toroidal power transformers will exhibit such good results.

          _-_-bear


PS. I forgot to mention that there is a company called Plitron that makes toroidal output transformers for hi-fi. These would be very good run as a reverse mod iron, using a Heising type connection only. If you read the fine print on their specs you find that the DC offset permitted is extremely low - many hi-fi hobbyists seem to miss this little bit of fine print that they try to hide! (ha!)


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on January 19, 2008, 09:35:17 AM
A tape core will work fine at almost 10 times the flux density. Do the math anything else needs a lot more turns or core area. Also the right tape core will work out to around 20 KHz. Ferrite cores start getting useful above around 40 KHz.
E-I core does the best job of moving heat out of the core area.
Hypersyl is easier to build because you just have to strap the two halves rather than building up the E-I stack while you worry about hypot issues. Round cores are the hardest to make because you have to transfer the wire to a machine to wind it. A round core concentrates the heat under the wire but is also best at concentrating the field inside the winding. The metal of the core can be the same in all configurations but I have never seen E-I core with 3 or 10 mil thick slices.
 


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: KD5MHQ on January 30, 2008, 05:11:18 PM
Ok, my turn...
Been reading this thread with rapt attention...way cool! I'm gonna purchase 1 of these torroids for my 60W AM rig. I've been really interested in torroid xfrmrs for power and have only imagined them for modulation...now I'm convinced. Thanx for the R&D and for posting it here.

Bob, KD5MHQ


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 31, 2008, 08:53:17 AM
Bob et. al.,

Please remember:

1. No DC on any of the windings (must use a Heising configuration)

2. Select the turns ratio to be close to what you need (it doesn't have to be exact, because your audio amplifier should be chosen to have more than enough audio power to modulate the transmitter).

For example, suppose you are driving a 5000 ohm transmitter load from an audio amplifier whose output is optimized for use with an 8 ohm load (i.e., 8 ohms gives the maximum power out of the amplifier). If you use a 19.2:1  transformer (230 volts => 12 volts), like I used, then the load seen by the amplifier will be 5000 / (19.2 x 19.2) ~ 13.6 ohms.

This is fine... but the amplifier may not deliver its peak rated output power into a 13.6 ohm load. So it is good to have an amplifier whose rated output power is somewhat higher than 50% of the input power of the r.f. amplifier you are planning to modulate.

3. The "secondary" of the transformer should be rated to deliver the peak voltage you need to modulate the transmitter. For example, the transformer I am using has a 230 volt (rms) rating for its "secondary". This corresponds to 230 x 1.414 ~ 325 volts peak. The Ranger operates at around 550 volts B+ when I run it at full power. Thus, it would have been better if the transformer were rated for a higher voltage than 230 volts (rms). Fortunately, the transformer seems to work fine, when I run the Ranger at full power. Part of the reason for this is because frequencies higher than 50Hz (the frequency at which the transformer is rated) produce proportionately less saturation of the core (which is the limiting factor with respect to the 230 volt (rms) rating at 50 Hz), and therefore the transformer can produce higher voltages than 230 volts (rms) provided I don't try to produce those voltages with a very low frequency sine wave.

Of note: I plan to try this approach in a scaled up experiment. I ordered a custom transformer from Antek with the following ratings:

1300 volt : 57.5 volts (57.5 volts is 1/2 of 115 volts, so John will have to cut the standard 115 volt primaries in half)

Turns ratio: 1300/57.5 ~ 22.6

Core VA rating: 800 VA

Maximum rated output voltage at 50 Hz: 1300 volts (rms) x 1.414 ~ 1838 volts peak

Application: Modified Heising modulation of my existing homebrew legal limit amplifier (which uses a Russian GS-35b triode), re-biased to operate in Class C, with the B+ reset to 1500 volts. Power output at carrier: 375 watts (1500 volts x ~400 mA on the input). Audio power required for 100% modulation (delivered to the r.f. amplifier): ~ 300 watts.

The modulation impedance of the r.f. amplifier will be 1500 volts / 400 ma = 3750 ohms. The load impedance seen by the audio amplifier will be 3750 / (22.6 x 22.6) ~ 7.3 ohms. I plan to use an audio amplifier that is rated to drive loads between 4 ohms and 16 ohms... with a peak output power capability of 400 watts (hopefully, borrowed from somewhere).

Best regards
Stu
 


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WBear2GCR on February 01, 2008, 12:43:19 PM
I look forward to your experiment Stu.

But I'll be very surprised if you manage to get any decent frequency response from the larger transformer. Sure, if you wrap feedback around it, you'll force a response - but that's not "good practice" imho, since that solution is never satisfactory. But, I guess we'll see.

I sent an email to that fellow via his website, but never got a response!
Perhaps he only responds to ebay inquiries??

                          _-_-bear


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on February 01, 2008, 03:35:05 PM
Bear

Here is a "thought experiment" for you (although I might actually try this on a smaller scale). The purpose of this thought experiment is to demonstrate how a single large transformer can provide 16x the audio power, with the same performance, as a single transformer.

I have already demonstrated that one of the small Antek 25 VA, 230VAC (rms) : 12VAC (rms) toroidal transformers does a great job as a modulation transformer for my Johnson Ranger (i.e. flat frequency response between 30Hz and 10kHz, linearity excellent down to 30Hz, 100% modulation of the Ranger when it has up to 550-600 volts on the plate of the 6146).

