Modulation Transformer

[N4LTA]

I am building a modulator for a single 4-400.

My original Modulation transformer was a Hammond 2220 rated at 175 watts. It was a little smaller than I liked but a good solid transformer.

I picked up a UTC S22 on ebay rated at 250 watts for $48 and thought I had found what I needed .

But - The S22 weighs half the weight of the Hammond and I am a little skeptical now.

Most of the Hammond parts seemed to be rated at CCS and this is a big beefy transformer with ceramic standoffs.

I trust sheer weight over nameplate dat. Anyone have any input. I really need 250 watts minimum for the 4-400.

[Jeff W9GY]

I ran an S-22 modulating a single 4-400 with a P-P pair of 4-125's.  It was OK while it lasted.  But, alas, the C-22 finally died.  Maybe just old age, but I'm more inclined to believe the power rating is overstated.

Jeff W9GY

[KE6DF]

Looking at old UTC catalogs for weight info:

            Weight   Power

S-22       21        250
VM-4      26        300
LS-67     33        260

So as you move up the UTC quality pecking order you get heavier transformers for the same rating.

If the Hammond was rated for CCS and if it was designed to handle DC current through the secondary, then if you use it with a modulation reactor to keep the DC off the secondary you should be able to go over the 175W ratings a bit.

Frankly, both of these transformers seem a bit lite for use with a 4-400 to me.

Another issue is voltage. If you look in the UTC catalogs at the tubes recommended for use with the S-22 you see tubes like 211, 203a, zb-120, and 838. All of these tubes have a max rating of 1500V on the plates. Now I don't see ratings for the secondary, but one has to think in those days people were intending to use a pair of similar tubes on the RF side.

So if you try to run a 4-400 at voltages it is happy with -- say 2500+ -- and you shoot for 130% modulation you are probably way over the voltages the S-22 was designed for.

I'm conservative, and plan to use my LS-67 for a rig with about 175W output -- with 1250v tubes -- and then eventually build a big linear if I want to go really QRO.

[N4LTA]

The Hammond probably weighs 30 pounds or a little more. I plan to run the 4-400 very conservatively at about 1900 volts looking for a power input of 500 watts. The plate current should be roughly 260-275 ma.

That gives me a modulating impedance of 7000 ohms.

The Hammond transformer has 1 1/2" ceramic terminals on the power input side and it looks like it is built for pretty high voltage. It is rated 275 MA DC.

[W2PFY]

A local Hamboner in this area tells me that when you use a mod transformer with a hysing reactor, you can run 3X the audio through it.On my Scratche  Apache I use  a
60 watt Sansui audio transformer hooked up backwards and coupled trough a hysing reactor of 30 henrys . I'm able to modulate 150 percent and some around here say it sounds as clean as a class E rig. My point is that you would probably be able to modulate 400-500 watts input transmitter with that Hammond transformer.

[k4kyv]

A local Hamboner in this area tells me that when you use a mod transformer with a hysing reactor, you can run 3X the audio through it.

I would also mount it on insulators and let the core and case float above ground, and provide spark gaps across the primary.

It may do more harm than good to put the spark gap on the secondary side.  The inductive kick induced back into the primary by the abrupt short on the secondary side may puncture the insulation. If the gap is on the primary side, the short occurs at the voltage source instead of at the load, and the transformer is protected before the spike reaches the winding.

Use two spark gaps, one from each end of the primary winding to the midtap, rather than just one gap from plate-to-plate across the whole winding.  With one single gap, you have the same condition as placing the gap across the secondary; since only one class-B tube works at a time across half the winding, the primary winding would act like an autotransformer and induce a voltage kick.

[N4LTA]

Finding a modulation reactor is very difficult. I have been looking for quite a while. It probably needs to be 40h and rated at 2.5 -3 KV.

I had planned to put it on insulators - I just got a bag of 12 insulators with 1/4-20 inserts yesterday from Storm Copper.

I am a Hammond distributor and can get an audiio output transformer for use as a mod transformer IF I could find a reactor. I have a big heavy 225 watt OPT here. If I had the reactor - I'd use it with a solid state driver.

[KE6DF]

If you connect the full primary in series with the full secondary on your s-22 it might make a good modulation reactor.

I am planning on doing that trick with a Kenyon 225 watt modulation transformer. Putting all the windings in series gives me about 70hy.

Still have the voltage issue. The stand-off idea would probably help with that and so you might be able to use the S-22 as the mod reactor.

