Externally modulating a Valiant

[WR6J]

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

[WR6J]

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

[W2BTK]

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

Very cool!  I just wanted to add that the original idea to use the antek toroidal transformers was from Stu, AB2EZ.  I believe there is a very good thread on the topic started by Stu somewhere around here...

[MikeKE0ZUinkcmo]

Well done!!

I did a somewhat similar Rube Goldberg modulation scheme for my ARC-5 transmitters many years ago using a dual primary 10V Filament transformer and an old SS stereo amplifier.    Worked pretty well.  

I've not tried this sort of thing with my Valiant or other transmitters though.

[W3SLK]

W2BTK said:

Very cool!  I just wanted to add that the original idea to use the antek toroidal transformers was from Stu, AB2EZ.  I believe there is a very good thread on the topic started by Stu somewhere around here...

You beat me to the punch there. Yes it is in the archives because I was doing some research with that less than a year ago. Something tells me (somebody mentioned it), that the mod transformer for the Valiant just doesn't have the 'snot' to properly do its job. I have a Valiant that was modified by Bob Raide, W2ZM(SK). He put a higher voltage plate transformer in to goose the 6146Bs he installed as modulators. I completely stripped the speech amp because there was an issue with AC hum getting into the audio chain. Thus why I got it at a reasonable price. One of these days I will return to it but the thought of using an outboard audio chain/amp is certainly on the table!

[WR6J]

Very cool!  I just wanted to add that the original idea to use the antek toroidal transformers was from Stu, AB2EZ.  I believe there is a very good thread on the topic started by Stu somewhere around here...

___

Thanks for posting and making sure Stu gets recognition for his work. I hope to try your approach on a higher power transmitter next year. Finding suitable chokes will be the challenge.

[WR6J]

Thanks Mike and Mike(y) for the interesting follow up comments. There's definitely a history to the use of the toroids for audio work.

I'm attaching a recording of my setup from a local SDR - I was checking into an AM net.

I hope everyone is having a good Memorial Day weekend.

Richard
WR6J

[N1BCG]

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.

While I'm a huge fan of handling peak limiting and bandwidth prior to the modulator input for tighter control, here's a popular three-diode negative limiter:

https://w5hro.com/attachments/wa1qix-pdf.415/

He uses a couple of Anteks and some very large chokes with the so-called "modified Heising" approach.

Ooof. That term needs to go away and never come back. In order to use the "Heising" name, *all* of these conditions need to be met:

1) There's no modulation transformer
2) The PA and modulator stages share a common power supply
3) The modulator is single ended
4) The current does not change with modulation
5) There's no D.C. blocking capacitor

I've always felt it was too much of a stretch to ignore these when simply adding a modulation reactor to carry the D.C. instead of the modulation transfomer secondary.

[KD1SH]

Susquehanna Hats!

Ooof. That term needs to go away and never come back. In order to use the "Heising" name, *all* of these conditions need to be met:

1) There's no modulation transformer
2) The PA and modulator stages share a common power supply
3) The modulator is single ended
4) The current does not change with modulation
5) There's no D.C. blocking capacitor

I've always felt it was too much of a stretch to ignore these when simply adding a modulation reactor to carry the D.C. instead of the modulation transfomer secondary.

[W3SLK]

KD1SH said:

Susquehanna Hats!

Quote from: N1BCG on Yesterday at 03:49:29 PM

Ooof. That term needs to go away and never come back. In order to use the "Heising" name, *all* of these conditions need to be met:

1) There's no modulation transformer
2) The PA and modulator stages share a common power supply
3) The modulator is single ended
4) The current does not change with modulation
5) There's no D.C. blocking capacitor

I've always felt it was too much of a stretch to ignore these when simply adding a modulation reactor to carry the D.C. instead of the modulation transfomer secondary.

Hey! Be careful there! I live in the Susquehanna Valley along the Susquehanna River and graduated from the school named after the great Iroquois Tribe, Chief Shikellamy!

[KD1SH]

https://www.youtube.com/watch?v=SHZA92LD-1I

KD1SH said:

Susquehanna Hats!

Quote from: N1BCG on Yesterday at 03:49:29 PM

Ooof. That term needs to go away and never come back. In order to use the "Heising" name, *all* of these conditions need to be met:

1) There's no modulation transformer
2) The PA and modulator stages share a common power supply
3) The modulator is single ended
4) The current does not change with modulation
5) There's no D.C. blocking capacitor

I've always felt it was too much of a stretch to ignore these when simply adding a modulation reactor to carry the D.C. instead of the modulation transfomer secondary.

Hey! Be careful there! I live in the Susquehanna Valley along the Susquehanna River and graduated from the school named after the great Iroquois Tribe, Chief Shikellamy!

[W3SLK]