Now, consider the arrangement... shown on the attached file... of sixteen (16) of those same transformers in a 4 x 4 array ( 16 x $8.00 per transformer = $128.00). This is, in principle, a quick and dirty way of doing the scaled up experiment... without having to wait for a custom transformer.

Each column (top to bottom) has four (4) transformers in series. The primaries are in series, and the secondaries are in series. So, each column represents a transformer with 48 volts in and 920 volts out. The turns ratio is still 19.2:1, just as in a single transformer (230/12 = 920/48 = 19.2). Each column, therefore, still represents an impedance transformation of 19.2 x 19.2 = 367 (just as for a single transformer).

Next, consider the equivalent circuit of a single column (see the attachment). The parallel capacitance of a single column is 1/4 of the parallel capacitance of a single transformer. [Four identical capacitors in series => 1/4 the capacitance of a single capacitor]  The magnetizing inductance of a single column is 4x the magnetizing inductance of a single transformer.

Next consider 4 columns in parallel (a total of 16 transformers). The turns ratio of four columns (or any number of columns) in parallel is the same as for a single column. Likewise, the impedance transformation of four columns in parallel is the same as for a single column.

The equivalent circuit of four columns in parallel has 4x the parallel capacitance of a single column, and 1/4 the magnetizing inductance of a single column.

Therefore, the complete array of 16 transformers has exactly the same parallel capacitance (1/4 x 4 = 1) and exactly the same magnetizing inductance (4 x 1/4 =1) as a single transformer.

The maximum voltage that the 4 x 4 array can produce at its secondary (which is limited by core saturation effects) is 4 times the voltage that a single transformer can produce (because a column consists of four transformers in series)

The maximum current that the array can deliver (limited by a combination of several effects) is 4 times the current that a single transformer can deliver (because there are four columns in parallel to share the total current).

Thus the array of 16 transformers has the same characteristics (turns ratio, frequency response, equivalent parallel capacitance across the output winding, equivalent magnetizing inductance across the output winding) as a single transformer... except the maximum power it can deliver will be 16 times as large
(4x the voltage and 4x the current) as a single transformer.

One large transformer (which doesn't cost all that much less than 16 small transformers) will perform at least as well as the array. It will weigh the same as the array, and it will be similar in physical volume (actually a little smaller) as the array.

Stu


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on February 01, 2008, 03:40:48 PM
Stu,
Check out the flat matrix transformer. The guy who held the patent Ed Herbert was my boss years ago. I think you can still find data on line.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WBear2GCR on February 01, 2008, 10:36:33 PM
Stu,

If your assumptions were correct for this situation, then there would have been no need to design Modulation Transformers differently than power transformers, eh?

Since there is/was a need to design transformers intended for wideband audio using different methods than power transformers, something must be missing from your analysis, one might surmise?

                         _-_-bear


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: Steve - WB3HUZ on February 03, 2008, 11:34:09 AM
Quote
wideband audio

This may be the flaw in your analysis. It seems you are thinking in terms of hi-fi audio transformers with high-end response out to 50k. Stu is looking to get response to 5 or 10k, all that's really needed to amateur radio voice work.


Just a thought....


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: Tom WA3KLR on February 03, 2008, 12:02:02 PM
Bear,

I think you are trying to (unknowingly?) equate the old iron-laminated transformer technology to the new powered iron (or ferrite) toroidal transformer Stu is using.  The toroidal core material must have a higher frequency response than the iron laminations, no surprise that this could be, I would think.  And we are comparing incidental use of power transformers as audio transformers.

I'd have to dig into my books to see if there is anything else governing the top end limit, but the core material, leakage inductance, and winding distributed capacitance are the main 3 factors.  I think all of these are inherently better with the winding you do on a toroidal core.  The E-I core really packs the wires together.

The toroid has more surface area for the windings so that all can be close to the core and spread out - wonderful.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on February 03, 2008, 12:09:42 PM
Better core material does have higher frequency response but it takes a lot more wire to get the same reactance so the leakage inductance monster will get you.
A good tape core will give you high permability like lron and good frequency response.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on February 03, 2008, 12:50:48 PM
Frank

I very much appreciate your inputs and comments. I am however puzzled by your last comment.

"Better core material does have higher frequency response but it takes a lot more wire to get the same reactance so the leakage inductance monster will get you."

Here's why I am puzzled:

A. The ferrite core material has a much higher permeability than any of the iron core materials. This is the most important difference between ferrite core transformers and iron alloy (e.g., hypersil) core transformers (tape wound or otherwise).

B. The much higher permeability of the ferrite core material means (by definition) that the B field is much larger in the ferrite core (vs. an iron alloy core), for a given H field. Therefore, to get the same output voltage (# of secondary turns x area of 1 turn x dB/dt) you can use fewer turns on the primary (i.e., less H field for the same primary current) and fewer turns on the secondary (even with less H field, the permeability of the material is so high that you still have a larger B field).

Example:

If the permeability of the ferrite core material is 81 x the permeability of a given iron alloy core material... then you can have 1/9th as many turns on the primary and 1/9th as many turns on the secondary (i.e., the same turns ratio). I.e.,

1/9th as many turns on the primary => 1/9th the H field (for the same current in the primary winding)

A permeability of 81 x that of iron alloy => the B field in the ferrite core = 1/9 (H field) x 81(permeability) =  9 x the B field in the iron alloy core (even though you have 1/9th the H field)

9x the B field => you only need 1/9th as many turns on the secondary to get the same output voltage. It also implies (of course) that the voltage on the input winding is the same (i.e. 1/9 x 81 x 1/9 = 1).

