Did some calculations for wire length using the AWG table. It makes a huge difference. Right now, #8 is from the breaker box 30 FT to the dryer outlet for the 4-1000 transmitter, which is what I'm plugging in the 3cx to for working on it. It's obvious that wire size will not do.

I should have mentioned that in doing my research I did find the work done by W2BTK. He uses a couple of Anteks and some very large chokes with the so-called "modified Heising" approach. I'm interested to try this when I build a 250W transmitter with a pair of 4D32s since I won't have a mod trans big enough for that project. This solution is a lot smaller and cheaper for the Valiant.

Richard

After experimenting with using a solid-state amplifier to modulate my small 6146B rig, I decided it would be fun to see how well I could modulate my Viking Valiant.

The accessory socket on the back was clearly designed to allow the Valiant to be used as a power supply and/or as an external modulator for another transmitter. However, it can also be wired to allow external modulation of the Valiant. I have a UTC VM-3 modulation transformer that is rated for 250W RF input, which is a good match for the maximum power of the Valiant. The plate impedance for AM is about 2000 Ohms. For the audio driver I used a 9V toroidal mains transformer (Antek AT-1209). I have used this as a push-pull output transformer in some of the experimental audio amplifiers I have built and found it to have remarkably good frequency response. It's rated at 100VA and seems to be able to cope well in the testing I have done. It is wired with the 9V input connected to the 8 Ohm output of my Crown PA. It has two 115V windings that I have wired in series to drive the VM-3. The resulting impedance is (230/9)^2*8 Ohms = 5225 Ohms. The VM-3 is a "Varimatch" transformer and can be wired for 5000:1730, which is an almost perfect match.

The results have been very encouraging with good frequency response and enough audio power for 150% or more modulation. I must be careful to avoid going to 0% modulation and plan to add a 3 diode negative peak limiter. Either that or use some kind of keep-alive voltage. Any advice in this area would be very welcome. When I get chance, I'll post an on-air recording of the audio.

I picked up the VM-3 for $50 and the 9V transformer was $26, so it's a very cheap experiment given I already have the audio chain.

Has anyone else tried externally modulating their Viking (or other transmitter?)

Richard
WR6J

Norm

Back in the early 90s I worked on a Suburban for a pirate that had been busted a couple times in the Miami area.  Cuban guy.

He ended up going mobile at 2.5kw FM Stereo.

I moved a few months after it was all running, never did hear if he got caught.  I know he cloned the amplifier for a friend of his after I moved. 

Had the capability to run off the 454 with an Electrodyne alternator or he had a big penny that hooked up to a flat rack that hung off the trailer hitch.

Back in the early 90s all of southern Floriduh was a pirates heaven.

That was my first vhf amp, too!

--Shane
WP2ASS/ ex KD6VXI

I think  it was Ramsey that got gigged for selling FM transmitters that weren't type accepted in the Broadcast rules, Part 73.  Years ago I worked for one of the FM antenna manufacturers and we used to occasionally get photos of our antennas or clones of them being used in unlicensed FM situations.  Some were on top of apartment buildings and a few were just a single bay sticking out a window.  The Commission would usually send a couple fellows out to explain things and tell them to stop but many just moved down the street to someone else's place and kept on going until the stuff got confiscated after subsequent visits.  It seems like the Miami area was ripe for that activity for some reason.
Now you can get a cheap exciter on Aliexpress and a 100 watt LDMOS amplifier for not much money.  Plug and play antennas are not common but almost anyone can figure out a dipole or ground plane.
About 60 years ago I had an AM transmitter that was potted in black plastic, not much bigger than an ice cube.  It ran on a 9 volt battery, had an input for a crystal microphone and an antenna terminal.  There was a screw that ran a slug through a coil to tune up or down the band.  Not much new under the sun and it's all good fun.
73 de Norm W1ITT

Didn't Ramsey get 'whacked' by the FCC for one of these units or was it their FM kit? I know Ron, WA3WBC had one going I think it put out close to a watt at the antenna, probably less radiated. Anyway, he had it linked to his 200 CD player that was loaded with blue grass and polka music. While he was out running errands he always had it dialed in.

Interesting stuff! Thanks for the comments.

I haven't been able to do a trapezoid test yet - I don't have an RF sampler (except the one that comes with the REA Modulation Monitor) and I'm not sure my PC based oscilloscope is up to the task.

I did try self-modulating the screen with a choke and it worked ok. I tried a 10H choke and also two in series for 20H. The extra choke didn't make any difference from what I could tell. The audio was lacking some bass compared with directly modulating the screen. I suspect that's because I required more audio power and exceeded the 15W rating of the ESE125 - leading to saturation at the lower frequencies. I could try again with my 1628SEA (30W).

What was interesting to me is how much modulating power was needed. With a choke in the screen supply I needed 45W audio to modulate an input power of 94W to the final - as expected. I then tested with the screen directly modulated, and found I needed 20W of audio to modulate the plate and 2W to modulate the screen. That means that one of the benefits of directly modulating the screen is that it reduces the audio power requirements by about half. This would be useful when building higher power rigs.

I found the video on using a capacitor to modulate the screen fascinating. It seems so simple - I will definitely give it a try on this test rig. I was surprised that the Johnson 500 doesn't have a dropping resistor to modulate the screen. How does it function? I also checked the Valiant schematic and couldn't see where the screen is modulated there either. Am I missing something?

I'll report back on the capacitor experiement.

Richard

Nice find.

Nice grab Steve. This subject seems to come up often when using tetrodes in Class C. Simplistic as it may sound, the venerable ART-13 modulation transformer used a tertiary winding to modulate the screen. Some have advocated the use of an audio choke in an effort to self-modulate the screen. This was pointed out to me when I was contemplating on using a 4CX250B(s) in Class C. Woody, N4MQ, found by adding a cap between the screen and the modulated B+ of his Johnson 500, he could increase his % of modulation. https://www.youtube.com/watch?app=desktop&v=yXZBU3OPZdc This was a topic of discussion between me and Dave, W2VW on 160M one night and as he pointed out, he would like to have seen what it looks like on the aforementioned trapazoid test.

And the other picture...