[W2PFY]

I use plain old filter reactors. One is 10 henrys and the other is 20 in series.I have the coupling cap on the one with the largest inductance.

It works.

[N4LTA]

I can get l hammond 10h filter reactors but they are rated at 1000v at 500ma - it would probably take 4 of them and that would be close to $300.00 and might not work?

Anyone got a source for a decent modulation reactor?

[W2PFY]

t would probably take 4 of them and that would be close to $300.00 and might not work?

Just a suggestion, start off with two and see how it plays  Everyone I know around here running under 250 watts are using just standard reactors.

[N4LTA]

The 1000 volt rating is what worries me on the Hammond chokes. I can mount them on standoffs but ??

[N3DRB The Derb]

I'm going to be using a S22 here soon, in a 2 hole HK257 rig. all of the S- series iron is on the light side but to be fair to UTC they were clearly adverted and spec'ed to be used for intermittent amateur and PA service action.

UTC clearly stated that if you needed more scrote, you needed the CG series or C series of commercial grade goodies.

having said that, I love s series iron. You just cant push em. BTW, if that was the recent S-22 offa ebay, I looked at it hard and my impression was that it had been rode hard and put away wet. i'd give that thang a little bake out in the oven at about 130 degrees before I put any coal to it. You can bet it got some dampness in it.

The early S-series iron with blue (first issue) or black labels and louvers are not sealed, the iron is just sitting there inside the case. the later models smooth light gray with no louvers and black labels are much better units sealed and compound filled. The best S-Series iron is the later models still with the connection info on the outside of the case.

I wouldn't buy ( meaning on ebay for $$$) any open case S iron unless it was NOS in the box and showed no signs of storage at all and the price is too good to not hit it. I have a lot of it.

The early blue label models have wonderful bluish-gray heavy wrinkle paint on them, but you usually never get to see one in good enough shape thats not filthy or rusty to know.

[N4LTA]

How much voltage are you going to run on the plate of the 4E27? If you ar elooking at 2000 or so. then your two holer will be very similar to my 4-400 single tube RF deck. I am thinking the hammond transformer that I have can handle the voltage and current. it weighs twice as much as the UTC S22.

Hammond has the SE output transformer 1642SE that would make a great modulation reactor. It's primary is 53 h at 300 ma . Hipot tested at 3500 volts. Weighs 28 pounds. Costs about $350.

I can get it at net but that's still a lot of money for a choke. Could run it with a 4-400 at 2500 volts at 275 ma for about  680 watts input - would need about 350 watts to modulate it. The hammond would probbaly handle that with no DC on the core. Have to go to 813s in the modulator and a new modulator power supply.

[N3DRB The Derb]

yeah probably 1900 jolts or so. with 2 KV each tube is good for 200 watts output. the nice thing is that you're doing that with only about 125 ma per tube so even 2 of them sucking down 250ma with a power supply that can really deliver honest 500 ma
you have enough left to run a common plate supply. 4-400 will do much more power of course, but then you have to go to 2 supplies.

I'm very confident the S-22 I have will be a good fit for the 257B's. It wont sound hi fi with big low end scrote but I never cared about that anyway. if it sounds good and can make 110% positive, I'm a happy camper. I'm a less is more guy - D-104 into the speech amp person.

[N4LTA]

Well keep us informed  as you progress - I thing I am going to start cutting metal on my modulator and power supply. I have a 1250 volt Hammmond 450 ma trnsformer to run the two 811As. I am going to use a 10H 500 Ma choke input and about 40 uF at 2000 volts on the filter. Will use three 1.5 KV 3 amp diodes in series in a full wave bridge (12 diodes total).

I will drive it with a Hammond 1615 15 watt high fi audio transformer in reverse and drive that with a solid state amp or maybe a dual 6L6 tube amp. Should take much to drive it.

[KM1H]

Have you tried this yet? Nothing wrong with a donut shaped choke if the price is right.

http://www.toroid.com/

Carl
KM1H

[ke7trp]

I get my Chokes from Robert  W0VMC.  He has a webpage. Give him an email. He always seems to come up with what I need at a very fair price.  Using a reactor will take any worrys about the iron out.

C

[N4LTA]

I have used quite a few toroid transformers from those guys. I have on hand a 1400VA core and a 300 VA core with just a primary wound on it (kit core)

I am not sure that they can handle the voltage. Might be able to strip the windings off and rewind the big core with teflon tape.

