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50 watt modulator 1964 ARRL Handbook




 
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Author Topic: 50 watt modulator 1964 ARRL Handbook  (Read 4817 times)
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KK4RF
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« on: October 10, 2019, 10:28:17 AM »

Gentlemen:
     I am trying to build the 50 watt modulator shown in the '64 ARRL Handbook. The circuit shows a 6AV6 speech amplifier, then a 6CG7 dual-triode, the first part is the second stage of the speech amplifier, the other triode section is the driver, which then drives through a 1:3 interstage transformer (they call for a Triad A-31X), which then drives the modulator comprised of 4 1625 tubes, 2 each in parallel push-pull arrangement. I have the modulation  transformer called for, a Stancor A-3893. I have an interstage transformer, a Stancor A-4812, which has a primary that has a center-tap, apparently for a push-pull driver stage. The modulator driver stage has a single plate driving the transformer primary. I am not sure this will work or not with the interstage transformer that I have.
     Any ideas or insights will be greatly appreciated. This is going to be my main winter project, so I am trying to get all my ducks in a row, so to speak. Thanks in advance, 73s,  Marty Harpen, KK4RF, Suffolk, VA
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KK4RF
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« Reply #1 on: October 10, 2019, 10:31:38 AM »

Gentlemen:
     I goofed. My transformer is a A-4212, not the A-4812 that I mistakenly  wrote a few minutes ago. Sorry for the mistake. 73s---Marty, KK4RF
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« Reply #2 on: October 10, 2019, 10:33:16 PM »

Gentlemen:
     I goofed. My transformer is a A-4212, not the A-4812 that I mistakenly  wrote a few minutes ago. Sorry for the mistake. 73s---Marty, KK4RF

Do you have a schematic of this modulator as not all of us have access to these vintage publications?

What is your LV B+?

Here is a suggested schematic.


Phil - AC0OB

* 6AV6 6CG7 Speech Amp Driver.pdf (121.21 KB - downloaded 153 times.)
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« Reply #3 on: October 12, 2019, 10:30:17 PM »

...which then drives the modulator comprised of 4 1625 tubes, 2 each in parallel push-pull arrangement....


Just curious, why 4, 1625's?

Even with two 1625's at a plate voltage of 400V in Class AB2, you can get 55 watts output.


* 807 RCA Long.pdf (1659.58 KB - downloaded 131 times.)
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« Reply #4 on: October 13, 2019, 02:11:30 PM »

I remember that modulator. Nice looking unit.
If you run AB1 the single ended driver should be fine.
AB1 PP-Par. 1625s for 50W is very conservative.
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« Reply #5 on: October 13, 2019, 04:24:31 PM »

Here is the SA, Driver, and Mod stage, Class AB1:

Phil - AC0OB

* 6AV6 6CG7 Speech Amp Driver.pdf (170.11 KB - downloaded 132 times.)
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KK4YY
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« Reply #6 on: October 13, 2019, 04:38:30 PM »

Even four 6L6's at 400V would get you to 50W in AB1 with that '64 Handbook design. About any of the 6L6 "family" should work fine at that voltage/power level. A real "run-what-ya-brung-in-AB1" amp.

If it were me, I'd use the 7027A - but only because I have 4 of them sitting here in search of sockets. Wink


Don
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« Reply #7 on: October 13, 2019, 08:44:39 PM »

With the interstage you have, it will not handle unbalanced DC.
That's the difference between the two transformers, assuming the same turns ratios.

The one with the CT "wants" push-pull.

A solution is to use a "parafeed" arrangement with the existing transformer.
That would be a choke to the plate of the tube (any suitable choke or transformer
winding is fine) and a cap to drive the primary of the interstage transformer.

Some selection of values for the choke and the series cap will be required to
avoid resonances in-band (usually at "bass" freqs). Design suggestions for
"parafeed interstage" circuits can likely be found online. Of course one can simply
drop in whatever you have and sweep input of the tube look at the secondary and observe the response too.

