10-20 Watt Plate Modulation

[G3UUR]

Jim,
      Took a look at your schematics of the modified G50 and noticed the diode connected to G2. I can see why you might get a bit of distortion from it. The action is probably a bit abrupt when it occurs! Here in the UK we have trouble getting push-pull output transformers for anything other than quite low power because the larger ones have been scooped up by the audiophools. That's why I was forced to buy a Hammond 125J for my 2E26 AM rig. For lower power AM transmitters (5 to 10 watts) I use a mixture of different transformers, including a 70-volt line transformer, push-pull output transformers, and in one 160m/80m tx a small 240V/115V autotransformer. They all work well as modulation transformers at low power. We have an 8-watt output limit in the phone section of 160m here!

Mike, I looked up the 829 in the Radiodaze catalogue, but they only list the 829B at 12 bucks. The transformer from Fair Radio is cheap enough, though, so for a bit more output power the 829 or 829B push-pull modulator and parallel PA might be a good way for John to go. The 2E26 will give about 12 watts out for 20 watts in on the lower frequencies, so would be in the right ballpark for him unless he wants it for VHF where it will give a bit less.

Dave. 
     

[KX5JT]

My "Aircraft Advisory" transmitter arrived last night.   It's going to be a small gold mine of parts.  My problem is I can not locate ANY documentation on it whatsoever.   The final is indeed a 2e26 but it doesn't look good.  The plate cap is breaking off.  No big deal, those tubes are still cheap.  The transformers..... I need a quick lesson on determining exactly what I have for transformers.  Any tips?  The plate supply should be easy enough to figure out (put line voltage on the primary and see what comes out on the secondary)  but how would I go about figuring out specs on the modulation xformer?  (I am assuming the smaller of the two large xformers is a mod xformer but I could be wrong about that too.... maybe it's a LV supply although there does seem to be a couple smaller filament supply xformers)

There is a label on the chassis cover...

Make:      Composite
Model:     RLH1AA
Freq:       122.8 mc
Power:    10 Watts

Google turns nothing up on this at all.

[WD5JKO]

John,

   Can you take a few pictures of that "Aircraft Advisory" transmitter and post them here?  Maybe we can help you unravel some of the mystery. It sounds like quite a project which might be fun too. So what other tube types are in there besides a 2E26? I think you might have to reverse engineer that unit, and draw a schematic with pencil and paper stage by stage. A learning project you wanted, well you got one..

   Dave, that diode to the 6146 screen grid on my G50 does abruptly stop the downward swing of G2, limiting G2 min at about 25-30 volts. This limiting does not limit the downward swing of plate voltage though, so the modified G50 is now incapable of 100% negative modulation. Without the diode in there, an envelope pattern on the scope would show frequent peaks where the downward swing hit and exceeded 100%. Listening on a monitor receiver, I could hear 'clicks' whenever the scope showed negative overmodulation. With the diode in place, the 'clicks' are gone, as is the over modulation. Look at the waveform for 85% modulation. You can just begin to see the slope change that starts at about - 80%.

http://pages.prodigy.net/jcandela/G-50/85_mod.jpg

here it is with ordinary speech:

http://pages.prodigy.net/jcandela/G-50/YEA_AM.JPG

   I was using an unamplified D-104 with no processing. The diode allows me to lean into the audio a bit to bring up the average without undue distortion.

   OK on 8 watts out on 160m for you in England. I grew up in Detroit, and prior to 2M FM taking off, 160M AM was king for local communication. Mobiles used 1806 KHZ, and Base stations used 1835 KHZ. A mobile running 10-15 watts output with an inneficient whip antenna could get an easy 30 mile radius. Listening to those guys inspired me to get my license.

Jim
WD5JKO

[KX5JT]

John,

   Can you take a few pictures of that "Aircraft Advisory" transmitter and post them here?  Maybe we can help you unravel some of the mystery. It sounds like quite a project which might be fun too. So what other tube types are in there besides a 2E26? I think you might have to reverse engineer that unit, and draw a schematic with pencil and paper stage by stage. A learning project you wanted, well you got one..

..................

Jim
WD5JKO

Well the transmitter is in pretty poor shape.  Really I was wanting the Transformers and some tubes and tube sockets.... In fact I have brought the thing with me here at the dispatch office and I am tearing it town between phone calls....

So far the power transformer is ....

Stancor PC8411
117 volt primary
375-0-375 CT @ 150ma DC Secondary
5.0V @ 3 amps                    "
6.3V @ 4.5 amps                 "

There is a filter choke (looks like the power transformer but it is about 2/3 the size and only has two leads coming out of it.)

