The Great AM Kluge

[W9ZSL]

I have lots of bits and pieces salvaged from an abandoned radio station.  I have a heavy-duty assembly, rack mount with space inside a steel enclosure measuring 13" W x 13 1/2" D and 7 1/4" H. It used to house a Gates broadcast cart machine mounted on pull-out rails. Looks like an exciter waiting to happen.

Enter the Kluge factor. I have a Heathkit VF-1 VFO w 10 V out.  Since I have rack space to burn (6' Gates/IGM rack), the VFO gets a rack-mount home of it's own. The valve is a  6AU6. An OA2 is included. The Exciter will be a 6AG7 followed by a 6L6 driving a 4-125A. I have an xtal for 3885. Other options are 160, 40 and 20 Meters AM. It all depends on what happens next with the Single-Sidewinders that want to stamp out AM. Don't get me started.

The  audio driver is up in the air. I'm covered with the 811A mod tubes. I think I finally found a sketchomatic for a driver using the UTC S-9 with a single 6V6 as the driver. Problem is the S-9 primary is a straight 15KCT and the plate Z of the 6V6 in the circuit I found is 12,500 Ohms. Sounds close enough for government work. All I need to know are the S-9 sec pin-outs. Ed, K9FWR got some tables re: ratios.  Help! Da Mad Mixer Also known as Tonemeister. It's this Recording Studio thing I do on the side. You will one day ask me how I got my AM audio to sound so natural. Simple.  Am I heading in the right direction of an insane 300 watt AM rig?  Mike/ ex K9ZSL/ Passage

[WBear2GCR]

Mike,

Sounds like the S-9 wants to be run P-P, given the CT.
So run it P-P.
Or you could "Parafeed" it and get the DC off it if you want to run it SE.

The xfmr in practical terms doesn't care much what the tube impedance is on the primary, since the secondary is going to see a grid that is rather high-Z until you draw some grid current when you go into B2 - assuming a pair of 811s that run in class B "naturally".

You may need to provide the secondary with some dummy resistance anyhow to get the response flat and free of ringing, and prevent a big crossover when the 811  suddenly demands current from the driver... so that value can be set to make the primary look like what the tube wants to see anyhow.

Once it goes into grid current on peaks the only issue is if there is enough current (snot) available from the driver tubes... well, I'm pretty sure that you can drive a pair of 811s into B2 with a P-P pair of 6W6, which is about 1/2 the power of a 6V6, and if for some strange reason you needed more power, you could shove in a pair of 6L6s, and if that wasn't enough some 6550s would fit too! 

Nothing wrong with using the IT transformer, but if you want better bandwidth, consider the "cathode follower driver"... doesn't need critical iron and will drive the pants off the 811.

My 2 cents on the subject... 

Btw, did any of those links have the S-9 listed??

             _-_-bear

[w3jn]

The UTC S-series mod xformers aren't noted for their hifi bandwidth.  They were made back in the day when the ARRL promoted midrange punch and garbage like high-level splatter filters.

[k4kyv]

The UTC S-series mod xformers aren't noted for their hifi bandwidth.  They were made back in the day when the ARRL promoted midrange punch and garbage like high-level splatter filters.

They are typical "ham radio quality" or "communications grade" audio transformers.

Somewhat better is the UTC medium quality, PA as in public address (later changed to CG as in commercial grade) series.

Their broadcast quality iron is the LS (linear standard) series, as well as the HA, A and O series.  The mod transformers would be included in the LS series.

The VM (later CVM) series of modulation transformers is actually part of the PA/CG  category.

[KE6DF]

Don's reply raises a couple of questions.

I have a Kenyon T-478 modulation transformer (225w) and I wondered where Kenyon stands in the audio quality pecking order. I also have a CVM-3 and am considering which to build a rig around first.

Another question I have concerns the "modified Heising" modulator design where the B+ DC is connected to the final through a choke and the audio is injected by coupling the modulation transformer to the tube side of the plate choke through a blocking capacitor.

I've read several times that one could get higher power out of modulation transformer using this design since the DC plate current is no longer flowing through the modulation transformer -- and therefore is not contributing to core saturation or heating.

But the question is: How much more power? Any rules of thumb out there? Could, for example, my 225W mod transformer put out 250 or 275W and therefore come close to modulating a (current) legal limit class c final?

I read one article that said that the audio power that a modulation transformer could put out is tripled by this design. So a 125W modulation transformer could put out 375W and modulate a 750W final. This seems way far fetched to me. I can't believe a triple.

[W9ZSL]

   Helloo! Well, here's what I'm going for. Got a bid on a UTC S-38 swinging choke 4-20 Hy at 55-550 MA. I scored a 12MFD @ 4KV to stick on the end of the S-38. Plate XFMR came out of a Gates BC-1T-250. It was later up-graded to a KW. I got the 250 watt Gates plate xfmr. Reads about 3600 VCT. Fed 4 x 833s. I should be able to get at least 1800 VDC no load into the S-38 and the filter cap to run the works into the 4-125A final. Now there is that negative tap where the UTC C-104 might be put to work for a secondary supply for the 811A modulators. Hey folks, When I design a rig, I do it like designing one of my recording studios. It's all resonance!   Dadah Daditdah.

[WBear2GCR]

I picked up a pile of late 1960s QSTs over the weekend at the local East Greenbush club's hamfester. The S series is advertised in the 1968 mags - so the specs will be in the later catalog that I linked before... FYI.

              _-_-WBear2GCR

[W9ZSL]

  Now we're getting somewhere! I sat down today and did a rough draft of the front o'da beastie. The rack is about 6' solid welded Gates IGM stuff. I decided not to paint it Packer colors. I'm going for late 50's early 60s Raytheon colors. They did their gear color combinations like Johnson. Dark maroon and gray V.S. Chocolate Brown and Tan/Sand. Controls were highlighted with the brown and white lettering. The panel back-drop was Tan. Striking as heck. Anyone remember Raytheon outside of the anti-missile missiles? This is the first rendering. Feel free to zoom in at will. Initial projects are the HV supply on the bottom, the Modulator and the Exciter. Any help will be so damn cool!  73 Da Mad Mixer