AMFONE group transmitter project

[W4VLJ]

Hi Everyone,

This is an extension to my earlier thread entitled "where to modulate".  I received so much great advice, I have decided to document my project on this forum, and ask for lots of help here.  Hopefully some of you will enjoy participating, and thanks in advance.

Here's where I am so far:

I am going to modulate the final amplifier.
I am going to use a pair of 4-400's to modulate another pair of 4-400's
I am going to design the transmitter for 1000W carrier so I am not stressing it at legal output power.

"pieces" I have so far:

3000V 1A power supply from a TV transmitter
5V 60A power supply, original application unknown
rack mount panel meters from a Henry amp, model unknown

Questions I have now:

1. What size modulation transformer should I be using?  I have heard 500W and 1000W?
2. Can anyone recommend a circuit for the preamp (mic amp?) and driver amp?  I have various handbooks from the 50's and 60's I am reading through.

Just some background - I am an EE, so I understand the safety precautions involved with this project.  However, being 48 I did not grow up in the "golden age" of equipment building, and I have spent my career working on low power, low voltage VHF/MW amplifiers (cell phones).  The hobby starting with the 60's equipment has been about all new devices, new math, etc., and I can actually see what I am doing without a microscope, which is why I enjoy the vintage equipment so much.  With this project I am stepping back in time a little further..

Thanks,

Mike

[w8khk]

Hello Mike, and welcome to AMfone! 

The tubes you have chosen for your rig will be conservative at the power level you wish to run, and should be easy to achieve a stable, clean sounding rig.

The size of the modulation transformer will likely be dictated more by what you find available than any other factor.  Anywhere between 500 and 1000 watts should be fine.  Depending upon the transformer specifications, you may want to also consider using a modulation reactor to eliminate DC current on the secondary of the mod transformer.

If you were using triode modulators, the driver design would be much more demanding, considering the varying load when the mod tubes draw grid current, in class AB2 or class B operation.  Tetrodes could be run either AB1 or AB2, so many options are available.   Here is an idea you may wish to research further:

I recall a QST article in the 50s or 60s that described a successful modulator design with a pair of 4-250s running class AB1 with about 3KV on the plates.  It produced in excess of 500 watts of clean audio with RC coupled receiving tubes driving the 4-250 grids.  The entire modulator, sans power supply, was contained on a single deck in the 19 inch rack cabinet.  Another advantage of eliminating the driver transformer and associated tubes is the ability to apply some negative feedback without cumulative phase shift issues over multiple stages.   If you cannot locate that article, I will try to help you find it as time permits.  Good luck on your project, I know you will receive lots of quality assistance on this forum.

[K1JJ]

Hi Mike -

4-400As  X 4-400As is a time tested lash up and should work FB.

Be sure to use well regulated screen and grid supplies for the modulators, assuming you run them as tetrodes.
A simple 50w zener of the correct voltage in the modulator cathode is all you need for a stiff grid bias.

Here's a simple MOSFET screen regulator circuit that I use in three different rigs. It will give you a solid, adjustable 300-400V for AB1/2 operation.  About 1/2 way down the page, look for the two MOSFET circuit just above the circuit marked "Circuit Number 2".

http://www.tubecad.com/2007/06/blog0109.htm


To eliminate the audio transformer (phase shift) problems and produce a solid audio driver signal for the 4-400As, then consider this solid state MOSFET audio driver linked below.  I use it with my present 4-1000A plate modulated rig (Fabio II) and works perfectly.  The good part is without audio driver transformers, I can tap the audio negative feedback from the mod xfmr secondary - and run it all the way back to the low level SS audio driver.  Usually, only one transformer can be used in a chain before NFB becomes difficult. (Just the mod xfmr in my case) Makes a big difference in performance to have a full NFB loop like this.

http://amfone.net/Amforum/index.php?topic=23632.0


Try to find a 1KW mod xfmr, if only for longevity and the ability to make an occasional mistake. There shud perfect BC  matches available for 4-400As. Use arc gaps on both the pri and sec - and even on the Heising reactor.