On the 807s or 1625s (1625s were ubiquitous after WWII, aka "cheap and plentiful") I'd
run ~650vdc on the B+.  Maybe higher. Given that in AB2 it is possible to get 200watts
from 4 x 807s, and at 650vdc you'd likely do the 140watts mentioned in the text, one
could opt to pull two tubes in a situation where the modulation power requirement is less than
half. (you'd get some mismatch to the plates, so less power)

Another option is to make a provision to set the PS to the higher B+ and then be able to
open the first cap in the PS making it a choke input supply, dropping the plate voltage
by ~0.707 x the B+ in the cap input situation. Likely you'd want to make the second cap
a bit heavier than they did back in '64...

Then you'd have 4 power level choices.

Don't forget to increase the breakdown voltages where applicable... adjust the bias circuit with the
higher B+...

Just some ideas...

Otoh one could "punt" and go with sweep tubes (not a common choice in '64).
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KK4RF
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« Reply #8 on: October 16, 2019, 06:23:52 PM »

Gentlemen:
     I was out of town for about the past 4 days and did not have internet access. I really appreciate all the replies and ideas. I have already decided to build a different 50 watt modulator, as I was able to locate an interstage transformer which ought to be perfect for the application. I've been busy reading up on all these different kinds of interstage transformers.  The modulator I am now scrounging parts for is the one in QST, in 1966, and was designed by Doug DeMaw. It uses a pair of 7027A tubes. Those tubes don't have an anode cap on the top and the tube sockets should be much easier to obtain and make holes for, compared with the 4 1625 modulator.
     Anyway, many thanks again to all who replied. It may be a few months before I get it all going. I have some other projects I am working on also.
     73s,  Marty, KK4RF, Suffolk, VA
     
     
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KK4RF
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« Reply #9 on: October 17, 2019, 09:32:27 AM »

Gentlemen:
     I have tried attaching the 2 different 50 watt modulator schematics. Hopefully they come through OK.
     My original question had to do with the interstage transformer. I had previously bought one at a hamfest and the primary has a center-tap and even designates for plates P-P. I realize that this transformer probably won't work with a driver circuit with just one tube (plate) driving it. I don't have any experience with this type of circuit.
     I carefully re-read all of your responses and I appreciate the time and thoughtfulness in posting them.
     I learn a lot from projects like these. Thanks again to all who have responded
     ---Marty, KK4RF, Suffolk, VA


* Scan_20191017.jpg (862.1 KB, 1700x2338 - viewed 314 times.)
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KK4RF
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« Reply #10 on: October 17, 2019, 09:35:30 AM »

OOPS!   only one of the schematics came through. Here is the other. This is the circuit I plan to build. Thanks...


* Scan_20191017 (2).jpg (436.05 KB, 1700x2338 - viewed 350 times.)
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KK4YY
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« Reply #11 on: October 17, 2019, 01:39:38 PM »

I think the big concern with the interstage transformer is that it has a gapped core to handle the unbalanced DC which will be present on the primary if driven by a single-ended stage. It may be possible to re-stack the laminations of a non-gapped core to achieve that, if you want to take the time. The performance will be different but may still be acceptable for your purpose.

In the past, I've used a small 15 watt multi-match modulation transformer as in interstage transformer. It had a gapped core (as is usual) and was way over-rated for interstage service. If you have the room on your chassis for one, that should work fine too.

There are so many different ways to get 50 watts of audio, it's nearly impossible to name them all. Sometimes deciding which one to build is the hardest part! Undecided


Don
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KK4RF
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« Reply #12 on: October 17, 2019, 01:59:36 PM »