C. Because the permeability of the ferrite material is much higher than the permeability of iron allow core materials... you have less leakage inductance (not more)... because the (cross sectional area) fraction of the total B field that is in the core (where the permeability is very high) will be much even greater (vs an iron core) than the (cross sectional area) fraction of the total B field that is in the air outside of the core.

Leakage = Cross sectional area of the H field that is not inside the core x 1 (i.e., the permeability of air) / [(Cross sectional area of the H field in the core x the permeability of the core) + Cross sectional area of the H field that is not inside the core x 1]


D. The only downside, which I have pointed out in each of my posts in this thread, is that the ferrite transformer cannot accommodate any significant amount of unbalanced dc in any of its windings (because the high permeability ferrite core material will saturate)

Best regards

Stu


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on February 03, 2008, 02:32:59 PM
Mack

Thanks for the kind words....

All:

Check out this commercial audio output transformer (rated at 70 watts). It is an audiophile transformer... with extremely tight frequency response specifications... and, at around $400.00, I don't think it addresses our needs. However, looking at the diagrams, it doesn't appear to be any more sophisticated than the filament transformers that Antek sells for a small fraction of that price.

http://www.plitron.com/audio_4004.asp

Note: see reference 5 in the notes shown under the specification table

Best regards
Stu


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on February 03, 2008, 03:40:19 PM
Here is a free version of the complete paper:

http://www.next-tube.com/articles/Veen/VeenEN.pdf

You might want to skip to Section 5 of the paper to get closer to the punch line (section 5.6). For those who are interested in the theory, the entire paper makes a good tutorial review of the issues. I skimmed it rapidly... but I plan to go back and read it more carefully in the future.

Stu


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on February 03, 2008, 05:03:24 PM
Stu,
What permeability values are you using for iron core. I'm thinking hi silicon steel is up around 12,000 but have not looked at specs in years. I think 3CB, F etc are much lower. Also I think 3C8 and F can't handle the flux density of grain oriented   high silicon steel. I could be wrong


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on February 03, 2008, 06:38:52 PM
Frank

I've been searching on-line, and I also looked in:

Lee, Reuben "Electronic Transformers and Circuits", 2nd Edition, John Wiley and Sons, Inc., 1955

[The entire book is available for download from

http://www.pmillett.com/technical_books_online.htm ]

On line, I find values of relative permeability (permeability / the permeability of vacuum) of around 4000 for "transformer iron" in:

http://en.wikipedia.org/wiki/Permeability_(electromagnetism)

i.e. The actual permeability of "transformer iron" = 5 x 10**-3 Newtons / Ampere squared.  The actual permeability of free space is 1.26 x 10**-6 Newtons / Ampere squared. The ratio is ~4000.

In Lee's book (e.g. on page 30, Fig. 27; in Section 15. Core Materials) I find "grain-oriented" steel shown to have a relative permeability that varies from 2000 (at a Flux Density of 20 Gauss) to a maximum of 35,000 (at a Flux Density of 10,000 Gauss). This figure also shows a relative permeability for "silicon" steel that varies from about 1000 (at a Flux Density of 20 Gauss) to a maximum of 10,000 (at a Flux Density of 3000 Gauss).

[See the copy of Figure 27, attached below]

It could be that the strong dependence of the permeability of the above materials on the Flux Density is a source of non-linearity in transformers made from these materials. This would lead one (perhaps) to design audio transformers from these materials to work at low Flux Densities (where the permeability doesn't vary as much with the Flux Density)... which also correspond to low permeabilities. [E.g., by including an air "gap" in the core]. On the other hand, power transformers made from these materials could be designed to work at much higher Flux Densities (and, therefore, higher permeabilities) because non-linearities would be much less critical for power transformers.

In Chapter 5 (starting on page 140) of Lee's book: "Amplifier Transformers", he talks about Harmonic Distortion (section 67 page 153). On page 167, in Table XIII, he shows how harmonic distortion caused by saturation of the magnetizing inductance of an audio transformer increases rapidly as the Flux Density rises above 500 Gauss in a transformer employing a silicon-steel core. Note, from Figure 27 (below) the relative permeability of silicon steel is about 6000 at a Flux Density of 500 Gauss

Meanwhile, the Ferrite transformers probably provide a more constant (and also very high) permeability over the range of Flux Densities at which they are operated.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on February 03, 2008, 08:39:27 PM
Very interesting. I seem to remember we did 60 hz. power transformers 12,000 to 14,000 Gauss. E,I core is pretty unstable and maybe that is why it was never used for switchers. Tape cores were used for low frequency switchers and I remember thinner tapes gave the best performance. So the gap must even things out and allow the DC offset. All this stuff is a real art.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WBear2GCR on February 04, 2008, 11:17:30 AM
Bear,

I think you are trying to (unknowingly?) equate the old iron-laminated transformer technology to the new powered iron (or ferrite) toroidal transformer Stu is using.  The toroidal core material must have a higher frequency response than the iron laminations, no surprise that this could be, I would think.  And we are comparing incidental use of power transformers as audio transformers.

I'd have to dig into my books to see if there is anything else governing the top end limit, but the core material, leakage inductance, and winding distributed capacitance are the main 3 factors.  I think all of these are inherently better with the winding you do on a toroidal core.  The E-I core really packs the wires together.

