Screen grid modulation questions?

[N2DTS]

Geo,
Do you need parts to build an AM rig?
What power level are you looking for?

In the US, most of the AM activity used to be around 7290, when the band was open, weekend mornings.
I have worked ve3owl? a number of times lately.

I have a bunch of parts for smaller type rigs I have no use for, share the wealth I say.
I could hook you up with tubes and mod trans if it helps.

Big stuff I dont got much of, but little parts I do.

Brett

[The Slab Bacon]

here is a PDF of the basic design from it's basic inception done for me by Steve a few years back. I have made a few minor changes since this drawing, but this is basically it.

[N2DTS]

Thats a neat design!

Brett

[The Slab Bacon]

Thats a neat design!

Brett

the only difficult item to get is the ARC-5 mod tranny. But there are other ways around it. I used it cause I had it. that rig's been in operation close to 10 years now.

[K1JJ]

Yep, interesting design, Slab.

Back in the early 70's I had no mod iron. I once ran a 4X1 in class C with composite screen and grid modulation.  I used two standard (small) 8 ohm to 5K?  audio output transformers. These can be found in standard tube radios, etc.  I hooked the 5K sec of one xfmr to the 8 ohm primary of the other in parallel. Then fed this junction with a stanadrd SS PA amp (8 ohms output) and hooked the opposite 8 ohms in series with the grid and the other 5K in series with the screen.   I called it composite screen/grid modulation at the time. It worked FB.

To test your peak efficiency - it's easy.  Just do a sustained whistle to get a peak power reading on your peak reading meter and also note the PA current and voltage. Then do "peak power out /  (E X I) in ."   I expect it will be in the 60%-65% area. If it's more, then all the better. Your plate color will probably not change much since it's an efficiency technique.

Anyway, the reason I'm wondering is I find I can bias my linear into deep class C operation (without a grid leak) and on AM ONLY, get a nice linear waveform out approaching 65% eff - but the dead carrier eff seems better, like what you are seeing..  Being a triode there's no screen to modulate, so I'm wondering how close they are in overall similarity.  The linear's overall plate heat is reduced compared to running the linear in class B.  The constant carrier acts like a bias to idle the tube outa class C, so to speak. I usually add diodes to the fil CT to bias it heavily.

T

[N2DTS]

Really?
Seems odd!
But if it works, put a switch on it to remove the bias for AM use, and allow the diode bias for ssb use?

In my new 4x813 amp, I have ten 3 amp 1000 volt diodes in series to give 7.5 volts bias on the cathode.
( I got 100 of those diodes for $5.00 off ebay)

I get very little resting current without any drive, 0 to 40 ma, I never looked closely.

You are using an adjustable bias source and cranking it way up and get a clean signal out with modulation?

Brett

[K1JJ]

Brett,

I have about 200  3A 1KV diodes on a plexiglas board with a rotary switch to short some out as desired. They go in the fil CT as bias.  I can select from class A all the way down to deep class C.

Try it.... add lots of diodes until you are way below resting current into C on your linear amp. Then drive it with some carrier until you are at the normal carrier output level. You will need more drive than a normal class B config, of course. Then run some audio tones through. You will find they look FB.  This will not work on ssb, of course, and cause severe cross over distortion.

The AM carrier gives the final a "resting idle" to form the audio around on the load line. I believe the carrier is operating in class C, which helps the dead carrier eff, though the audio peaks are probably still in the class B eff category due to having to be linear.

Brett, please try it and take some careful measurements. If it works like I think mine does, then it will be worth publishing it around so that the many AMers using linears can increase their dead carrier efficiency. Right now, class B linear carrier eff at 25%-33% really sux...

T

[w4bfs]

I found that when I tried the controlled carrier modulation, what was "modulation" was sort of a very abrupt analog to digital reaction, where my analog signal (audio) was being interpreted into a stream of highs and lows (as I saw on the scope). Sort of like a Pulse Width Modulation.

But the effect on the sound quality was absolutely horrible! It looked great on the plate current monitor but it was impossible to call it modulation. Digitization seems more accurate a description.



What I ended up trying was placing a 15k resistor in series with the first 6DE7's (the tube I ended up using was not a 6DE7, it was a 6BZ7/6BQ7, which is what I had on hand to try) cathode-ground connection and reduce the 22 Meg resistor down to something less than 1 meg. This let the first tube behave in a more linear fashion. But it was still not good enough to give me more than perhaps 40% modulation.

Somehow I'll have to do something to improve my situation. Just not sure what now.

73s
geo

hi John ....just got your message ... just refreshed my memory with the WC3k mods to a dx 60

I think there may be a typo on the carrier control pot value ... 50 ohms seems way too low .... more like 5k or 50k

notice that the 6de7 stage a cathode is grounded .... this forms grid leak or contact bias which varies with audio level in and shifts plate voltage q point .... lift the cathode and place a 2k pot to ground and this will allow you to vary bias from grid leak to about 2 volts which should keep the plate q point steady and defeat the controlled carrier aspect...

ok Geo ....the 6bz7/6bq7 is a small twin triode for low voltage / low power applications, not suitable for this job .... what you really need to have as the cathode follower that feeds the pa screen is a low mu triode or a tetrode connected as a low mu triode, quite similar to the discussions about driver tubes for class B modulators that have been here lately ... so what do you have ?  ....