There's a small "Triad AX5 Input Xformer" marked
100 ohms primary Red Blue
P.P. Grids Green
C.T. Black

There's another small transformer about the same size as the Triad with no markings.

There's another transformer a bit bigger without markings.

Tubes pulled are all RCA
5U4 rectifier
2E26
6AQ5 x 3
12AU7 x 2
12AT7
5763
6BH6 x 4
0B2
6BJ6


Also have a few other things, nice ohmite pots... 1meg, 50K and 5K .. a relay.... all the tube sockets of course....

No, I don't want to "ressurect" this transmitter but I believe I have a few nice items to work with on something new.   I saw someone asking 75 bucks for the Stancor PC8411 and I certainly paid WAY less for all of this. 

[Gito]

Hi

When Jim used a power trans former as a choke and than reading Am Amazing Transformation By Tim Smith/WA1HLR.

There's an article using power transformer as a modulator transformer ,which can used a normal push pull Amplifier to modulate a normal class C transmitter.
So the Audio amplifier is more Efficient,can be class AB1,AB2 or class B
The modulating impedance is the square of the voltage turn ratio

Gito

[KX5JT]

Hmmm..

117 volt primary
375-0-375 CT @ 150ma DC Secondary
5.0V @ 3 amps                    "
6.3V @ 4.5 amps                 "


Okay I'm going to print and carefully read Tim's Amazing Transformation.

[Gito]

Hi

Remember the power input of the transmitter + power of modulator Power  (50% of the input power of the transmitter + looses in the transformer ) = must not exceed the power rating of the "Modulation Transformer" in this case +/- 100watt,so the transformer is not saturated at high modulation.
Also remember the voltage capability of the transformer isolation paper.So some Technician left it floating/isolate it from ground,so no high voltage is "close" to ground

I make a little modification for a 1 to 1 ratio Impedance of the circuit

Gito

[Gito]

Hi

Sorry I miss calculate the impedance ratio of the last picture I attached,

A 500 -0-500 seconder winding with a 115 v winding connected as modulator trafo like the circuits above has 1000 :615  is about 1.7 so the impedance ratio is 1.7X1.7 =  2.89

A 200 -0-200 seconder winding with a 115 v primair winding connected as mod.trafo has 400:315 is about 1.3 so the impedance ratio  is about 1.69 .

at the picture .the voltage ratio is 2:1 so the impedance ratio is 4

Sorry

Gito

[WD5JKO]

Well the transmitter is in pretty poor shape.  Really I was wanting the Transformers and some tubes and tube sockets.... In fact I have brought the thing with me here at the dispatch office and I am tearing it town between phone calls....

So far the power transformer is ....

Stancor PC8411
117 volt primary
375-0-375 CT @ 150ma DC Secondary
5.0V @ 3 amps                    "
6.3V @ 4.5 amps                 "

There is a filter choke (looks like the power transformer but it is about 2/3 the size and only has two leads coming out of it.)

There's a small "Triad AX5 Input Xformer" marked
100 ohms primary Red Blue
P.P. Grids Green
C.T. Black

There's another small transformer about the same size as the Triad with no markings.

There's another transformer a bit bigger without markings.

Tubes pulled are all RCA
5U4 rectifier
2E26
6AQ5 x 3
12AU7 x 2
12AT7
5763
6BH6 x 4
0B2
6BJ6

Also have a few other things, nice ohmite pots... 1meg, 50K and 5K .. a relay.... all the tube sockets of course....

No, I don't want to "ressurect" this transmitter but I believe I have a few nice items to work with on something new.   I saw someone asking 75 bucks for the Stancor PC8411 and I certainly paid WAY less for all of this. 

John,

   What a treasure chest full of parts. I bet one of those unidentified transformers is a modulation transformer. It might be worth looking at the wiring of the 6AQ5 plates to see if two of them go to a transformer. They might also use all 3 6AQ5's in parallel as a class A single ended modulator. If you haven't clipped all the wires yet, a little CSI work might pay off.

   You can take the unidentified transformer(s), and figure out which terminals have ohm-meter continuity. A low resistance usually means few turns, and a high ohm reading usually means many turns. You can easily excite the transformer with something like a 6.3v filament transformer, and then calculate turns ratio using a AC DVM looking at the voltages across each winding.

   I bet that with the parts you have already, you can make the foundation of a nifty little transmitter. Keep us posted on your progress.

