Cathode Transformer Modulation

[N0WEK]

I've got a 100 watt Bogan PA amp around here (PP 6146s or Parallel PP 6146s) that has the 70 volt tap that should work; sounds like a good use for it. Of course it'll have to get in line with getting the radio room built first.

[VE3LYX]

No you actually understand it. The advantage of using the transformer is you don't suffer the voltage loss or drop across the tube as the transformer winding makes that connection and then you wiggle that from the primary winding with your modulator/speech amp deal. The disadvantage is it takes more umph to do. With the tube completing the circuit you have the voltage drop across the tube which is why you dont want a big hd tube as some would think but a small one with low internal resistance and if you need more current carrying ability you add more little tubes in parallel IE 2 to4 6v6s or 6l6s if absolutely necessary. It is very effective and the voltage drop across the mod tube can easily be totally compensated for in PS design. IE if you want 600volts on the finals and you are dropping 200 volts across the mod tube then an 800 volt supply would get you back on track. I have tried a lot of stuff with this over the last couple years. You understanding is very good from what I see having actually done a lot of work experimenting with this. I use the direct tube method series cathode mod on both my ARC5s and on my bare essentials Cw rig when I want it on AM. I have also several times used it on my 45 tube TNT before I got into loop mod. It was harder on the TNT because of the direct heated cathodes in both the TX and the modulator but providing them each with their own independent heater supply cured that. It is a very practical way to take any rig with kathode keying and put it on AM. All the palaber about mod %s and this and that become irrelevant because plain and simple it is straight forward and it works well. After a couple of years of trying both I have retired the transformer one and now only use my direct tube ones of which I have built three and have a fourth under construction.
don
Inspired by the thread I fired up the 80M ARC5 and went on air twice today. Still happy. klappt gut

[Steve - K4HX]

Series and cathode modulation (as defined by Jones) are two different things. One is the equivalent of plate modulation and the other is a combination of grid and plate modulation.

The 1940 Radio Handbook spoke of Cathode Modulation as being with a transformer in the ground circuit of the Final Amplifier Cathode(s) and called the direct tube placed there as "Series Modulation' of the Cathode.

My question - In Cathode Modulation, what is an advantage of using the transformer in the Cathode circuit, in stead of the tube?  (of course, there can be no PWM through a transformer)

A possibility that I am considering is that with the transformer it works with less power loss thana with the tube (power coming from the modulator - but only something like 20%).  That the additional tube placed in series requires consideration of the dissipation lost in the tube so a much high voltage is required (based on the peak voltage limits of the tube(s) used - a type of Series Resistance form).

[Another paper on "Series Modulation" used the tube(s) as a Plate Modulation approach, not using a transformer (nor Heising), but simply having the tube control the flow of current through it to the Plate of the Final Amplifier (being placed between the Final Plate and the HV Power source - cathode of the Modulating tube going to the Plate of the Final Amplifier) - again - a higher voltage is required to compensate for the extra tube.]

or - am I missing something? - vital? -

[W9LCE]

In one of my sources (not recorded [horrible] - from AMFone?) - I seemed to see a difference on cathode modulation, based on where the grid receives its bias. 

If I understand the statement - - If a bias voltage is obtained from a separate source (ground? - PS? - at least - tapped down on a cathode transformer), then the grid receives some modulation through the cathode - but if the bias was "floating" - bias obtained from the cathode above the cathode modulator(?) - so that no change occurs in the voltage relationship between cathode and grid - then the modulation was essentially "Plate Modulation" - just achieved in the cathode. 

From this, do I understand that the Cathode "Plate Modulation" would give a larger Final Amplifier Power level (54% to maybe 75%?), because of the differences between grid dissipation and total tube dissipation?

Had I better forget about my favorite 807 (or even the 6146) and go to triodes (811? - 572b?) - just to forget about the "confusing" screen? or doesn't cathode modulation have problems with the screen? (because it includes the screen?)