[AMRadio] Home brew AM


Brian Carling bcarling at cfl.rr.com
Sat Oct 2 08:54:36 EDT 2004


Thanks for the info Jim.

I will add this to the Boatanchors Directory...
There are several other AM "Cathode Mod" links there too
http://www.af4k.com/Boatanchors_Directory/

On 1 Oct 2004 at 21:43, Jim candela wrote:

> 
> Guys,
> 
>     I have accumulated a couple of circuits for series cathode modulation.
> One is for a 6146 final, and the other for a 807 final, and both use a sweep
> tube or two for the cathode modulator.
> 
> 	Using this technique for QRO AM at 375 watts carrier output will be pretty
> inefficient since the modulator tube will likely be class A. It could be
> class D or E switch mode with a good Low pass filter. I believe that some
> early KA1SI PDM rigs used some big tubes this way. As for the filament
> transformer, a high standoff is really no big deal so long as you wind it
> yourself and insulate accordingly. Heck I once made a 208 VAC to 60 VCT at
> 200 amps where I needed 120 KV insulation primary to secondary. Layers of
> Mylar sheet (like shingles) and transformer oil did the trick. Could also
> maybe use a ferrite horizontal fly back transformer (custom wound) for the
> filament. Why use 60 hertz? Could be 50 Khz so long as the filament is at
> the right temperature.
> 
> Here is a link to two different cathode modulated AM rigs:
> 
> http://pages.prodigy.net/jcandela/Modulator/
> 
> 
> Regards,
> Jim
> WD5JKO
> 
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net]On Behalf Of Brett gazdzinski
> Sent: Friday, October 01, 2004 7:42 AM
> To: 'Discussion of AM Radio'
> Subject: RE: [AMRadio] Home brew AM
> 
> 
> Clay,
> Sounds like a cool idea, and I always thought it should work well
> since there is no mod iron.
> 
> I would go for a lower output using lower voltage though, with different
> tubes.
> 4500 volts is a lot to expect a filament transformer to handle, and if it
> arcs over, it could be into the 110 ac side!
> 
> The 450th is a big triode, so I don’t think you can run it single ended
> pie net output, you would need to do 2 250th tubes in push pull or
> something,
> but the 450th would do the cathode modulator quite well.
> 
> It would be interesting to look at what available tubes would work
> with this type of circuit.
> There are some nice modern tubes to play with, 3cx800's and other
> big triodes, or, if it could work, tetrodes.
> Not sure how cathode modulating a tetrode would work out.
> 
> If you could cathode modulate a pie net output tetrode, that would open
> up a lot of possibilities.
> Maybe find one that runs a lower voltage but is good for some power...
> 
> Brett
> N2DTS
> 
> 
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Clay W7CE
> Sent: Thursday, September 30, 2004 12:52 PM
> To: Discussion of AM Radio
> Subject: Re: [AMRadio] Home brew AM
> 
> 
> One option that isn't discussed often because it is considered inefficient
> at high power is series cathode modulation.  This is accomplished by putting
> the modulator tube between the RF amplifier tube's cathode and ground.  The
> only apparent inefficiency is that you run the RF amp at one fourth of it's
> Class C CW power rating.  I'm currently considering building such a rig
> using a 450TH for the RF amp and another 450TH for the modulator.  At 4500V,
> the 450TH is good for about 1500W out on CW, and with the modulator tube
> added will drop to about 375W of carrier.  On modulation peaks it will hit
> the tubes CW power level of 1500W.  The tube will be doing fairly light duty
> at this power level since it can handle 1500W CCS output at 4500V, and
> should last a very long time as a result.  Based on the price and weight of
> 450TH's vs. large mod transformers, I think it's a good trade off.  In
> addition, the fidelity and audio response will be limited primarily by the
> audio driver, not the mod transformer.  On the negative side, I have to run
> the plates at 4500V vs. about 2500V for plate modulation.  The filament
> transformer for the RF amp will also need to be hi-pot tested to 4500-5000V
> (anyone willing to part with a pair of 7.5V, 12A filament transformers that
> will work for this?).
> 
> Any thoughts and advice on this approach will be appreciated.
