The AM Forum

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.

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

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.

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.

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.

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.

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.

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.

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.

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?

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

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

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.

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.

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.

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.

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

http://www.toroid.com/

Carl
KM1H

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

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

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!

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

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.

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?

"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

No - don't worry about send the thread on a tangent. I'd like to see some of the experts jump in.

pat

It always works a LOT better to have way more audio power than you need.
Things are much cleaner that way, and you dont have any problem getting well over 100% positive modulation.

I like as much audio power as rf output power as a rule of thumb.

On the other hand, some guys on the air lower the modulation percentage for a better cleaner sound.
I dont agree with that, I like 90% neg modulation and the positive peaks can go wherever they want, that usually sounds good.

Excessive positive modulation can distort in the detector on the receive side, and very few people have gear that handles that well, so I suppose that would sound loud but semi-crappy in most shacks.

If you pay attention to the 1500 watt pep rule, I wonder if there is much difference at the far end if you run 250 watts carrier and 1500 watts pep, or 500 watts of carrier and 1500 watts pep???

Anyone ever run tests of this? What sounds better? What works better?


I cant say I am a big fan of the S series iron, special crap for hams is what I thought it stood for, different from the well made commercial stuff.
Even the commercial CVM 5 mod transformer is not very good, although it is good for the power.

Brett

While we are discussing UTC modulation transformers, I have often wondered about the concept that if one takes a modulation transformer designed to support DC on the secondary (like the CVM or S series), that one can deploy these transformers in a modified heising arrangement and then run them at 3 times the power.

So, by that theory, a CVM-4, with it's 300w rating and gapped core, could put out 900 watts of audio if the DC was run through a heising reactor?

This sounds way too high to me. Perhaps moving the DC off would give some kind of a boost to the 300 watt rating, but three times????

One of the reasons I question this is the 260W spec for the LS-67 transformer from the same vendor weighs more than the CVM-4, is designed with no gap for use with a heising reactor. And it is only spec-ed at 260W??

That three times number doesn't seem to add up.

If people said 20% more or something like that, it would sound more reasonable.

The DC on the core takes a lot of core capacity that can't be used for modulation. If you look at the Hammond SE transformers - the 75 watts one is  as large as the 280 watt push pull transformer.

As an example the largest Hammond SE 1642SE is rated 75 watts and weighs 28 pounds

The largest Push pull transformer is rated 280 watts and weighs 28 pounds

One has 300 Ma DC on the core and one has no net DC - That a difference of 370%  - so the three times may very well be true.

Pat
N4LTA

Saturation is one thing, but more power means more current and potentially more voltage. I'd look at the voltage specs on the CVM unit before cranking up the power.

I found the specs on the Hammond 2220 transformer. It was a "professional series" transformer with ceramic standoffs and mica insulation. I would bet it would do fine with a modulation reactor at 300 watts or so.

It weighs 28 pounds.

BTW KM1H - Toroid Corp just called me and said they could not build a reactor with a 2500 volt rating. The engineering guy said they did not have a meter that could measure 50h. That was somewhat of a surprise.

I believe it's the diff between ICAS and CCS, as well.  Three times, for slightly greater than SSB duty cycle. 

I did play with removing DC from solid state amplifiers broadband xformers.  It DID help, and efficiency went up.  SO, I would tend to agree... From what I remember, I got about 20 percent more power, PEP, out of a circuit when removing DC from the xformer.

--Shane

It takes 6dB to make an S-meter move one increment, so I guess the difference from 250W carrier and  a 500W carrier would not make much difference.

But an interesting thing was brought to my attention about the P.E.P thing. You could have a 1KW carrier and only modulate for a P.E.P. of 1500 watts. Hows about those apples???

FRED

Tube specs were quoting theoretical power and not the actual power to the load. They also did not include distortion power.

QST articles regularly stated to run substantially more than half the RF input and even quoted mod iron losses as around 20%.

Heck, Im running 304TL's to modulate 250TH's as well as a four pack of 813's. We wont discuss what reaches the antenna ;D but be certain its fully modulated.

Clipping the highs isnt going to do anything for the peak power of male human voice as its already falling off at 3 kc. The true power is in the lows. Cheap iron already kills the lows so if you have limited power put it into "communications" audio until you can score the right iron. Dont get carried away with lots of processing either with cheap iron, it doesnt like the high average power. If people tell you the audio sounds restricted just tell them its a Collins 8)

Im using a wet 7500W mod xfmr with the 304TL's, no worry about saturating ::) but Id still like to find something that doesnt need a cherry picker to move. Dont laugh...it was cheap!
All the shaping is done at low level ( 150-3500cps) and with just a pi-net splatter filter at high level. I can easily go 150% on the positives and see nothing nasty on the spectrum analyzer.

Before that I was using 810's and a 600W xfmr to modulate the 250TH's and even at 900W RF input I couldnt get the clean processed levels I wanted. Thats now with the 2 x HK-354 RF deck which is running at 600W and works fine.

In the late 50's I had the classic PP 250TH's with 810's and didnt know then what I was missing until I tried duplicating it a few years ago. And went thru a big awakening and actually read the old articles with understanding this time around.

Carl
KM1H