[N2DTS]

I love when someone builds something, so I plan on sending geo a crap load of parts.
I have lots of small bits like mod transformers, tubes, sockets, meters, resistors, etc.

I will go through the junkbox and see what I have, I think I have a nice 120 watt multimatch mod trans, a bunch of old hifi type tubes I will never use, some dx100 parts, lots of 812 tubes, some 4D32 tubes, but no sockets for the 4d32's I think, a bunch of 6146's, plenty of 6as7's?, some 6b4's, and lots of other odds and ends type tubes.

I have a LOT of old meters.

So he could do plate modulation with something, lets say he uses a paair of 6146 tubes in the RF section, what to use as modulators?
I have some 807's I think, 6146's would work but tend to sound crappy, some little zero bias triodes and nix the screen and bias supplies?

Brett

[K3ZS]

I am plate modulating a Globe Chief (2 X 807) with an all transistor Radio Shack 40W PA amp I picked up at a garage sale for $3.   The mod transformer is from an old Johnson business band amplifier that used 2 X 2N3055 to modulate a 12V 6146.    I wonder if you could use an output transformer backwards with a solid state amp if it had adequate voltage and current rating.

[VE3GZB]

ok Geo ....the 6bz7/6bq7 is a small twin triode for low voltage / low power applications, not suitable for this job .... what you really need to have as the cathode follower that feeds the pa screen is a low mu triode or a tetrode connected as a low mu triode, quite similar to the discussions about driver tubes for class B modulators that have been here lately ... so what do you have ?  ....

I tried the circuit afterwards using a 6AS7. Not much difference in results.

[Gito]

Hi
Reading ,how you try to screen  modulate your transmitter,reading the ARRL hand book,it is the same as choke coupled modulator on the plate but used it at the screen .
in tha ARRL hand book it is called Clamp -tube modulation . but the choke is replaced by a resistor .

Just a though ,You used an 24 v 5 watt lamp as the Plate meter,is it possible that the Resistance of the lamp ,is high enough .So when the plate current of the 6146 gets high when You modulate the screen.There 's much drop in the Plate voltage of the 6146.
So it 's acting like a"negative feed back" on the RF output.

Gito.

[VE3GZB]

It's a possibility but the lamp doesn't get bright/hot enough to illuminate to the 24 volt level it's designed to. So I don't think it's a major issue at this point.

[Gito]

Hi

Reading Your Article,modulating  the screen,it's the  right way,
so I"m still curios ,it is still possible that there is a" resistance" in your B+ or,it's the power supply that's goes" up and down" when you modulate your transmitter so the Effect is still " negative feed back" on Your RF output,when You modulate

Gito

[N2DTS]

I dug up a utc vm2 (new in box) a bunch of 807's and 1625's, some sort of utc s series driver transformer, and a bunch of other parts i have loads of, including some meters (ditch the bulb).

A lot to box up, so it might take me a while to ship it.

Brett

[N2DTS]

I took the boxes to fedex, and they wanted $230.00 to ship this stuff!
I then went to the post office, and it was not much cheaper.

I had no idea it cost so much to ship some junk, its not far from NJ...
Is there no way to get it to Canada without breaking the bank?

Brett

[KX5JT]

I had no idea it cost so much to ship some junk, its not far from NJ...
Is there no way to get it to Canada without breaking the bank?

Brett

Catapult?  

[VE3GZB]

I took the boxes to fedex, and they wanted $230.00 to ship this stuff!
I then went to the post office, and it was not much cheaper.

I had no idea it cost so much to ship some junk, its not far from NJ...
Is there no way to get it to Canada without breaking the bank?

Brett

You've got a PM.

[The Slab Bacon]

Anything over 150-200lbs it is definately MUCH MUCH cheaper to use truck freight.
they charge you by hundredweight. The initial minimum is kinda pricey, but after that it only costs a few more bux per 100lb after that.

Skip (K7YOO) sent me a "package down" here for Derb (and 1 piece for myself).
We had it shipped to my shop so I could unload / load it with a forklift.
The crate was somewhere over 400lbs and the shipping was somewhere around
$125 IIRC

                                                     The Slab Bacon

[N2DTS]

Well, I have 2 boxes, one is 40 pounds, the one with the iron in it, the other is under 20, (tubes and parts).
For that, $230.00!

Maybe because its going to Canada?

Brett

[VE3GZB]

Scary!! Hmm, well then, don't worry about sending anything then, ok. I appreciate the offer very much, but these high prices today are killing everyone.

[Gito]

Hi

You have a 6146 transmitting tube and a 6L6 audio tube,here's a schematic diagram of Globe Scot 65 -plate modulated,Hopped it can be  used in your transmitter with a little change here and there,in the globe Scot 680 it used a 6AG7 as the oscillator.
 
Gito,

[VE3GZB]

Gito - thanks! I'll see what I can do with it! I'm also using a 6AG7 osc. so it looks very hopeful!

73s
geo

[VE3GZB]

Gito - the lower part of the schematic's parts list is missing, do you have the lower half of it?

[Gito]

Hi Geo

Looking at the schematic I Attach.it looks complete to me,but i"ll attached another one


Gito