Consider a solid state screen, grid and plate fault shutdown circuit. Look at my Fabio II thread for info on how to use opto-isolators and cheaply available relay boards to pull this off.  My 4X1 is bullet-proof to mistakes now. Any current surge in the plate, screens or grids of either the modulators or finals shuts down the PTT sequencer.

Look at page 8 and 9 for more details of the shutdown fault system:
http://amfone.net/Amforum/index.php?topic=33816.0


Consider vacuum variable caps for the plate and loading.  With turns counters you will be able to QSY bands quickly as well as make it more compact and handle max power.

I'll add some more comments as you figure out what parts become available.

Good luck!

Tom, K1JJ

[wa3dsp]

Tom,

 Curious about the shunt regulator vs. a series regulator? It seems the shunt regulator would be a lot less efficient. I use a series fed FET in my Collins 516F2 mods and I have used this circuit in many other places with great success. I use a similar circuit to regulate the low B+ on my Ranger.

http://www.crompton.com/hamradio/collins/Collins 516F2 Modification Schematic.pdf

Also, why would one need a 500 watt or more mod transformer? If you intent is to stay within the amateur AM legal limit then a 250 watt transformer would be more than enough. If you like to build things you can not legally use on the air then its a different story.

[W2VW]

e run either AB1 or AB2, so many options are available.   Here is an idea you may wish to research further:

I recall a QST article in the 50s or 60s that described a successful modulator design with a pair of 4-250s running class AB1 with about 3KV on the plates.  It produced in excess of 500 watts of clean audio with RC coupled receiving tubes driving the 4-250 grids.  The entire modulator, sans power supply, was contained on a single deck in the 19 inch rack cabinet. 

1947 or '48 QST. "500 Watts Audio From AB1" or somesuch.

[Steve - K4HX]

Or any KW BC TX that used 4-400s.

[W4VLJ]

Thanks guys, keep em' coming.

I wanted to discuss my power calculations to address Doug's question, and make sure I did it right.  Doug I found your website really useful, thank for posting such a good resource. 

Key points:

-Keep in mind I said earlier that I want to over-design this so it runs happy and has a long life. 

-I plan to use dipoles for 160, 80, and 40 meters, so unity gain on the antennas.


Being primarily a microwave engineer I tend to think in dBm's so please forgive me if I oversimplify.  Here is my thinking:

Max power = 1500W PEP = 375W carrier (PEP/4) = 55.74dBm

Adding 3dB margin for VSWR, radio to antenna losses, antenna efficiency, and to ensure no distortion I come up with 58.74dBm= 748W, so my goal of 1K carrier is a little high.  1000W is only 1.26dB higher.  I will definitely run the transmitter backed off (I plan to monitor with a rack mount Bird 43 and a Gates modulation meter).  I'm thinking of this design along the line of a BC transmitter designed to run indefinitely, bulletproof.  Sorry, but that's the engineer showing through.

I'll be searching through the QST archives for the articles mentioned, thanks for the tips.

73,

Mike

[kb3ouk]

Or any KW BC TX that used 4-400s.

And here's pretty simple one too, the Bauer 707: bama.edebris.com/download/bauer/707/bauer707sch.pdf

[Steve - K4HX]

Indeed. KISS. Get the mod tranny and reactor from a kW BC TX and you'll have plenty of head space.

[wa3dsp]

Mike,

 Well unlike commercial, amateur is TX power output, the ERP does not enter into the calculation. So legally you can run 375 watts carrier 100% AM modulated output. I would say you could achieve that output with no more than 500 watts input on all HF bands. I suspect you are planning on using it on 40 meters or below so  500 watts would be a conservative input power. Plate modulated you need 50% of the input power, thus 250 watts. Given a little headroom and safety factor I would say 300 watts, maybe 350 maximum is a good number. Definitely not more than 400.

