analog series cathode modulator

[W2INR]

In discission with Steve QIX a few weeks ago I was talking about a Ranger 2 that I picked up at NEAR-Fest last fall. Steve had recommended going to series cathode modulation and eliminating iron all together.

I am looking for sugestions, schematics, designs on  how to go about this. What tubes, voltages etc.

Any ideas?

G

[WA1GFZ]

Gary,
You could get away with a FET at the Ranger plate voltage of around 600 volts. Heck an FQA11N90  on a good heatsink would work. Your output would be around 15 watts carrier. You would need about double voltage to run the ranger at full power which would require a tube. I bet an triode connected 813 as a cathode follower would work if you had a 10 CT transformer that could float up to at least 2500 volts. A 1500 volt power supply would be perfect. Also a FET could be used to screen modulate the Ranger but not sure if you would get much over 15 watts carrier. That would be even easier on a FET. Then of course there is cathode modulation where you could put the FET right on the chassis next to the tube but it would need a new 6.3 volt transformer for the final with the cathode connected to the CT.
It doesn't get cleaner than class A.

[w4bfs]

a series modulator with adequate B+ will produce top quality audio but a series pass cathode follower will require an enormous ac grid swing (800 Vpp or so) to fully modulate the ranger's final ... did some looking into this and just about requires a decent driver transformer which is hard to find here ... might could use an audio output but the audio quality would likely be compromised ... don't have to transfer but a little power ... its about the insulation !

this is why putting the modulator in the cathode of the pa makes sense .... there have been reports in the past that beam power tetrodes don't work well cathode modulated but I think that the levels of grid and screen modulation had not been optimized in those designs, IMHO ...73 ...John

[WA1GFZ]

You will not fully modulate the Ranger with a cathode modulator unless you increase the plate voltage.
 Also you only need enough voltage swing to cut off the tube (100% negative). I bet no more than 100 volts on the cathode.
The 6DQ5s in my PDM V2 are cathode driven with a FET and they shut off will maybe 100 volts on the cathode and the supply is at 1550VDC.

[KE6DF]

Attached is a cathode modulator circuit from Orr's book.

It doesn't use a transformer -- not even a power transformer.

[W7TFO]

I have the parts to assemble an AM rig using a 851 triode for use on 160 using topology very similar to this circuit, and somewhat akin to the recent ER article. 

The modulator section has 6SJ7's and 1625's in the lineup. 

The RF side has a Meissner Signal Shifter driving a single Taylor T-55.

HVDC from a bridge of 8008 MV rectifiers.

More to come.

[Steve - WB3HUZ]

Why?

In discission with Steve QIX a few weeks ago I was talking about a Ranger 2 that I picked up at NEAR-Fest last fall. Steve had recommended going to series cathode modulation and eliminating iron all together.

I am looking for sugestions, schematics, designs on  how to go about this. What tubes, voltages etc.

Any ideas?

G

[W7TFO]

Probably because there is altogether enough efficient & reliable AM carrier generation via silicon in this world.

[steve_qix]

Hmmmmm...  well, it depends on how much power you want at carrier.

I am currently experimenting with a low power (defined as less than 100 watts - 5 to 50 watts in this case) output transmitter.  It uses a very clean analog series modulator (class H in this case - the analog modulator is 80% efficient at carrier).

The purpose of this low power transmitter is a very clean, multi band (160/80/40) SIMPLE, inexpensive, high fidelity driver for a linear.  It can also function as a stand-alone low power transmitter.

In this case, I am running 28 volts at around 2.2 amperes at carrier.  However (and this is the important part), in order to be able to modulalate 200% positive (a requirement), the power supply voltage is around 95 volts.
How does this pertain to the Ranger?

If using the same type of system with the Ranger, if you want to run 600 volts at carrier, the high voltage power supply must supply more than 1800 volts (to give you 200% positive modulation capability).

I have never seen a class H modulator with tubes, but maybe its day has come !!!!   Slick, slick, slick !!  I would imagine at least 75% efficiency could be obtained with such an arrangement.

This is not completely thought out at the moment, but off the top of my head I can imagine a 6DQ5 operating as a series modulator (cathode follower), working off an 800 volt supply, supplying 600 volts DC to the final amplifier at carrier.   Let's say 100mA plate current, so that's 20 watts of dissipation in the modulator.  The 6DQ5 can do that.

Now for the peak tube.  It's going to take an additional 1000 volts or so (for a total supply of at least 1800 volts) to give 200% positive capability.   Perhaps a bridge around the power transformer into a capacitor input filter would do it??

From my measurements, the average current flowing through the peak tube, under normal modulation conditions is going to be about 1/2 the carrier current.  That's AVERAGE current.  The AVERAGE voltage drop will probably work out to be about 1/2 the positive peak supply voltage, or around 500 volts.  So, this would work out to be around 25 watts of average dissipation (under normal modulation).  Remember, these are just off the top of my head calculations.  But, at carrier you will get around 75% series modulator plate efficiency.  Not bad.

So, that's an approach to think about 
With straight class A, using all of the same voltages as above, 1800 volts with a 600 volt carrier, the power dissipation required of the modulator will be 120 watts (assuming 100mA carrier current @ 600V).  This yields an efficiency of 33% (180 watts power into the modulator, 60 watts out to the RF amp).

The class A or class H modulators can be implemented as cathode followers, eliminating the need for extensive modifications to the Ranger input circuitry and metering.  A cathode follower modulator is also applicable to many RF amplifier implementations.
Ok, who's going to build it?      I've done it with MOSFETs at low voltage (100 volts), but never with tubes or high voltages.  But, I can't see why it wouldn't work.  A 75% efficient (at carrier) analog modulator with no transformers is a beautiful thing!  Oh, did I mention it's all DC coupled? WOW!

PS

As an asside, this COULD be done with high voltage IGBTs (they are available up to 2500 volts).   A 3 stage class H arrangement could also be possible with standard MOSFETs.  The voltage across any single MOSFET would never exceed 600 volts.  Hmmmmmmmmmmmmmmmm....  I've got an AF67 around here just begging for this circuit........ 

[steve_qix]

.

[Steve - WB3HUZ]

Why?

[Ralph W3GL]

  STEVE - HUZ

      WHY? 

[Steve - WB3HUZ]

Exactly. G is off the air and out of radio. Why worry about modifying a transmitter?

[K1JJ]

http://www.amwindow.org/tech/htm/602mod.htm


Why go through all the trouble to generate big, clean audio to drive a linear?

A simple NE-602 solid state balanced modulator (full write-up in the link above) can be mounted in most any ricebox or used as the heart of a new rig. It will produce  DC to 100khz+  audio with precision at whatever I.F. frequency you desire - and will do 300%... 400% - or as high as you want to run your positive audio peaks.  ($5 and a handful of parts)

I have one running in my FT-102 driving a wide range of linears and feel it's the cleanest AM rig I've ever run to date.

For an AM driver of 50w carrier or less, efficiency is not really an issue  - especially when driving a big linear is the ultimate goal.

Just another way to smoke a roach.

T

[N3DRB The Derb]

Gary aside,  most of your listening audience is hearing you on 40 to 50 year old BA's that have signal to noise ratios of 40 db and audio + detector distortion levels of 10 to 20%.

I think the next USEFUL step in AM evolution after class E, pdm, etc.  needs to be in easy mass non destructive conversions of  B'A's  into something that could take advantage of super high modulation and low distortion. AM has done enough with transmitters, the receiver side of things need to catch up.