Gito, I like your posts and attachments. We seem to be redefining Heising/Plate modulation by offering circuit variations to an old idea.

Jim
WD5JKO

[G3UUR]

John,
        I'm a bit puzzled by the make of the transmitter you've just acquired. Was it really as stated in your earlier post or was it Comco? They made quite a bit of aircraft comms equipment. I don't know much about the subject, but an old friend of mine runs a museum full of the stuff. He has lots of info on airborne gear, but doesn't have a PC or scanner and isn't on the internet, either. Your haul of components from this transmitter is pretty good and one of the transformers should be for modulation, as Jim mentioned. You'll probably find that two 6AQ5 tubes were used in the modulator if the transmitter is only rated for 10 watts. The Triad transformer is for a carbon mic to drive two 6AQ5 tubes in push-pull. The 5763 may have been used as a driver for the 2E26, but would make a nice 12 watt PA itself.

The push-pull Heising arrangement shown in Gito's post is a Collins design, I think. They used it in a 1950's design using, of all things, a pair of 2E26 tubes modulating an 829. The other arrangement with the 115V winding feeding the PA via a capacitor would need the PA to be run at a very low HT voltage to achieve full mod. The arrangement with the 115V winding stacked on top of one half of the push-pull winding is a much better design and would give the required step-up for 100% mod of a PA running at the same HT voltage as the modulator tubes.

A good arrangement for push-pull output transformers or CT power transformers used as mod transformers is to run the modulator tubes at a slightly higher HT voltage than the PA and couple to the PA HT supply through a capacitor with an AF choke in series with the supply, as in one of Gito's diagrams - sorry I can't be specific but the diagrams don't show up in posts on the topic summary when you're writing replies.

Dave.

[KX5JT]

John,

   What a treasure chest full of parts. I bet one of those unidentified transformers is a modulation transformer. It might be worth looking at the wiring of the 6AQ5 plates to see if two of them go to a transformer. They might also use all 3 6AQ5's in parallel as a class A single ended modulator. If you haven't clipped all the wires yet, a little CSI work might pay off.

.........
Jim
WD5JKO

Yes it is a nice score of parts!!! But in my haste to see what I had I did clip wires and pull parts.   Still it shouldn't be too difficult to figure some of this stuff out.  Part of my problem now is I have too many potential projects!!!  I need to priortize and sort them out in some order that makes sense.  You know how when you first get fired up about something, in my case AM and toob equipment, you want to do everything all at once?  That's where I find myself.  So I think I need to figure out an order that makes sense.

a.)  Modify my working station a bit.  I have a modified DX-60 that I think I might like to change the controlled carrier to pure screen grid modulation.  The SB-200 could probably use a little more bias by adding a couple diodes since I am using Svetlana 572B's.

b.)  I have a clean Johnson Viking I that tunes up properly according to the manual all the way  to the point of applying plate voltage.  I want to solid state the powersupply and work through it's issues and get it on the air as my first plate modulated rig.

c.) Build up this low power plate modulated "exciter" for an amp from scratch based on the parts from this Aircraft Advisory rig.

d.) Build a legal limit plate modulated rig after I have been successful with the lower power one.  Someone advised maybe to skip the lower power one and just learn by doing the big one.

[KX5JT]

John,
        I'm a bit puzzled by the make of the transmitter you've just acquired. Was it really as stated in your earlier post or was it Comco? They made quite a bit of aircraft comms equipment. I don't know much about the subject, but an old friend of mine runs a museum full of the stuff. He has lots of info on airborne gear, but doesn't have a PC or scanner and isn't on the internet, either. Your haul of components from this transmitter is pretty good and one of the transformers should be for modulation, as Jim mentioned. You'll probably find that two 6AQ5 tubes were used in the modulator if the transmitter is only rated for 10 watts. The Triad transformer is for a carbon mic to drive two 6AQ5 tubes in push-pull. The 5763 may have been used as a driver for the 2E26, but would make a nice 12 watt PA itself.

.....
Dave

Hi Dave,

There is a name plate under the black wrinkle flip top lid that clearly states

Make:      Composite
Model:     RLH1AA
Freq:       122.8 mc
Power:    10 Watts

All the info posted about "modified Heising" is really great stuff.  I've looked at Timtron's document on the Amazing Transformation as well as Steve HUZ's A Quick and Not So Dirty Path to Hi-Fi Audio which apparently outlines the same modulation technique but with an existing audio amp.  Very juicy and pertinent.  I am learning a lot!

John KX5JT