Since you like to talk in dB you can appreciate that minor or maybe not so minor power differences are often imperceivable at the receive end.  You need to double power for a 3dB or 1/2 S unit increase. So the difference between running 375 watts and 300 watts or even 250 watts AM would in most cases not even be noticeable at the receive end. I laugh when someone tries to push a pair of 572B's to 800 or 900 watts instead of the recommended 600 watts. You are just burning up energy and hardware for virtually no change at the other end. I suppose its a kudo to say you are running maximum power but you could say you are and back it off to 300 watts and no one would know the difference.

Once you get up into high powers you have to expend a great deal of energy for little return. The signal would increase the same amount going from 100 to 200 watts as from 100,000 to 200,000 watts !!!

I appreciate that you want to build some real tube equipment but check this out for a neat full power AM station -

http://k7dyy.com/products.htm

 

[KD6VXI]

I believe he is saying g he wants to ensure he has 375 carrier at the feeding to the antenna.  This would necessitate having approximately 3 dB overhead, assuming 3 dB deadline loss due to length, compounded by set.

At least, that's what I took his math for.....

Would this be legal, is the question of the eve......

--Shane
KD6VXI

[wa3dsp]

No, it would not be legal. It is output at the transmitter. ERP is not used whether it be loss or gain.  On HF this is usually a detriment at 10ghz with a 20 foot dish it is an asset.

[steve_qix]

The modulator design really depends on how good you want the transmitter to sound.  My suggestion would be to stay away from anything in old handbooks, QST articles, etc. because all of this will be marginally designed, and for the most part communications quality (sometimes, not even that) audio.  Yes, a lot of people have build these and they sound "ok", but not great.  Again, it depends on what you're after.

Another criteria is how much positive peak capability you want.  Old broadcast designs for class AB modulators should give you at least 110% positive, and some can get to 125% (assuming the right modulation transformer, of course!!).  Triodes (or in most cases triode connected tetrodes) will give you a greater plate voltage swing, and therefore more modulation capability for a given modulation transformer.  A low phase shift system with reasonably unmitigated positive peak capability will show positive peaks of 150% and even more at times with less than 100% negative modulation using a male voice into a good condenser or similar microphone.

Probably the most important part of the modulator is the audio driver.  Direct coupled is FAR superior.  This is a new design.  Stay far away from driver transformers (or really any transformer in the audio driver at all).  Avoid R/C time constants in the audio path (if there are any) that are too small.  ANY low frequency phase shift in the audio range (I use 5hZ as my bottom when looking at phase shift characteristics) anywhere in the system will affect your asymmetry (if you care about this).  High frequency phase shift will limit your ability to put negative feedback around the modulator.  There will always be some high frequency phase shift, particularly if there is a modulation transformer involved, but with a proper driver, you will be able use a reasonable amount of negative feedback.

Anyway, a few thoughts    What you do should be dictated by what you want as the end product and that, of course is a personal choice.

Regards,  Steve

[W4VLJ]

Hi Everyone, Mike here.

So a little more definition on the amplifier; I am very concerned with the quality of my transmit audio, so based on my research so far I believe I want to use a modulation reactor.  Rob (W0VMC) pointed me in that direction, and I found a great article on AM Press / Exchange.  Unfortunately it's a two part piece and AMFone only has issue 102, not 103.  Does anyone else know of a place to find these old copies?  Is the publication in business anymore?  Everyone in agreement I should use a mod. reactor?

73,

Mike

[K1JJ]

Hi Mike,

Once you get your AM rig and modulation running well, you may want to fine tune it with NFB, various modifications, etc. We all love to tinker.

The way to measure and get definitive data is to run some Total Harmonic Distortion (THD)  and Intermodulation Distortion (IMD) tests. This is easy to do by injecting various audio tones thru the rig and sampling some RF. You can use a $25 SoftRock module and computer, a Flex, other SDR, spectrum analyzer, etc. to display the spectrum.

Here's a thread that shows actual AM THD measurements and thoughts:

http://amfone.net/Amforum/index.php?topic=33555.0


And a thread about measuring linear amp IMD:

http://amfone.net/Amforum/index.php?topic=33505.0


I run all my rigs thru these tests and have made some meaningful improvements that would never have shown up when using other simpler test methods.


Good luck with the project!

T