Don,
     Thanks for the help. The interstage transformer I ordered is made by Hammond. I believe it is a 154A (if I remember correctly) It had the same specs as called for in the Doug DeMaw modulator from QST in 1966.
     I've always wanted to make a plate modulator with a real modulation transformer, and if I can get something in the 50 watt range, that would be great. I built the PineBoard transmitter by Bob Heil from QST a couple years ago and it actually worked very well. I ran mine through a linear to get about 50 watts out. Still, though, I want to build a modulator that I can try with different transmitters. I'd like to be able to plate modulate a Command Set transmitter for 75 meter AM.
     It's all fun. 73s,  Marty, KK4RF
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« Reply #13 on: October 17, 2019, 08:10:40 PM »

one thing to consider .... can you find another rare difficult to find transformer when one of the tubes decides to arc internally due to loss of vacuum or other gas buildup .... this is one of the reasons when you find one of these old beauties that they may have an open winding

decide on the tradeoff between less parts count with the transformers and more parts using more tube stages but nothing unobtainum at replacement time
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KK4YY
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« Reply #14 on: October 17, 2019, 08:55:36 PM »

I think you may have gotten the Hammond 124A. I believe that has a gapped core which can be used with a single-ended or push-pull driver. It can also be disassembled and cross-laminated for improved low frequency response if you use a push-pull driver. Now, I've never done this, but I've heard tell...


Don
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« Reply #15 on: October 17, 2019, 11:33:28 PM »

Don and Beefus,
     Thanks for the comments. Finding the transformers to build a modulator is the hardest part of building a modulator. I found a beautiful, NOS, multi-tap mod transformer, still in the box. 60 watts, at the Orlando hamfest a couple years ago and now I am finally going to start building a modulator. This will be a fun project. I am not a great metal worker, but what the heck.
     I will keep you guys posted of the progress, but it may be a couple months. 73s,  Marty, KK4RF
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« Reply #16 on: October 21, 2019, 10:29:20 PM »

Hi Marty,

Just to have out there, I should mention my first proper modulator, which was in this class. It was the modulator for my first TCS Transmitter, which was tore up pretty bad. I called the final super modified result - The Little Ranger. That was my first AM transmitter and this had to be back in around 1986. Later on I would get a pair of unmolested TCS twins and I still have them. But this first one was given no quarter. I put both 1625's in parallel and removed the transformer and used it for the driver in my external modulator. Hey its is 1:1, big and had a nice center tap. That's right the TCS mod tranny became the driver phase splitter. Oh and removing that transformer made way for an inboard power supply!

The mod iron in the external modulator was a Fair radio job that was $6.95 at the time and it was a 3:1 if I remember so it did a good job handling two 1625's modulated by another pair...

So the lineup was a 6J5 mic amp. into a 6SN7 into a pair of 1625's in Push-Pull AB. And I had feedback to the cathode of the first section of the 6SN7, and the second section was the driver. So both feedback and and Negative Cycle Loading as taught to me by Steve QIX (at the time KA1SI). Oh and I remember very clearly that Steve saw me grab the TCS under the pines at Deerfield.

Mike
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KK4RF
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« Reply #17 on: October 22, 2019, 10:53:56 AM »

Mike,
     Thanks for the ideas on your modulator for the TCS rig. We operate a fully restored TCS transmitter/rcvr on board the USS Wisconsin Battleship,, harbored  in Norfolk, VA. Just using it on CW but it has a beautiful modulator section built in.
     Still collecting all the parts for the modulator. I've not  ever built a modulator  with 50 watts of audio before. Hoping I can get it to sound clean without hum. Plan to try it with one of my ARC-5 type transmitters, or perhaps with one of my old Globe Chief transmitters.
     Read your WW2 spy radio article in Electric Radio. Excellent writing, Mike. Keep up the good work.  73s,  Marty, KK4RF
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« Reply #18 on: October 24, 2019, 07:46:55 AM »

Should you decide on a parafeed driver, instead of the frustration of looking for a gapped core transformer (or modifying lams) you can easily avoid resonances by using a 10K 10W resistor for the plate load. I modified a Johnson Ranger and a couple of Valiants that way to eliminate distortion in the bottom end when speech amps were modified for AM Gangsta audio.