The toroid has more surface area for the windings so that all can be close to the core and spread out - wonderful.

Tom,

I am unaware of anything other than tape wound cores being used in power iron. If there is some new core material being used for this, I'd like to read about it online, got an URL?

I wrote an email to Antek asking them about the core material, and no answer came back. Maybe I will try asking them via their ebay address...

------------------

Pliotron does make toroidal "audio output transformers" but if you read the fine print they can handle nil DC offset, making the idea of using them for PP service a joke! (even though people do...). Obviously without DC on the core, it's a different matter.

For a good overview of the issues in transformers, always refer to the "bible" - Radiotron Designer's Handbook.

                _-_-bear


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on February 04, 2008, 11:38:26 AM
magnetics makes tape cores. They have been around for many years.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on February 04, 2008, 11:55:01 AM
Bear

You wrote:

"Pliotron does make toroidal "audio output transformers" but if you read the fine print they can handle nil DC offset, making the idea of using them for PP service a joke! (even though people do...). Obviously without DC on the core, it's a different matter."

I believe the requirement is for nil unbalanced DC.

I agree that this is still a challenge, but in push-pull, the average (a.k.a. DC) currents going to each of the tubes flow in opposite directions in each half of the associated winding. While, traditionally, one might not worry too much about the balance of the average current flowing in each half of the push-pull circuit, with modern technology it would be relatively (compared to decades ago) easy to implement a simple feedback circuit to balance out the average plate currents in each tube (e.g., using Hall-effect devices). One could adjust the grid bias of one tube relative to the other to achieve this.

As an aside, I wouldn't mind having a way to separately measure the plate currents of the 810 modulator tubes in my KW-1... but that hasn't reached the top of my "to do" list.

You also wrote:

"I wrote an email to Antek asking them about the core material, and no answer came back. Maybe I will try asking them via their ebay address..."

I think that John, of Antek, being a small entrepreneur, is focused on delivering off-the-shelf products to folks like us, and on answering questions from prospective customers who are likely to place large orders. He sometimes answers my E-mails (asking to place an order for a custom transformer) and sometimes I get no response. I don't mind, because my orders are of little significance to his bottom line.

There is some information on Ferrite materials here. (See, for example, materials: W and H):

http://www.bytemark.com/products/ferrmat.htm

There is also a tutorial on the use of ferrite materials in transformers here. (See, in particular, the discussion of the use of ferrite materials in wideband transformers):

http://users.catchnet.com.au/~rjandusimports/tut_2a.html

Also, please refer to the table at this url:

http://users.catchnet.com.au/~rjandusimports/ft_mat_1.html

Note that the difference between the initial permeability (low values of Flux Density) and the maximum permeability (high values of Flux density) is fairly modest in ferrite material H... compared to the variation in the permeability vs. Flux Density in the grain-oriented steel and nickel steel curves I posted as a JPEG attachment in my prior post. This confirms that ferrite core materials can be used over a wide range of Flux Densities, where their permeabilities are very high, without the transformer suffering from the nonlinearities that might result from the use of (for example) grain-oriented steel or nickel steel (when used at Flux Densities where their permeabilities are high).

Best regards
Stu




Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: w3jn on January 08, 2010, 04:56:21 PM
BUMP for WB4BFS


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: N4LTA on January 08, 2010, 06:30:47 PM
Several months ago I asked a toriod supplier about making a modulation transformer and was told  that they could not do it due to DC current. I didn't discuss modified Heising.

I am just starting to test a 60 watt transmitter using a Hammond 123JSE as a traditional modulation transformer. It is physically large and I suspect it would do 100 watts in a modified heising arrangement.

The problem that I have run up against with the modified Heising,  is the chokes are harder to find than modulation transformers. Hammond has 10h at 500 ma but 4 of these is large and heavy and not cheap. I sell Hammond products and it is still expensive even at dealer cost. (If you need some Hammond stuff at a discount - let me know)

I may try 4 of them on my 4-400 rig that is under construction.

Pat
N4LTA


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WQ9E on January 08, 2010, 06:45:36 PM
Pat,

I had planned to use the Hammond chokes in series for a project last year but I came up with a traditional choke first.  But please let us know how it works out as the Hammond units are nice, readily available, and not that expensive. 

It is nice to know that there is a Hammond dealer on board!

Rodger WQ9E


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: KK4RF on January 08, 2010, 06:58:37 PM
Stu,
     Could you put your experomental AM rig on the air this weekend? How about checking into the Old Military Radio net this weekend on 3885 early Saturday morning? I would love to hear what it sounds like.
                       ---Marty, KK4RF---


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on January 08, 2010, 07:32:22 PM
Stu,
A better answer for your question. Looking at the tape itself. The thinner the tape the higher the frequency response. I have seen tapes as thin as 3 mils. This reduces eddy current losses and will blow the doors off an EI transformer core for audio response. Many years ago I worked on early switchers and we could make a very nice 15 KHz square wave using tape cores. I bet a tape core will make a good audio transformer but not cheap to wind. Magnetics Co. has some good application notes that go back 30 years on selecting the right tape core. I build a CD ignition that would throw a 1/2 inch spark. It had a 300 watt inverter wound on a tape core. It wiped out a stock coil in a couple weeks.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 08, 2010, 07:49:47 PM
Hi

