The AM Forum

Hello Friends,

I am working on the design of my first AM transmitter.  I started with the power supply, went up into the RF stage and am now working on the modulator design. I have built CW transmitters before, but this will be my first time building a modulator with my transmitter. I am going with plate modulation and am considering my options.  I am considering a modulator from an old ARRL handbook that uses two 807 tubes.  The RF Final section of my transmitter will also use two 807 tubes.

I have a question about how to calculate the turns ratio that I will need in my modulation transformer.

I know that the formula I need to use to get the information is turns ratio = 
the square root of Z1/Z2.

The plans for my modulator (from an old ARRL handbook) tell me what the impedance will be for the modulator.   

However to calculate the turns ratio I also need to know the impedance of the RF transmitter. 

Here is where I have a few questions that I am sure you fellows who have been doing this for a long time know, and consider very basic, but presently have me stumped.

QUESTION 1)Is the impedance of my transmitter for the purpose of this calculation the same as the impedance I used when designing my pi output tuning network?   If that is the case, then my impedance is the 1900 ohms for the two 807s in parallel that I needed to design the pi network to match to the RF output to the 50 ohm antenna.

I understand the formula for calculating the impedance of the transmitter to be the basic Ohm’s Law formula for resistance. I can solve that if I know voltage and current. My transmitter is rated at 200 mA and people who have built similar transmitters say the plate should dip to 180 mA at resonance.  My high voltage supply is 750VDC.  The high voltage to the modulator is also 750VDC. I am using the same supply for the modulator that I use for the RF section since my 750 VDC supply is 450mA. 

That leads me to two more questions:
QUESTION 2 A.) Which number do I use for current in the ohm’s law formula, the 200mA it is rated at or the 180mA it will dip to at resonance? 
QUESTION 2B.)  Which figure do I use for voltage? 750 Volts or 1500 Volts since I will be applying power to the modulator and the RF stage? Or is there some other voltage number I should use that I am not thinking of?

Thank you for your help with this! I appreciate your time and willingness to help me learn something new, very much!  73, Carl W8WZ

http://amfone.net/Amforum/index.php?topic=26630.0

Great information on this subject from Don K4KYV on this site.

Al VE3AJM

Carl,

The output tubes are in parallel and the modulator is push pull, all 807's.

By dividing 750V/180 mA or 200 mA you get something around 3500-4K. If the Modulator is P-P and it is on the same 750V, generally the impedance on that side should be 2 times this value or 7K CT. But these tubes are only running about half the current each (50W out) so the impedance will be somewhat higher than 2X.

So 3:1  or 10K CT to 3500  or 9K CT to 3K would be my guess.

The Apache modulation transformer uses 11K to 3K which is 3.7:1. It uses different tubes for the modulator than the RF but it is not that different of a setup.

Mike WU2D

RCA says that a pair of 807s in push-pull audio service with 750 volts on the plate will have the following effective plate-to-plate load resistance:

Class AB1 - 12000 Ohms,
Class AB2 - 7300 Ohms.

Plate load is simply the plate voltage/plate current.  Plate load for the Pi Network is different, that's plate voltage/2xplate current.  It is the PA plate load that dictates everything.  You work backwards from the PA plate load to the modulators.

180ma or 200ma, doesn't matter, close enough.  With 750 volts at 180ma your plate load would be about 3750.  750 volts on the 807 modulators requires a certain plate load (most likely 8-10K).  Knowing the two impedances you can figure the impedance ratio, the turns ratio is the sqrt of that.  Looks like about 2-1 step down, turns ratio about 1.4-1.

Building a xmtr is great, don't over think everything.  You will most likely have to make adjustments to the design as available parts permit.

Fred

you can certainly do 807s modulated by 807s as many designs have but the question I would ask is this - what you really want to do or would you want a higher performance design ?

a better choice would be a modulator tube with more emission reserve than an 807 / 6146 / 6l6 which are all from the same family ... some sweep tubes (ex: 6dq5) are inexpensive and offer distortion reduction, aotbe

Thanks Everyone!  This is VERY VERY helpful information to me! I'm really learning some things here, which is very fun.

I have seen some DX 100 modulation transformers for sale at a fair price recently and wondered if that modulation transformer would work for my design.  Does anyone know the turn ratio of that transformer?

I know  The DX100 uses a pair of 1625's in push pull in its modulator.  Since those tubes are the same as my 807's except for the fil voltage, I assume its modulator has an impedence of around 7300 ohms as it is using around 750 volts.   My RF deck is using a pair of 807s in paralell instead of the 6146's in the DX100 but I believe the voltage and current is the same in both designs.  If I can use the DX100 transformer in this project, I can cross one big step off my list, but before I buy that iron, I would appreciate an affirmation that it will work, or words of caution that it will not from those who know better than I do.  Thanks! Carl W8WZ