> 
> Clay  W7CE
> 
> 
> ----- Original Message -----
> From: "Brett gazdzinski" <Brett.gazdzinski at mci.com>
> To: "'Discussion of AM Radio'" <amradio at mailman.qth.net>
> Sent: Thursday, September 30, 2004 9:03 AM
> Subject: RE: [AMRadio] Home brew AM
> 
> 
> That would be hard, but not impossible.
> You would need to read up on mod transformer construction,
> find something to use as a core, and wind loads of wire on it,
> keeping in mind the very high voltages that it would have to withstand.
> 
> You can buy mod transformers, peter dahl makes new ones, ebay, etc.
> Old surplus is best, old military rigs, broadcast rigs, old ham
> rigs...
> 
> I am using mod transformers that were built by RCA in the 1940's
> for some sort of ship board radio for the Dutch navy I have been told.
> 
> Without any mod iron, you would have to go to some sort of
> screen modulation with large tubes.
> 
> Brett
> N2DTS
> 
> 
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of GGLL
> Sent: Wednesday, September 29, 2004 7:55 PM
> To: Discussion of AM Radio
> Subject: Re: [AMRadio] Home brew AM
> 
> 
> Hello people, I agree with the nice look and smell of "glass" AM. Few years
> ago, I made my CW/AM transmitter, with an 807 plate/screen modulated, 50
> watts
> CW, 40 watts AM output. Started with a triode/pentode  oscillator/doubler
> VFO,
> then converted to solid state oscillator/buffer, followed by a tube driver
> (6BQ5) and finally the 807 to a pi tank output.
> Making my own plate modulated transmitter (200..300 watts out) is a buzzing
> idea in my mind since a couple of months. But here the most difficult item
> to
> get is the modulation transformer, does someone know a method to build it by
> 
> myself?.
> 
> Many thanks in advance
> Best regards
> Guillermo - LU8EYW.
> 
> Brett gazdzinski escribió:
> > Steve,
> > What do you need, everything?
> >
> > Here's a partial list of what you need:
> >
> > 3 Chassis 17x3x14,
> > 3 panels 19x10.25?,
> > 3 sets of side supports,
> > Power transformer to give 1500 volts at 500 plus ma,
> > Choke at 500 plus ma, two is better, swinging for the modulator,
> > Rectifiers (I like the k2aw? bricks),
> > Modulation transformer,
> > Bleeder resistors,
> > Big relay and time delay relay for step start if you go that route,
> > 4 Tube sockets,
> > Filament transformers,
> > Killowatt plug in tank coils and its socket and swinging link,
> > Push pull grid coils for the bands wanted,
> > Push pull grid tuning cap (150 to 200 pf per section, low voltage)
> > Modulator grid drive transformer 8 to 10,000 ohms roughly at 20 or more
> > watts,
> > 2 Neutralizing caps,
> > 4 811a tubes,
> > Grid leak resistor (big wire wound pot works well),
> > Insulated shaft couplings,
> > Plate tuning cap, dual 100 pf 5000 volts or better,
> > Meters for grid current, mod current, plate voltage, plate current,
> >
> > A large pile of wire, high voltage wire, screws, nuts, knobs, connector
> > strips, line cord, switches, lights, panel bushings,fuse and holder or
> > breaker,
> > spacers, ceramic standoffs, rf connectors, etc.
> >
> > I could give you some stuff, and sell you some stuff most likely, and
> > MIGHT be able to get you a mod transformer.
> >
> > Brett
> > N2DTS
> >
> >
> >
> > -----Original Message-----
> > From: amradio-bounces at mailman.qth.net
> > [mailto:amradio-bounces at mailman.qth.net] On Behalf Of
> StephenTetorka at cs.com
> > Sent: Wednesday, September 29, 2004 9:52 AM
> > To: amradio at mailman.qth.net
> > Subject: [AMRadio] Home brew AM
> >
> >
> > Thanks for the replies.
> >
> > The 812/811 combo was sorta what I had in mind.
> >
> > Getting the components shouldn't be too hard if I keep searching.
> >
> > Let me start here...anyone have any suitable parts?
> >
> > About the E class...read a few things about it...sounds most
> > interesting...but MOSFETS don't have the fragrance of warm vacuum tubes on
> a
> > chilly winters evening nor the soft orange glow cast through the rig
> > perforations onto the shack walls to keep one company...nor the humming
> > vibrations of pumped RF being piped to the outside antenna.
> >
> > 73
> > Steve WA2TAK
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