As an recap (I've been using the rigs described below for over 1 year... this is an old thread that has been resurrected):

I've been using this approach to modulate my Ranger (via the 9-pin connector of the back) for quite some time now. I use a pair of Hammond 10H, 300 mA, chokes, connected in series (for the Heising reactor); and a 25VA-rated Antek transformer with a pair of 115 volt primaries and a pair of 12 volt secondaries. The secondaries are in parallel, and the primaries are in series... so the step up ratio is 230: 12 = 19.2 . The transformer costs $9.95 + shipping. The Ranger has a modulation resistance of 5000 ohms (600 volts B+ / 120 mA of average plate current). With the 19.2 step up transformer, the audio amplifier sees a load impedance of 5000 / (19.2 x 19.2) ~ 14 ohms. I feed the transformer with the 8 ohm (nominal) output of a solid state audio amplifier. I use an audio amplifier capable of delivering 120 watts into 8 ohms... because solid state audio amplifiers are relatively inexpensive these days. [No need to use an amplifier that can only deliver the required modulating power, with no room to spare]. The 25VA Antek transformer has not had any problems handling the power.

On my high power (legal limit) plate modulated 2 x GS-35b transmitter, I use the following: An off-the-shelf Antek step-up transformer rated at ~ 1kVA, with a pair of 115 volt primaries and a pair of 800 volt secondaries. The primaries are in parallel, and the secondaries are in series... so the step up ratio is 115:1600. This wasn't quite enough of a setup ratio (and Antek didn't have anything with a higher step-up ratio available off-the-shelf at that time)... so I feed this with another Antek transformer, rated at around 1 kVA, with with a pair of 115 volt primaries and a pair of 62.5 volt secondaries. This gives me an additional 2:1 step up in front of the other transformer.. Even with two transformers in series: 62.5 volts => 115 volts => 1600 volts (a total step up ratio of 25.6:1), these transformers behave as if they are ideal transformers (the magnetizing inductance is so high that it does not affect the low frequency performance, and the winding capacitance is so low that it does not affect the high frequency performance) for frequencies between 30 Hz and 10 kHz. I believe that Antek now offers a single transformer that would give me the same step up ratio. I use a Peter Dahl 50H 300 mA Heising choke in this application. In this transmitter, the modulation resistance is 1700 volts B+ / 300 mA average plate current = 5667 ohms. With the 25.6:1 step up ratio, the audio amplifier sees a load impedance of 5667 ohms / (25.6 x 25.6) = 8.65 ohms. I feed the transformer an audio amplifier capable of delivering 600 watts into 8 ohms. Obviously, we are talking about very high voltages... so extreme caution is required in putting all of this together and using it. The output of the Antek step up transformer (driven by the audio amplifier) is more than 2000 volts peak. The voltage across the Heising capacitor is more than 3700 volts peak (1700 volts of B+ on one side, and less than -2000 volts peak on the other side). The 50H Heising choke extends the low frequency cutoff down to 30Hz, but one really doesn't need that much inductance. I.e., in the case of the Ranger, I am only using a 20H Heising reactor, even though the modulation resistance of the r.f. output stage is about the same as it is in my high power transmitter. It is, however, important that the Heising reactor can handle the very high voltage across it; and between it's winding and ground.

In both cases, the performance is excellent, in terms of bandwidth and linearity. I usually monitor the output of the audio chain, and the output of my r.f. sniffer, simultaneously, on a dual trace digital scope. I scale and shift the audio trace to sit right on top of the top portion (envelope) of the rf trace. They track each other perfectly.

If you hear me on the air, and if I am not using one of the above transmitters, I will be happy to switch over to one of them to let you hear it for yourself.

Best regards
Stu


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: N4LTA on January 08, 2010, 08:03:11 PM
You can purchase a toroid core from toroid corp of MD with no secondary for very low cost and wind your own secondary.  The largest is rated at 1400 Va and is about $95 . 

It is huge.The cores come with a 120 volt primary prewound and taped They have 5 or so sizes.  I have used the big one for a power transformer for a 700 watt SS RF amp and have a smaller one for an audio amp (modulator) power supply.

Pat
N4LTA


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on January 08, 2010, 09:03:28 PM
Stu,
I bet you could add more turns using say #22 teflon wire to increase the output voltage at the expense of a bit more c between turns.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: w4bfs on January 08, 2010, 09:11:48 PM
BUMP for WB4BFS

Thanks John ... I enjoyed our qso on 40 mtr today ... your signal peaked at 40 over ... the TMC is very strapping here .... tnx fer all the dope about it

Hi Stu ... how are you doing? ... on the dual gs35 tx, have any winding insulation issues surfaced ?  ... are you using spark gaps or some form of neg peak limiter to prevent arcing ?    I am considering using some of these toroids on several tx projects


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: w3jn on January 08, 2010, 09:18:50 PM
Indeed, was FB!

What brought this up was a nice QSO today with KA1KAQ and WB4BFS, who mentioned the thread and said he couldn't find it.  I said I'd bump it to the top for him.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 08, 2010, 09:29:20 PM
John

Happy New Year!

No problems with arcing etc. on the 2 x GS-35B rig

I'm not using any spark gaps.

Note: The solid state amplifier acts as a voltage source... provided the current through it doesn't exceed the limits set by its protection circuitry. On negative peaks... when the modulated B+ goes through zero volts.... the audio amp is already sinking all of the current flowing in the Heising reactor. If the output of the audio amp goes further negative... it sees a high impedance (differential impedance) ... but it doesn't care. I decided that a negative peak limiter would serve no useful purpose in this configuration.

