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Solid State High V. Series Modulator




 
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Author Topic: Solid State High V. Series Modulator  (Read 9687 times)
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W1FYO
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« on: May 27, 2011, 01:48:45 PM »

A couple  of years  ago  I decided to plate modulate my Ameco TX86 transmitter since the internal screen modulator left something to be desired.
Trying to find a mod transformer was difficult and  expensive . The price of iron continues to go  up.   After looking at the alternatives ,I decided to go with a solid state series modulator for cost and simplicity , the caveat being efficiency and a B+ supply of around 1200 volts .  Mouser lists a  power mosfet device of 1500v , the STW4N150 , and comes in a TO247 package for a little over 6 bucks ( at the time it was $6.10). The TO247 package has the lowest thermal resistance compared to the TO220 version . Three devices are used in the class A modulator ,  a voltage amplifying stage followed by two source followers . The source followers are mounted directly to heat sinks that are floating at the B+ level  and  are isolated from the enclosure using ¾” ceramic standoffs  . I didn’t want a fan so I used a heatsink for each of the source followers . They measure 2 x 2 x 7” and have 7 fins . Probably would get away with one heatsink if I used a fan. A small heatsink I extracted from a computer P.S. is used for Q1. The temperature for Q2,Q3 heatsinks  topped out at 55 deg C. during  an old buzzard transmission.
  The modulator makes use of negative feedback and has a pot for adjusting the output voltage which I set to around 450 V. My  homebrew preamp provides an audio level  of 20 v pk- pk to drive the modulator output to 1000v . What I like about this modulator is its simplicity and the clean audio it produces . With the  frequency response that is flat out to 100 khz and beyond . I  limit the high end frequency in the preamp . Since the modulator acts as a voltage source , impedance matching is not an issue . I also use it with my old ARC 5 and gotten good reports .  This is a reliable  modulator that could be a good alternative for the 50 to 60 watt class of transmitters .   


* series mod station.jpg (291.63 KB, 1599x892 - viewed 1024 times.)
* series mod.pdf (15.17 KB - downloaded 787 times.)
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WD5JKO
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WD5JKO


« Reply #1 on: May 28, 2011, 10:49:52 AM »



Bob,

   Good deal getting around that increasingly unobtanium iron dilemma. I like the headroom and bandwidth your circuit provides. Of course being class A, scaling this idea up has efficiency concerns which might lead one to a class D switcher design with PWM board, and filter.

   With that additional headroom, I wonder how you deal with limiting modulation peaks? I don't know what that Ameco uses for an RF tube, but my modified class A modulated Gonset G50 uses a 6146 RF tube. If I hard clamp the 6146 screen voltage to ~ >=25v (but never less), I get a soft clamp on the plate modulated waveform such that negative modulation is limited to just under 100%, and yet the positive peaks go unimpeded. Back off the gain a tad, and the diode limiter never kicks in.

Jim
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W1FYO
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« Reply #2 on: May 28, 2011, 02:12:38 PM »



    Jim,
      The Ameco TX86 uses the same tube as your gonset , a 6146 ,was designed for mobile use back in the 60's.
Limiting the downward v to the screen looks like a great idea . With no mod transformer  you wont get the distortion products from
the transformer when the plate current suddenly goes to zero . Planning to build an upscaled  version  to modulate a sweep tube(s) final.
Have most of the parts for that project . I noticed the price of the mosfets is starting to go up.
Bob W1fyo   
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« Reply #3 on: May 28, 2011, 02:52:02 PM »

That's an interesting use of the V MOD Level pot.
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« Reply #4 on: November 09, 2011, 01:16:46 PM »

Yes, what are we doing?


Addendum: I think I see; it is a DC controlled feedback system for setting operating voltages of the FET gate bias, which in turn sets the Mod level. I like it.

Phil - AC0OB
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gerry_w1id
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« Reply #5 on: November 09, 2011, 03:13:02 PM »

Looks good. The TX-86 is an interesting little transmitter which is seldom seen. I don't have it in front of me now but I think the face is about 6 inches square and slightly deeper. In other words a very nice compact little rig. You can substitute almost any variation of the 6146 for final tube. The heater string can be wired for either 6.3 or 12.6V. If you don't have 600V available, you can use a 2E26 with 300V. The only thing I would watch out for are those tiny little bypass caps around the power connector. They look to be on the edge of their voltage rating and there is not much room in there to use larger ones. My HB power supply is good for about 550V high voltage with no load. The TX-86 modulator seemed to do a pretty good job for what it is. I was able to get a clean modulation waveform out of mine into a dummy load. Load/tune & drive was a bit tricky as there is no tune provision. In other words, you gotta be fast and accurate. And how about that bouncy panel meter?
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WA1GFZ
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« Reply #6 on: November 12, 2011, 07:49:50 PM »