I'll check with W0VMC

Pat
N4LTA

[w5omr]

There's still the failing belief in the myth that if you run a 500w transmitter on AM, plate modulated, you only need 250W of Modulator.  This is true and the math bears it out, *if* you're modulating the rig with a sine-wave.  We humans don't speak in sine-wave.  The complex wave pattern that comes out of our vocal chords is rich in harmonics and is (check it with a scope) rather asymmetrical in shape.  This natural asymmetricalness (new word?) of our voice requires -more- modulator power, to properly modulate the final stage.

By following the correct polarity all the way from the microphone element through the EQ/mixer/speech-amp/driver-stage/modulator, you can have positive peaks reaching well beyond the 2-to-1 ratio.

100% modulation, the way *I* see it, is when the negative cycle hits the base-line on the scope.  Let the positive peaks run up to wherever they may.

Be careful though.  Not that anyone around *here* would do such a thing (*cough, spit, sputter*) but the law says "1500w pep at the antenna".  A 300w signal, modulated with a sine-wave to 100% and 2-to-1 ratio on the scope produces 1200w pep.  With proper attention paid to your audio circuit, from microphone to modulator, your asymmetrical peaks -could- hit 4-to-1 ratio on your scope.  With a 300w carrier, you're now looking at instantaneous peaks of much larger than 2.5kWpep!

[N4LTA]

Actually - I have seen the opposite stated - That voice on average requires less power than a sign wave.

I know that an audio amplifier can deliver more peak energy with speech or music than it can deliver sign wave energy.

[w5omr]

Actually - I have seen the opposite stated - That voice on average requires less power than a sign wave.

I know that an audio amplifier can deliver more peak energy with speech or music than it can deliver sign wave energy.

check here
http://www.qsl.net/wa5bxo/asyam/aam3.html
Don/K4KYV, Bacon, John/WA5BXO and Tim/W5TOB contributed

[KE6DF]

A 300w signal, modulated with a sine-wave to 100% and 2-to-1 ratio on the scope produces 1200w pep.  With proper attention paid to your audio circuit, from microphone to modulator, your asymmetrical peaks -could- hit 4-to-1 ratio on your scope.  With a 300w carrier, you're now looking at instantaneous peaks of much larger than 2.5kWpep!

I've seen this analysis before, and it makes good sense, but it brings up another point related to this thread.

How does one pick modulation transformers and tubes given this asymmetry?

The transformers and tube (class B specs) data sheets all assume sine waves.

So will a 300W spec-ed transformer work to modulate a final running 600 watts given the modulator really has to put out a lot more power than 300W on peaks?

Same question with tubes. If the tube you use, for example, an 811A as a modulator with 1250v on the plates, will that work to modulate a 600 class C stage given that the class B spec sheet says an 811a at the voltage is good for 310W? Or do you need more tube?

The specs for AB1 use of tubes for modulators seem particularly problematic in this regard -- especially if driven by a resistance coupled low power stage -- directly from a phase inverter using something like a pair of 6sj7's as is commonly described in old handbooks. Doesn't seem like there is any upside as these circuits can't drive the grids positive -- yet such circuits are used on many broadcast TXs so they must work.

[N4LTA]

That argument is confusing. The modulator that I plan to use can deliver a minimum of 250 Watts sine wave rms power. If it is called to deliver more instantantious power, it can do so.

The Radio Handbook and older ARRL Handbooks say that unclipped speach requires only a 1:4 ratio - where a sine wave requires a 1:2 and clipped speech less than 1:2

It is confusing to talk peak in one place and average sine wave modulator output at the same time.

It a lot like rating an audio amp at 800 watss peak music power when it can really deliver barely 200 watts RMS Sine wave..

In my situation - I have a variable high voltage supply on the 4-400. If needed, I'll crank down the variac until I can get what I need.

Many if not most of the commercial am rigs had just 50% audio power of the final input power. Some had less.

Correct me if I am wrong?

[KE6DF]

"That argument is confusing. "

I'm not arguing with you. I'm confused too by these discussions of asymmetrical peaks.

I'm wondering how one insures sufficient peak power from a modulator.

It seems like people think, build, and spec things differently now days as compared to the 1940's and 1950's when most of these transformers and tubes were designed and spec-ed.

In the old articles, it seemed sometimes like people went out of their way to clip the peaks and filter out low frequency and high frequency voice components.

And now days, we are trying to faithfully reproduce what 1950's ham filtered out.

Sorry if I'm sending  your tread off on a tangent.

dave