Best regards
Stu


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: w4bfs on January 08, 2010, 09:41:23 PM
ok Stu ... this is good stuff ... I just visited the Toroid co of Md website and their toroid kits look interesting ( thanks Pat ) ... might just be the ticket for a WELL insulated application as a modulator ... is the shipping tape (both clear and colored) we all use for shipping, mylar tape and would it be suitable for hv insulation ?


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WU2D on January 09, 2010, 08:56:42 AM
Great thread and real possibilities for compact rigs and mobiles! I use a backwards 6.3 V 5A filament transformer driven by P-P Mosfets in my ARC-5 mobile. It was not so flat! I employed modified Hiesing and feedback to straighten the response. I am sure that this kind of approach would help here too.

Mike WU2D


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: N4LTA on January 09, 2010, 10:39:59 AM
They (Toroid of Md) supply 3/4" wide thin mylar tape with the cores - it has no glue. They warn about winding too many turns and high voltage - so to go higher in voltage than a couple hundred volts you need more insulation probably.

I have used Scotch HV tape - it also has no glue - used for taping high voltage electrical power connections -  on RF toroids to 2000 Watts in tank circuits. It should work well and is like fiberglass cloth. I also used teflon tubing and inserted the magnet wire into the teflon tube. You can buy it on the net for exactly that purpose.

Pat
N4LTA


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: KM1H on January 09, 2010, 11:16:09 AM
The toroids Ive seen look far from precision wound which brings up the question...

Can stranded Teflon insulated wire be used? It is readily available in scrap yards for next to nothing, has silver plated wire (thats just a chemical reaction reason) and sizes 20 to 28 seem very common often in full reels.

Trying to stuff a half mile of solid wire down a teflon tube doesnt appeal to me....Im lazy ;D

Mylar tape comes in different widths and thickness so a single layer of sufficient voltage standoff with a proper overlap should be OK. Ive used this in regular transformers when I had access to a small local transformer company. Then the owner had the audacity to have a massive stroke and die. :'(

Carl
KM1H


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: WA1GFZ on January 09, 2010, 03:04:30 PM
Carl stranded wire works fine as long as you have plenty of room to fit the Teflon insulation. EI core you quickly run out of room. It is also easier to bend around a core when you are using large size wire.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: w1vtp on January 09, 2010, 06:35:09 PM
Quote
Therefore, I think it is helpful to go back to the theory, rather than relying on the design rules that were based on assumptions which might not be valid for our objectives (which are to have fun).


I was going back to the theory. It shows that using a larger cap produces overshoot. It has little or nothing to do with frequency response requirements.

Steve

What you said about overshoot was revealed during Paul, WB2SKC's PSpice modeling for my EICO 720 experiment.  We opted for a compromise of 2 MFD in conjunction with the Hammond 20 Hy choke -  using a 4 MFD revealed a big overshoot at the bottom end.

Al


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: w4bfs on January 16, 2010, 10:32:05 AM
This thread has been most thought provoking, thanks to all who have responded ...

The Antec 3500 volt between primary and secondary windings, no initiate any spark specification appears to be in response to UL / CSA voltage isolation specifications which led to insulated split bobbin EI lamination construction some years ago ... the question that is still most bothersome (to me, anyway) is the insulation between the secondary windings.

I am assuming that these dual winding secondaries are wound simultaneously in a bifilar fashion in a single pass (?) with some type of shuttle passing machine ... if this is so, then putting windings is series would tend to put high voltage stress on the wire insulation ... it seems to be a reliability issue to me  ... if the secondaries are single wound with attention paid to voltage gradient (with additional insulation ) then my 'objection' becomes a non-issue.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 16, 2010, 12:48:13 PM
On my high power transmitter, I am using an Antek transformer (as a step up / modulation transformer) that is rated at 120 or 240 volts AC in, and 800 or 1600 volts AC out.

In my application, with less than 1800 VDC for B+, I need less than 2250 volts (peak) output from the transformer to obtain 125% modulation on positive modulation peaks (i.e. B+ x 1.25).

1600 volts AC corresponds to 1600 x 1.414 volts peak, which corresponds to 2262 volts peak.

So... I'm below the rated output voltage of the transformer if I keep the modulation peaks to less than 125%

In my Ranger configuration, I'm using a transformer (backward-connected) rated at 120 or 240 volts AC in, and 12 or 24 volts AC out.

240 volts AC corresponds to 240 volts x 1.414 = 339 volts peak.

I'm running the Ranger with 500 volts B+ on the plate of the 6146 (when I run it in its full power mode). Therefore I need 625 volts from the Antec transformer to achieve 125% modulaton peaks.
In the case of the Ranger, I am exceeding the rating of the transformer by a significant factor. It is working fine, but... to be on the safe side (and also to obtain less saturation of the core at low frequencies)... one might choose a transformer with a 450 volt AC (or greater) rating on the output winding, while still having a sufficient step-up ratio (i.e., ~20:1).

Stu


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: flintstone mop on January 17, 2010, 04:20:38 PM
Velleman makes a kit amplifier using toroids for the output. Nice looking stuff
Fred

(http://site.tubedepot.com/k4040_f1.jpg)


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: w4bfs on January 22, 2010, 04:09:34 AM
John

Happy New Year!