I don't like operating tubes near or at their limits. Rather than something in the 6L6 family I used 807s or their octal sweep tube equivalents, 6BG6s. You can get a good 75W out of them, so at the 50W level they just loaf along and last longer.
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KK4RF
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« Reply #19 on: October 25, 2019, 07:14:48 AM »

Warren,
     Thanks for the ideas there. I just received my Hammond 124A from Antique Electronics Supply so I am still collecting parts. Still need a couple of chokes, a 20 Henry unit (for a filter following an audio clipper stage) and a 1 Henry unit(in the power supply stage.) I have a nice aluminum chassis that should be strong enough for all the heavy components.
     I am also doing a functional restoration on a Viking Ranger at this time as well. It puts out power on CW and the mod section sounds OK into a nearby receiver. Still quite a few way out of tolerance resistors. I am doing Phil Salas' mods for the ranger.
     Anyway, lots to do here. Thanks for all the help. A lot of you guys understand modulators much better than I, and you know how to modify circuits to allow  you to make a modulator with components on hand.
     Thanks again---Marty, KK4RF
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« Reply #20 on: October 25, 2019, 07:44:49 PM »

Uhhhh... you probably don't want to use a 7024 for the modulator tubes?
Today that is a relatively expensive tube.

Better to go with a pair of 6L6GC or 6550s - imports will work ok, fine.

You'll not want to use the clipper or the C-L-C filter either. So, you are going
to be modifying the circuit. Nothing to be afraid of, but you will be making some
changes.

The modulator is the SAME as a "PA" amplifier, except... except it uses a modulation
transformer at the output, not one that matches an 8 ohm (or so) speaker!

Turning one page in my 1965 Handbook, one sees a "6146" modulator circuit.
Take out the 6146s and put in 6550s or 6L6GC tubes and voila! pretty much
what you are looking for!

You can "figure" your match by working the "ideal" for the transmitter tube's Z, then
back via the turns ratio of the mod iron, to the primary of the mod iron. You
use the simple formula that converts turns ratio to impedance ratio... then look
at the plate Z of the mod tubes VS. plate voltage. That will tell you where
to put the PS B+ for most ideal operation...

Of course the old school method of sizing the modulator power at 1:1 to the
transmitter power today is "out the window". Today you'll want a somewhat
over powered modulator, a 3 diode negative peak modulation circuit so that you
can drive >100% positive and not "hit the baseline". In which case use oversized
tubes with extra B+ and ur all set! Cheesy

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« Reply #21 on: October 25, 2019, 07:47:54 PM »


Oh, Warren KB2VXA, better to do "parafeed" with a choke, not a resistor.
Performs better, and wastes less heat.

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« Reply #22 on: October 25, 2019, 09:19:34 PM »

Fortunately, the Stancor A-3893 modulation transformer that you have is a multi-match unit. So, after you set it up the wrong way, you can change the taps and set it up the right way. Wink
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« Reply #23 on: October 26, 2019, 09:16:06 AM »

Gentlemen:
     Thanks for the additional thoughts and ideas. The 50 watt DeMaw modulator includes a clipper circuit. I intend to give it a try, but if I have trouble with it, would probably wire it out of the circuit. We'll see what transpires. The circuit calls for 7027-A modulator tubes. I do have a set of 6550 tubes here. Will see how things go.
     The modulation transformer is a multi-match unit and I figured there will be some trial and error before I get things just right.
     All of these mental machinations...Hoping they stave off dementia---will see how things go. Thanks again for the ideas---Marty. KK4RF
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« Reply #24 on: October 26, 2019, 11:32:46 AM »

WARNING: Do NOT use the clipper circuit!!!


Please check your ARRL handbook, the modulator that follows the one you originally posted
is about what you actually want, just dump the 6146s for the 6550s and adjust the bias and
screen voltages per the tube manual specs. "A 120 watt modulator" is the title. Note that
at 500vdc that article shows 75 watts? Extra power/headroom is your friend. If you must,
you can drop the B+ more...

Going from 50watts to 100 watts is exactly 3dB more. Not that much. FYI.
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