I read that you can get 4000 volt IGBTs. At the time Digi Key had an $80 price on them. It was also in a TO247 package.
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KK4RF
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« Reply #7 on: November 13, 2011, 02:06:39 PM »

Bob,     
     I heard your ARC-5/series modulator on the Old Military Radio Net yesterday on 3885. The audio sounded very nice and you seemed to have plenty of RF as well. I was quite impressed. How do you hook up the modulator to the ARC-5? Sorry about such a basic question but I'm in the dark on this type of modulation. Good work there, Bob.   ---Marty, KK4RF---
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W1FYO
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« Reply #8 on: November 29, 2011, 02:01:00 PM »

Marty,
       Thanks for the FB report. With my arc5 , I simply place the modulator between the B+ supply and the HV line to the BC457 and set the voltage to around 450 volts. The current draw with the 1625's is around 100 ma for about 35 watts out . A separate regulated supply of 220 volts is used for the oscillator. This  old arc5 has become my favorite 75 meter
transmitter,  now  I just have to get it to tune down to 3700.
Bob W1FYO
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W9BHI
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« Reply #9 on: November 29, 2011, 02:39:02 PM »

What changes to the circuit would be required to have this modulate say...
a Valiant II?
Just thinking out loud.
Don W9BHI
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K5IIA
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« Reply #10 on: November 29, 2011, 02:43:14 PM »

yes i have 30 pounds worth of stuff modulating my valiant. power supply for 811's, fil transformer, tubes and bc 610 mod transformer.  driver transformer and a solid state pa amp. sure would be nice to just do a little circuit like that and feed with a preamp.  Roll Eyes
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WD5JKO
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WD5JKO


« Reply #11 on: December 01, 2011, 09:04:25 PM »



The problem with this circuit as you scale up the power is poor efficiency. This is because the modulator runs class A. If efficiency is not a concern, then go for it. Here are some examples:

1 807 or 6146:
B+ to modulator:1Kv, B+ to RF amp set to 450v, RF amp runs 450V @ 100ma or 45w DC input, and at 70% eff. puts out 32watts. The power supply must put out 100 watts. The modulator dumps 550v @ 100ma or 55 watts of heat. The overall efficiency is 32%.

2 or 3 6146:
B+ to modulator set to 1500v, B+ to RF amp set to 600v, RF amp runs 600v @ 250ma or 150w DC input, and at 70% efficiency we get 105 watts carrier. The power supply must put out 1500v @ 250ma or 375w. The modulator dumps 900v @ 250ma or 225 w of heat. The overall efficiency is 28%. The B+ to the final was set low deliberately to allow for higher than 100% positive peaks; to about 150% at clip point.

So doing things with a series class A modulator give you about the same efficiency as a linear amplifier on AM.

But it should sound marvelous!

Jim
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kb3ouk
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« Reply #12 on: December 01, 2011, 10:29:17 PM »

i've been thinking of ideas for a transmitter and i get stuck at the modulator section. this looks like the perfect way to go. and it seems like high voltage iron is a little easier to find than modulation transformers are.
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WD5JKO
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WD5JKO


« Reply #13 on: December 03, 2011, 11:31:20 AM »


For higher power examples, the efficiency issue is discussed on Steve's Class E site with an analog class H modulator concept:

http://www.classeradio.com/modulate.htm

Some variation of the original circuit in this thread might be readily applicable to a situation such as the Johnson Valiant, or Vikings.

Jim
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kb3ouk
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« Reply #14 on: December 04, 2011, 10:30:18 AM »

ok, i'm curious, what all in this circuit would need to be changed/modified to use it with lower voltage/higher current to modulate a solid state final?
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« Reply #15 on: December 04, 2011, 11:50:25 AM »

I have some IXLF 19N 250A (2500V, 19A) devices here, and the intent was to build a class H modulator for a Valiant.  I may now do this project or a variation thereof, because I've been using the Valiant on 10 meters, and would like to be completely transformerless.

From my calculations, it will take around 600 watts of dissipation in the upper class H to avoid exceeding the S.O.A. of the device under all operating conditions over the entire audio cycle for class H. 

On the other hand, using PWM only one device would be required as the switch.  It may be that I go PWM rather than class H just to avoid any heat production.  A source follower with a floating driver is the most practical implementation for an existing, common cathode RF amplifier such as is in the Valiant.

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