No problems with arcing etc. on the 2 x GS-35B rig

I'm not using any spark gaps.

Note: The solid state amplifier acts as a voltage source... provided the current through it doesn't exceed the limits set by its protection circuitry. On negative peaks... when the modulated B+ goes through zero volts.... the audio amp is already sinking all of the current flowing in the Heising reactor. If the output of the audio amp goes further negative... it sees a high impedance (differential impedance) ... but it doesn't care. I decided that a negative peak limiter would serve no useful purpose in this configuration.

Best regards
Stu

so it seems that the solid state modulator output stage acting thru the step up transformer then cap coupled to the modulation reactor can 'snub' some voltage peaks that it didn't generate .... this sounds like damping factor to me ... can be a good thing

been thinking about this, some .... Tom, K1JJ also found out in a related thread that his solid state power amp could generate some large spikes at the end of transmitting to receiving mode .... this tended to fire mod xfmr spark gaps .... I believe he had to relocate his solid state amp away from the rf pa to eliminate the problem ... sounds like a line power transient or rf bombard

could be like the bucolic aphorism 'closing the barn door after the horse got out'

consider the inductor carrying current (ex: dc relay coil) ... when the current is shut off, a large magnitude reverse polarity voltage 'spike' is generated which is snubbed with a reverse connected diode ....end of problem

perhaps a series reverse connected resistor/diode combination (ala reverse peak limiter) across the modulation reactor does several good things .... helping prevent splatter (with the keep alive circuitry) and negative spike snubbing when at end of transmission .... this is likely more important with tube modulators since they tend to have lower damping factors

the connection to this thread is my attempt to increase system reliability by reducing stress on the modulator system


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 22, 2010, 09:51:20 AM
Beefus

Let's review the two "rainy day scenarios" that would cause arcing of a modulation transformer (or its associated spark gaps):

Scenario A: Overmodulation

The output of the modulation transformer drives the modulated B+ on the plate of the rf output tube to zero volts (relative to the cathode of the tube). The rf output tube cannot conduct in the reverse direction (it acts like diode with respect to current flowing from plate to cathode). If the modulator tries to drive the modulated B+ below zero, the load on the modulator will be higher (because the r.f. tube will not conduct in the reverse direction). The higher load on the modulator means that each additional mA of current produces a higher change in the voltage. Thus the modulator now has the ability to produce a higher voltage across the input and the output of the modulation transformer than it would if the rf tube were still conducting.

In this scenario, there is no sudden interruption of current (whether in Heising configuration or not). There is only an increase in the load impedance on the modulator when the plate current decreases to zero. This increased load on the modulator may result in excessive voltage being produced across the modulation transformer if the modulation being applied (in the negative direction) is large enough.

Scenario B: Loss of rf drive to the rf output tube.

If the rf drive to the rf output tube suddenly disappears when the transmitter is "on", then the output rf tube will not conduct current. Any current that was flowing from plate to cathode in the output rf tube, at that instant, will be interrupted. Because of the presence of large inductors (the Heising choke and/or the modulation transformers input and output windings)... this interrupted plate current will need to be diverted to somewhere. It cannot just stop, instantaneously. The interrupted plate current will be diverted into the modulation transformer... and, from there, into the output side of the modulator. If the modulator has a large output impedance (as in the case of a typical tetrode-based, class B modulator), the diverted current will produce a large voltage (much larger than the normal output voltage of the modulator)... which will cause a spark to occur across the modulation transformer's windings, or across a spark gap (if present), or across the modulator's output tube, or wherever the weakest point is for the diverted plate current to flow.


Now, getting back to the issues you are raising.

If the modulator has an output impedance that is intentionally matched to the modulation impedance of the rf stage... [which is the standard engineering practice in many traditional vacuum tube transmitters; because it maximizes the output power that can be delivered by the modulator, and therefore (for a given modulation power requirement) would minimize the cost of the modulator tubes, and the cost of the power supply for the modulator]... then scenario A will result in a doubling of the load on the modulator during excessive negative modulation peaks... and therefore twice the change in voltage across the modulator and the input and output of the modulation transformer during excessive negative peaks. If the components are marginal, in their ability to handle the voltages they need to handle... then damage to the modulation transformer might occur. [Note, in a tetrode-based modulator, without any feedback between the output and the input of the modulator, the peak output voltage swing is limited by the plate voltage; but the output impedance is very high] If we are using a design in which the output impedance of the modulator (after taking into account the turns ratio of the modulation transformer) is significantly lower than the modulation impedance of the rf stage, then the effect of the loss of the modulation resistance of the rf output stage, on excessive negative peaks, will be small. You still need to be concerned, independent of the diode effect produced by the r.f. stage, as to whether your modulator is putting out excessive current, and therefore, excessive voltage across the modulation transformer and across the modulated rf tube, when you modulate more than 100% in either direction. In my legal limit transmitter, the solid state audio amplifier (modulator) has a source impedance (after taking into account the step-up transformer) that is much lower than modulation resistance of the tube: less than 4 ohms x 25 x 25 modulator source impedance v. 1800 volts/300mA rf stage modulation resistance => less than 2500 ohms modulator source impedance v. 6000 ohms rf stage modulation resistance. On 100% positive peaks, the modulated B+ voltage across the tube is 3600 volts. If I were to push this above 4000 volts, I would be entering the territory where the tube is known to suffer catastrophic failures. This has, of course, nothing to do with negative peaks.

