modulating a vintage transmitter with a Crown DC-300?

[KJ4RIG]

While searching this website and the links, I ran across this great article....
"A Quick and Not So Dirty Path To High Fidelity Audio" by Steve, WB3HUZ.
I really like his approach of not doing extensive irreversible mods to vintage equipment, and being able to achieve great audio without doing any real harm really appeals to me.  I guess it's the "car collector" mentality that I have. 
Since I have had a couple of them since the early '70s, I have thought for years about what a great modulator a Crown DC-300 might be in a plate modulated AM transmitter.  Of course, I had no practical application where I could try it, until....a couple of weeks ago when I got my ham license! 
After reading Steve's article, which deals with vintage transmitters of 100 watts or less, it got me to thinking what vintage transmitter would be a good candidate for trying this with a modulator capable of almost 600 watts...
Another big question I have is where I could find a suitable modulation transformer, with an 8 ohm primary, for, say a 500 to 1kw watt transmitter.
Has anyone tried something like this?
Guy
KJ4RIG

[WD5JKO]

Guy,

   The Crown DC-300 is one of those old Brute force SS amps that are nearly indestructible, and a great source for AM modulation. I once used a Crown M600 in the same way with very good results. Boy the M600 sure is heavy! These AMPS are nearly a perfect voltage source with Rs = 0. I suppose you will use the bridge configuration?

   In my case I used a Harmon Kardon Citation V audio output transformer in reverse as a modulation transformer. That transformer was a real mule, rated at 40 watts RMS down to something under 10 hz. I don't recall the exact mathematical relationship, but if you limit your modulation power under 100 hz with a transformer rated at 10 hz, the maximum audio power you can run through the transformer is much higher. In fact this transformer took everything the M600 could put out, 150hz and up! I am not saying to get rid of everything below 100hz, but instead to equalize such that those frequencies are attenuated 6-10 db below the level at 1 Khz. This little trick makes your mod transformer look much bigger, and another benefit is that is that folks across the country can actually copy your audio, i.e. you will be heard, and not sound like a muffled "woofer".

   Keep in mind that you will need to DC isolate the RF Pa plate current from the Hi-Z secondary of this transformer, so the modified Heising circuit will be needed with a large choke, and isolating AC capacitor.

  If your voice has much asymmetry, running that Crown into a near DC short circuit will cause a DC current to flow into the transformer primary (4,8, or 16 ohm taps). This will use up some of the Crown power supply ampere budget. Crown in their manuals for the big amps recommend a series capacitor between the AMP and the load (if under 2-3 ohms DC). One possibility is dual 10,000mfd caps back to back with the + ends together to make a poor mans AC capacitor placed in series with the load. Remember that large loudspeakers usually have a DC resistance that is just under the AC impedance rating, so no series Cap is needed there. When you drive say the 4 ohm tap of a transformer, the DC resistance of that primary might be just a few tenths of an ohm..

   I think you will have better results if the full winding is driven (common, and 16 ohm tap) so long as the turns ratio works out for your RF system modulated. I would concentrate more on turns ratio than trying to match impedances. This is especially true since the big Crown doesn't care what it looks into so long as it has the voltage and amps to drive a given load.


Good Luck with your Crown!
Jim
WD5JKO

[KJ4RIG]

Hi Jim,
Thanks for the reply.
Yes, I was thinking of using it in bridging mode.  I just checked the specs at Crownaudio.com and a DC-300A will deliver about 350 watts into a 16 ohm load in that configuration.  So a 500 watt transmitter or something close to that, would be a great match....hopefully with plenty of reserve in the power supply for some asymetrical modulation.  Anyone with ideas about specific models of transmitters that I should be on the lookout for, please jump in!
I'm going to use some sort of broadcast audio processing, maybe even an Optimod AM if I can find one for a reasonable price.  I'd also like to get a real broadcast modulation monitor.  There's something about AM modulation monitors that has always appealed to me on some sort of primal level...LOL.
I guess I'll have to see what transmitters are out there and which one I get to get too specific on the modulation transformers and such, but I sure do like the "sound" of this!
Guy

[KC2IFR]

I have used the crown DC300 is sound reinforcement apps and found that if pushed a little to hard it would DC. It would put the full rails dc voltage across the speaker.......hence the name DC-300
Be careful with that amp.

Bill

[W2VW]

Where were you guys when I was trying this stuff? I used a Precision Power 600 watt mobile amp as an audio driver for a pair of 4-1000s in grounded grid. The thing kept kicking itself out until I used a pair of large caps back to back in series with the outputski.

[KB3DKS]

I have used the crown DC300 is sound reinforcement apps and found that if pushed a little to hard it would DC. It would put the full rails dc voltage across the speaker.......hence the name DC-300
Be careful with that amp.
Bill

  I will second that caution Bill! I have personally seen the results due to a customer turning the amplifier on and full up before turning the preamp on.
DC'd the woofers right out of the magnet gap. Blew the amp too...
  The DC 300 Series is unballanced high imp input with, I believe, an AC/DC input coupling switch on the "A" series. It is best to use a low to high input transformer or bridging tranformer for safety. These amplifiers have been used as vinyl recording lathe drivers and also servo positioning drivers in the DC mode.

I believe that the service manuals are still on file somewhere from when I was an authorized shop.

Have a great New Year !!!
Bill, KB3DKS in 1 Land

[N2DTS]

Eric, wb2cau did that for many years, about 30 years ago?

I think he used some sort of power transformer as the mod transformer, and he sounded very good.

Not sure of the power level he ran...

Brett
 

[flintstone mop]

I think Tom VU, K1JJ is using a SS amp to modulate (?) or a speech amp(?) a pair of 4-1000's.
He was having issues with some sort of "kick-back" also.

Fred

[WD5JKO]

Guy,

   I am attaching some sketches and a brief write-up of what I did with my Crown M600 mated to a Johnson Viking I (4D32 RF Tube). This was the beginning of an ultra modulation circuit I developed. I later used a Harmon Kardon audio output transformer described in my first post.

   Sometime later, I added another modulation transformer to act as an audio derived "boosted B+" such that whistling into the Mic brought the B+ from 750 to 1250 volts prior to AC modulation. I did this by rectifying (FW bridge) the second transformer's output, and filtering with 100 Mfd @ 450V, and routing the unmod B+ through there. This circuit add on boosted the unmodulated carrier from 100w to 200w, or you could say about a 3db boost.

   With the second transformer providing the boosted B+, the diodes in the ultra-modulation circuit had a less important role, except during the boosted B+ charge-up time.

   The effect to most folks receiving this rig running a single 4D32 at 800w PEP output AM was the audio was very clear, not wide, and extremely LOUD.

   You might call this, "super modulation with progressive negative cycle unloading with carrier boost".  

Turning off the boosted B+ (a switch), and cranking down the audio to no more that 80% modulation, the super modulation circuit acts as if it is not there, so for casual QSO with good signals the Viking one was acting like a stock rig. When conditions worsen, or to be "battle ready", throw a switch, crank up the level control, and the QSO continues...

To do this with a DC-300, you might need two of them..

All the talk of the DC 300 putting DC to the load causing carnage goes away when you AC couple the output like I did between the M600 and the primary of the modulation transformer. That fear evaporates while those that don't like pumping AC through back to back electrolytics continues..

Jim
WD5JKO