With respect to loss of rf drive to the rf output stage (scenario B), one has to look carefully at the output resistance of the modulator. As mentioned, in a tetrode based Class B modulator (e.g., a Johnson Ranger) the interrupted plate current of the rf output tube has no place else to go... unless it generates an arc. Increasing the plate voltage on the tetrode modulator tubes (caused by the inductors trying to maintain the interrupted current) has too small an effect on the plate current through that tube.  Therefore, scenario B implies a serious problem, that cannot be mitigated by a negative peak limiter. You need to ensure (or hope that) the drive signal does not turn off while there is plate current flowing. It is possible that the bias on the grid of the 6146 (in the Ranger) will increase (less negative) fast enough (if the rf drive is lost) to keep the plate current flowing.  

If the modulator has a reasonably low output resistance (1/ the change in modulator current that results from a change in modulator output voltage) then it can absorb the interrupted (rf stage) plate current that occurs in scenario B, without producing a large voltage change at its output.


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: w4bfs on January 22, 2010, 02:22:47 PM
great stuff, Stu ... if I am following along does not scenario B occur at each end of transmission if supply B+ is present unless some significant turn off sequencing is involved ? 


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on January 22, 2010, 05:25:20 PM
Beefus

Scenario B only occurs if there is current already flowing in the Heising reactor  (a large inductor with lot's of energy stored in its magnetic field) or (alternatively) unbalanced DC flowing in the secondary of the modulation transformer (a large inductor with lot's of energy stored in its magnetic field)... which is suddenly given no place to go (except to flow into the output impedance of the modulator or to arc across the easiest arcing path that allows the current to continue to flow).

If the transmitter is off, there will be no significant current flowing in the Heising reactor or the secondary of the modulation transformer (unbalanced DC or otherwise). Therefore there is no sudden disruption of existing current associated with the turn-on transient process. On turn-on, the current in the Heising reactor or the secondary of the mod transformer will slowly build up.

In the case of the Heising design, when the rf tube first begins to conduct, at turn-on, it will draw current from the Heising capacitor (if the other end of the Heising capacitor can deliver any current), or at least attempt to do so. This will cause the voltage on the tube side of the Heising reactor to drop below the B+ value. This will create a voltage across the Heising reactor that will cause the current in the Heising reactor to start building up to its steady state value, in an orderly way.

Violent effects should only occur if you try to suddenly discharge the energy stored in the electric field of a capacitor (not relevant here); or if you try to discharge the energy stored in the magnetic field of an inductor (by turning off the current that is flowing in that inductor).

Best regards
Stu


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: w4bfs on January 27, 2010, 02:17:49 PM
hi Stu ... I didn't know you taught and from the discussion I assume its engineering level courses ....so should I call you doctor Stu ? .... I have been thinking about this discussion and perhaps we are saying the same thing with different words .... I fully agree that no snubbing is required with high damping factor modulators (most solid state) unless enough energy were reflected back to exceed ratings or abilities of the ssa .... after talking with Reed W2CQH , he suggests the use of diode / resistor snubbing would likely be useful with a vacuum tube modulator, especially pentodes , due to much lower damping factors and might be able to obviate the need for spark gaps to protect mod iron...

wish I had time for more but I am an income tax preparer and am busy now ....73 ....John



Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: w4bfs on February 18, 2010, 10:27:33 AM
with apologies to those who may not be interested in re-surrecting this thread ONE MORE TIME ... finally got around to doing some hi pot testing on a small Antek toroid (dual 120 V to dual 12 V @ 25 VA ) ... the construction of this unit appears to be dual primary wound on first then layer of mylar insulation then dual secondary .... seems to be adequate insulation between primary and secondary and is spec'd so ..... the hi pot test (not spec'd by Antek) between the primary windings (one end to one end) showed leakage starting at about 1200 vdc ....this seems to indicate formvar( or enamel ) insulation only .... the construction method does not indicate a varnish dip of completed transformer .... this is probably ok for a 600 V class modulated 6146 and is the application suggested so far

I wonder how the higher voltage transformers are assembled ... are the paired windings wound together or one at a time with additional insulation ? ... assuming this is so is dangerous and could lead to premature meltdown :'(


Title: Re: Inexpensive toroidal transformer works as a modulation transformer!
Post by: AB2EZ on February 18, 2010, 12:11:01 PM
John

Good input! I'm not surprised that the two, unconnected, 120VAC windings of this 25VA transformer are showing leakage current starting to flow between them... when tested with 1200 volts between one winding and the other. When the windings are placed in series, the transformer is rated at 240 VAC end-to-end... but the voltage between the two windings would be 120 VAC. This corresponds to 170 volts peak. It is, however, good to know, specifically what the limitations of this transformer are. The larger 800VA transformer that I am using with my legal limit, plate modulated transmitter is rated at 800 VAC across each of the two separate output windings (1600 VAC when the windings are placed in series). That corresponds to a difference between the two windings of 1/2 x 2263 volts peak. I would assume that a hi pot test of the same kind that you conducted with the small transformer would again confirm a significant margin between the peak voltage that would be present between the secondaries in series, and the voltage at which leakage current flowing between them would occur. Antek now offers a toroid rated at 2000 VAC (output windings in series), which corresponds to difference between the windings of 1/2 x 2828 volts peak.

Stu
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands